4) MICROCONTROLLER FEATURES
Thanks to Robin Getz of National Semiconductor who supplied some of
the material in this section.
CMOS - Complementary Metal Oxide Semiconductor
This is the name of a common technique used to fabricate most (if
not all) of the newer microcontrollers. CMOS requires much less
power than older fabrication techniques, which permits battery
operation. CMOS chips also can be fully or near fully static,
which means that the clock can be slowed up (or even stopped)
putting the chip in sleep mode. CMOS has a much higher immunity
to noise (power fluctuations or spikes) than the older fabrication
techniques.
PMP - Post Metal Programming (National Semiconductor)
PMP is a high-energy implantation process that allows
microcontroller ROM to be programmed AFTER final metalization.
Usually ROM is implemented in the second layer die, with nine or
ten other layers then added on top. That means the ROM pattern
must be specified early in the production process, and completed
prototypes devices won't be available typically for six to eight
weeks. With PMP, however, dies can be fully manufactured through
metalization and electrical tests (only the passivation layers
need to be added), and held in inventory. This means that ROM can
be programmed late in production cycle, making prototypes
available in only two weeks.
Von-Neuman Architecure
Microcontrollers based on the Von-Neuman architecture have a
single "data" bus that is used to fetch both instructions and
data. Program instructions and data are stored in a common main
memory. When such a controller addresses main memory, it first
fetches an instruction, and then it fetches the data to support
the instruction. The two separate fetches slows up the
controller's operation.
Harvard Architecture
Microcontrollers based on the Harvard Architecture have separate
data bus and an instruction bus. This allows execution to occur
in parallel. As an instruction is being "pre-fetched", the
current instruction is executing on the data bus. Once the
current instruction is complete, the next instruction is ready to
go. This pre-fetch theoretically allows for much faster execution
than a Von-Neuman architecture, but there is some added silicon
complexity.
CISC
Almost all of today's microcontrollers are based on the CISC
(Complex Instruction Set Computer) concept. The typical CISC
microcontroller has well over 80 instructions, many of them very
powerful and very specialized for specific control tasks. It is
quite common for the instructions to all behave quite differently.
Some might only operate on certain address spaces or registers,
and others might only recognize certain addressing modes.
The advantages of the CISC architecture is that many of the
instructions are macro-like, allowing the programmer to use one
instruction in place of many simpler instructions.
RISC
The industry trend for microprocessor design is for Reduced
Instruction Set Computers (RISC) designs. This is beginning to
spill over into the microntroller market. By implementing fewer
instructions, the chip designed is able to dedicate some of the
precious silicon real-estate for performance enhancing features.
The benefits of RISC design simplicity are a smaller chip, smaller
pin count, and very low power consumption.
Among some of the typical features of a RISC processor:
- Harvard architecture (separate buses for instructions and
data) allows simultaneous access of program and data, and
overlapping of some operations for increased processing
performance
- Instruction pipelining increases execution speed
- Orthogonal (symmetrical) instruction set for programming
simplicity; allows each instruction to operate on any
register or use any addressing mode; instructions have no
special combinations, exceptions, restrictions, or side
effects
SISC
Actually, a microcontroller is by definition a Reduced Instruction
Set Computer (at least in my opinion). It could really be called
a Specific Instruction Set Computer (SISC). The [original] idea
behind the microcontroller was to limit the capabilities of the
CPU itself, allowing a complete computer (memory, I/O, interrupts,
etc) to fit on the available real estate. At the expense of the
more general purpose instructions that make the standard
microprocessors (8088, 68000, 32032) so easy to use, the
instruction set was designed for the specific purpose of control
(powerful bit manipulation, easy and efficient I/O, and so on).
Microcontrollers now come with a mind boggling array of features
that aid the control engineer - watchdog timers, sleep/wakeup
modes, power management, powerful I/O channels, and so on. By
keeping the instruction set specific (and reduced), and thus
saving valuable real estate, more and more of these features can
be added, while maintaining the economy of the microcontroller.
EEPROM - Electrically Erasable Programmable Read Only Memory
Many microcontrollers have limited amounts of EEPROM on the chip.
EEPROM seems more suited (becuase of its economics) for small
amounts of memory that hold a limited number of parameters that
may have to be changed from time to time. This type of memory is
relatively slow, and the number of erase/write cycles allowed in
its lifetime is limited.
FLASH (EPROM)
Flash provides a better solution than regular EEPROM when there is
a requirement for large amounts of non-volatile program memory.
It is both faster and permits more erase/write cycles than EEPROM.
Battery backed-up static RAM
Battery backed-up static RAM is useful when a large non-volatile
program and DATA space is required. A major advantage of static
RAM is that it is much faster than other types of non-volatile
memory so it is well suited for high performance application.
There also are no limits as to the number of times that it may be
written to so it is perfect for applications that keep and
manipulate large amounts of data locally.
Field programming/reprogramming
Using nonvolatile memory as a place to store program memory allows
the device to be reprogrammed in the field without removing the
microcontroller from the system that it controls. One such
application is in automotive engine controllers. Reprogrammable
non-volatile program memory on the engine's microcontroller allows
the engine controller program to be modified during routine
service to incorporate the latest features or to compensate for
such factors as engine aging and changing emissions control laws
(or even to fix bugs!!). Reprogramming of the microcontroller
could become a standard part the routine engine tune-up.
Almost every application could benefit from this type of program
memory - If a modem's hardware supported it, you could remotely
upgrade your modem from Vfast to V.34, or incorporate new features
such as voice control or a digital answering machine.
OTP - One Time Programmable
An OTP is a PROM (Programmable Read-Only-Memory) device. Once
your program is written into the device with a standard EPROM
programmer, it can not be erased or modified. This is usually
used for limited production runs before a ROM mask is done in
order to test code.
A OTP (One Time Programmable) part uses standard EPROM, but the
package has no window for erasing. Once your program is written
into the device with a standard EPROM programmer, it cannot be
erased or modified. (Well, sort of - any bit that is a one can be
changed to a zero - but a bit that is a zero cannot be changed
into a one).
As product design cycles get shorter, it is more important for
micro manufacturers to offer OTPs as an option. This was commonly
used for limited production runs before a ROM mask in order to
test code. However, one problem with Mask ROM is that
programming, setup, and engineering charges make it economical
only when the systems manufacturer purchases large quantities of
identically programmed micros. Then when you discover THAT bug
(and find it and fix your code), you have quantities of *old
buggy* micros around that you have to throw away. Not to mention
that lead time (the time when you submit your code to the micro
manufacture, to the time you receive your micro with your code on
it) can be at least 8 weeks, and as bad as 44 weeks.
Software protection
Either by encryption or fuse protection, the programmed software
is protected against unauthorized snooping (reverse engineering,
modifications, piracy, etc.).
This is only an option on OTPs and Windowed devices. On Masked
ROM devices, security is not needed - the only way to read your
code would be to rip the microcontroller apart with a scanning
electron microscope - and how many people really have one of
those?
Although - and this is a manufacturer's little know fact - when a
silicon manufacturer makes your ROMed microcontroller - they have
to test it in order to make sure that it is programmed properly.
(You should see what a spec of dust does on a mask :-) In order
to test this, they must be able to read out the ROM and compare it
to the code you submitted. This mode is known as test mode. IN
TEST MODE YOU CAN READ OUT THE ROM OF ANY DEVICE. Anybody who
tells you different, does not know what they are talking about -
or is lying. This is usually not a big deal because test mode is
***VERY*** confidential, and (usually) only known by that
manufacturer (i.e. you cannot put a device into test mode by
accident). Test mode is ONLY applicable with ROMed devices.
Low voltage parts
Since automotive applications have been the driving force behind
most microcontrollers, and 5 Volts is very easy to do in a car,
most microcontrollers have only supported 4.5 - 5.5 V operation.
In the recent past, as consumer goods are beginning to drive major
segments of the microcontroller market, and as consumer goods
become portable and lightweight, the requirement for 3 volt (and
lower) microcontrollers has become urgent (3 volts = 2 battery
solution / lower voltage = longer battery life). Most low voltage
parts in the market today are simply 5 volt parts that were
modified to operate at 3 volts (usually at a performance loss).
Some micros being released now are designed from the ground up to
operate properly at 3.0 (and lower) voltages, which offer
comparable performance of the 5 volt devices.
Now, why are voltages REALLY going down on ICs? Paul K. Johnson
(of Hewlett-Packard) explains:
There are a few interesting rules of thumb regarding transistors:
1) The amount of power they dissipate is proportional to their
size. If you make a transistor half as big, it dissipates
half as much power.
2) Their propagation delay is proportional to their size. If you
make a transistor half as big, it's twice as fast.
3) Their cost is proportional to the square of their size. If
you make them half as big, they cost one quarter as much.
If you make a transistor smaller, you improve the power, speed,
and cost. The only drawback is that they are harder to make.
(Hey, how hard can it be for HP, IBM, Motorola, National, etc?
ed.) Everybody in the world wants to make transistors smaller and
smaller, the advantages are enormous.
For years people have been using 5 Volts to power IC's. Because
the transistors were large, there was little danger damaging the
transistor putting this voltage across it. However, now that the
transistors are getting so small, 5 Volts will actually fry them.
The only way around this is to start lowering the voltage. This
is why people are now using 3 (actually 3.3) Volt logic, and lower
in the next few years. It isn't just because of batteries.
Brownout Protection
Brownout protection is usually an on-board protection circuit that
resets the device when the operating voltage (Vcc) is lower than
the brownout voltage. The device is held in reset and will remain
in reset when Vcc stays below the Brownout voltage. The device
will resume execution (from reset) after Vcc has risen above the
brownout Voltage.
Idle/Halt/Wakeup
The device can be placed into IDLE/HALT mode by software control.
In both Halt and Idle conditions the state of the microcontroller
remains. RAM is not cleared and any outputs are not changed. The
terms idle and halt often have different definitions, depending on
the manufacturer. What some call idle, others may call halt, and
vice versa. It can be confusing, so check the data sheet for the
device in question to be sure.
In IDLE mode, all activities are stopped except:
- associated on-board oscillator circuitry
- watchdog logic (if any)
- the clock monitor
- the idle timer (a free running timer)
Power supply requirements on the microcontroller in this mode are
typically around 30% of normal power requirements of the
microprocessor. Idle mode is exited by a reset, or some other
stimulus (such as timer interrupt, serial port, etc.). A special
timer/counter (the idle timer) causes the chip to wake up at a
regular interval to check if things are OK. The chip then goes
back to sleep.
IDLE mode is extremely useful for remote, unattended data logging
- the microprocessor wakes up at regular intervals, takes its
measurements, logs the data, and then goes back to sleep.
In Halt mode, all activities are stopped (including timers and
counters). The only way to wake up is by a reset or device
interrupt (such as an I/O port). The power requirements of the
device are minimal and the applied voltage (Vcc) can sometimes be
decreased below operating voltage without altering the state
(RAM/Outputs) of the device. Current consumption is typically
less than 1 uA.
A common application of HALT mode is in laptop keyboards. In
order to have maximum power saving, the controller is in halt
until it detects a keystroke (via a device interrupt). It then
wakes up, decodes and sends the keystroke to the host, and then
goes back into halt mode, waiting either for another keystroke, or
information from the host.
Multi-Input Wakeup (National Semiconductor)
The Multi-Input WakeUp (MIWU) feature is used to return (wakeup)
the microcontroller from either HALT or IDLE modes. Alternately
MIWU may also be used to generate up to 8 edge selectible external
interrupts. The user can select whether the trigger condition on
the pins is going to be either a positive edge (low to high) or a
negative edge (high to low).
UART
A UART (Universal Asynchronous Receiver Transmitter) is a serial
port adapter for asynchronous serial communications.
USART
A USART (Universal Synchronous/Asynchronous Receiver Transmitter)
is a serial port adapter for either asynchronous or synchronous
serial communications. Communications using a USART are typically
much faster (as much as 16 times) than with a UART.
Synchronous serial port
A synchronous serial port doesn't require start/stop bits and can
operate at much higher clock rates than an asynchronous serial
port. Used to communicate with high speed devices such as memory
servers, display drivers, additional A/D ports, etc. Can also be
used to implement a simple microcontroller network.
SPI (Motorola)
An SPI (serial peripheral interface) is a synchronous serial port.
SCI
An SCI (serial communications interface) is an enhanced UART
(asynchronous serial port).
I2C bus - Inter-Integrated Circuit bus (Philips)
The I2C bus is a simple 2 wire serial interface developed by
Philips. It was developed for 8 bit applications and is widely
used in consumer electronics, automotive and industrial
applications. In addition to microcontrollers, several
peripherals also exist that support the I2C bus.
The I2C bus is a two line, multi-master, multi-slave network
interface with collision detection. Up to 128 devices can exist
on the network and they can be spread out over 10 meters. Each
node (microcontroller or peripheral) may initiate a message, and
then transmit or receive data. The two lines of the network
consist of the serial data line and the serial clock line. Each
node on the network has a unique address which accompanies any
message passed between nodes. Since only 2 wires are needed, it
is easy to interconnect a number of devices.
MICROWIRE/PLUS (National Semiconductor)
MICROWIRE/PLUS is a serial synchronous bi-directional
communications interface. This is used on National Semiconductor
Corporation's devices (microcontrollers, A/D converters, display
drivers, EEPROMS, etc.).
CAN & J1850
CAN (Controller Area Network) is a mutiplexed wiring scheme that
was developed jointly by Bosh and Intel for wiring in automobiles.
J1850 is the SAE (Society of Automotive Engineers) multiplexed
automotive wiring standard that is currently in use in North
America.
Both of these groups have the "NOT INVENTED HERE" syndrome and
refuse to work with each other's standard. The standards are quite
different and are not compatible at all.
The CAN specification seems to be the one that is being used in
industrial control both in North American and Europe. With lower
cost microcontrollers that support CAN, CAN has a good potential
to take off.
Analog to Digital Conversion (A/D)
Converts an external analog signal (typically relative to voltage)
and converts it to a digital representation. Microcontrollers
that have this feature can be used for instrumention,
environmental data logging, or any application that lives in an
analog world.
The various types of A/D converters that can be found:
Succesive Approximation A/D converters -- This the most common
type of A/D and is used in the majority of microcontrollers. In
this technique, the converter figures out each bit at a time (most
significant first) and finds if the next step is higher or lower.
This way has some benefits - it takes exactly the same amount of
time for any conversion - it is very common - (and therefore very
cheap). However it also has some disadvantages - it is slow - for
every bit it takes at least one clock cycle - the best an 8-bit
A/D can do is at least 8 clock cycles (and a couple for
housekeeping). Because it takes so long - it is a power hog as
compared to the other types of A/Ds.
Single Slope A/D converters -- This is the type of converter that
you can build yourself (if the microcontroller has a couple of
analog blocks on it). Your single slope A/D converter would
include Analog Mux / comparator / timer (8-bit timer = 8 bit A/D -
16-bit timer = 16 bit A/D) with input capture and a constant
current source. The only microcontroller (that I know of) that
has all of this on it is National's COP888EK.
First Step is to clear the timer to 0000 and then start it. It is
a simple matter to hang an external capacitor, and charge it with
the constant current source (linearly because of the current
source) when the voltage on the cap exceeds the sampling voltage,
the comparitor toggles, stops the timer - and voila - you have the
voltage in uSecs - with 16-bit accuracy. The only drawback is you
can't really expect 16 bits (14 yes) - the conversion time varies
quite a bit, and it is SLOW.
Delta-Sigma A/Ds converters -- This type of A/D converter is found
on higher-end DSPs. These are the hardest to understand of the
A/Ds because it just makes a best guess (a little National
Semiconductor humor here :-). Delta sigma A/Ds can be broken down
into two main parts.
The modulator which does the A/D conversion and the filter, which
turns the output of the modulator into a format suitible for the
microcontroller (or DSP).
The modulator is very simple - it just compares the input voltage
to the average of the last 100 (or so) modulator outputs and
decides if the input is higher or lower than the average. This
happens millions of times a second, resulting in a high speed
single-bit datastream of 1s and 0s who's *average* is equal to the
input voltage. Becuse the ouput is only a one or a zero, there are
very few sources of errors. This is the main reason that
delta-sigma A/Ds are **very** accurate.
The filter comes after the modulator ... and this filter is
essentially a big DSP block. It must take the very high speed
stream of ones and zeros and turn it into a slower speed stream of
16-bit (or greater) words to be used by the microcontroller. This
process is called decimation and the filter is often called a
"comb filter". Another digital filter follows this stage and
rejects unwanted frequencies. This filter performs a similar
function to the anti-aliasing filter required in many traditional
A/D appliactions, but it does it at an unprecedented level of
performance and at low cost. This is the other major benefit of
delta-sigma A/Ds.
Flash A/D -- This is the basic architecure for the fastest
category of A/Ds. The flash converter involves looking at each
level that is possible and instantaneously saying what level the
voltage is at. This is done by setting up comparators as
threshold detectors with each detector being set up for a voltage
exaclty 1 LSB higher than the detector below it. The benefit of
this architecture is that with a single clock cycle, you can tell
exactly what the input voltage is - that is why it is so fast.
The disadvantage is that to achieve 8-bit accuracy you need 256
comparators and to achieve 10-bit accuracy you need 1024
comparators. To make these comparators operate at higher speeds,
they have to draw LOTS of current, and beyond 10 bits, the number
of comparators required becomes totally unmanageable.
D/A (Digital to Analog) Converters
This feature takes a Digital number and converts it to a analog
output. The number 50 would be changed to the analog output of
(50/256 * 5Volts) = .9765625V on a 8-bit / 5 Volt system.
Pulse width modulator
Often used as a digital-to-analog conversion technique. A pulse
train is generated and regulated with a low-pass filter to
generate a voltage proportional to the duty cycle.
Pulse accumulator
A pulse accumulator is an event counter. Each pulse increments
the pulse accumulator register, recording the number of times this
event has occurred.
Input Capture
Input Capture can measure external frequencies or time intervals
by copying the value from a free running timer into a register
when an external event occurs.
Comparator
One or more standard comparators can sometimes be placed on a
microcontroller die. These comparators operate much like standard
comparators however the input and output signals are available on
the microcontroller bus.
Mixed (Analog-Digital) Signal
We live in an analog world where the information we see, hear,
process, and exchange with each other, and with our mechanical and
electronic systems, is always an analog quantity - pressure,
temperature, voltage, current, air and water flow are always
analog entities. They can be digitized for more efficient
sorting, storage and transmittal, but the interface - the input
and output - is almost always analog. Thus the essence of analog
electronics lies in sensing continuously varying information,
shaping and converting it for the efficiency of digital processing
and transmission, and reshaping the digital data to an analog
signal at the other end.
Mixed analog-digital devices are being used increasingly to
integrate the complex functions of high-speed telecommunications,
or the real-time data processing demanded by industrial control
systems and automotive systems. Start looking for
microcontrollers that have analog comparators, analog
multiplexers, current sources, voltage doublers, PLL (Phase Lock
Loops) and all sorts of peripherals that you thought were analog
only.
Polling
Polling is not really a "feature" - it's what you have to do if
your microcontroller of choice does not have interrupts.
Polling is a software technique whereby the controller continually
asks a peripheral if it needs servicing. The peripheral sets a
flag when it has data ready for transferring to the controller,
which the controller notices on its next poll. Several such
peripherals can be polled in succession, with the controller
jumping to different software routines, depending on which flags
have been set.
Interrupts
Rather than have the microcontroller continually polling - that
is, asking peripherals (timers / UARTS / A/Ds / external
components) whether they have any data available (and finding most
of the time they do not), a more efficient method is to have the
peripherals tell the controller when they have data ready. The
controller can be carrying out its normal function, only
responding to peripherals when there is data to respond to. On
receipt of an interrupt, the controller suspends its current
operation, identifies the interrupting peripheral, then jumps
(vectors) to the appropriate interrupt service routine.
The advantage of interrupts, compared with polling, is the speed
of response to external events and reduced software overhead (of
continually asking peripherals if they have any data ready).
Most microcontrollers have at least one external interrupt, which
can be edge selectible (rising or falling) or level triggered.
Both systems (edge/level) have advantages. Edge - is not time
sensitive, but it is susceptible to gitches. Level - must be held
high (or low) for a specific duration (which can be a pain - but
is not susceptible to glitches).
Interrupts are critical when you are controlling anything (this is
what microcontrollers do). If you misunderstand any of the terms,
and design your systems with the way you *think* it works - not
the way it *really* works - it will effect system performance. It
may also work for a very long time with no problems, and then all
of a sudden fail. Check your datasheets - these descriptions are
the correct ones (or are at least supposed to be), but that does
not mean that they are agreed to by the silicon manufacturers, (or
by the marketing guys that they employ, and who write parts of the
data sheets.)
4 bit microcontrollers usually have either a polling or
non-vectored type of interrupt scheme. 8 and 16 bit
microcontrollers usually have some type of vectored arbitration
type of interrupt scheme. 32 bit microcontrollers usually will
have some type of vectored priority type of interrupt scheme.
Again, check your data sheet to make sure - or ask a
manufacturer's rep if you aren't sure.
Maskable Interrupts
A maskable interrupt is one that you can disable or enable
(masking it out means disabling the interrupt), whereas
non-maskable interrupts you can't disable. The benefit of
maskable interrupts is that you can turn off a particular
interrupts (for example a UART) during some time critical task.
Then, those particular interrupts will be ignored thus allowing
the microcontroller to deal with the task at hand. Most
microcontrollers (as well as most microprocessors) have some type
of Global Interrupt Enable (GIE) which allows you to turn off (or
on) all of the maskable interrupts with one bit. NOTE: GIE
usually does not effect any NMI (Non-Maskable Interrupts)
Vectored Interrupts
Simple (non-vectored) interrupts is one of the simplest interrupt
schemes there is (Simple = less silicon = more software = slower).
Whenever there is an interrupt, the program counter (PC) branches
to one specific address. At this address, the system designer
needs to check the interrupts (one at a time) to see which
peripheral has caused the interrupt to occur. Code for this may
look like (on a COP8):
IFBIT UART,PSW ; If the UART bit has been set
JP UART_Recieve ; Jump to the UART receive service routine
IFBIT T1,PSW ; If the timer has underflowed
JP Underflow ; Jump to the underflow service routine
... and so on
This can be *very* slow - and the time between the interrupt
happening and the time the service routine is entered, depends on
how the system designer sets up their ranking. The peripheral
that is checked last takes the longest to process. Most
microcontrollers that have fewer than 3 - 5 interrupts use this
method. The benefit of this is that the system designer can set
the priority - The most important peripheral gets checked first -
and you get to decide which peripheral that is.
Vectored interrupts are a little easier to set up, but the system
designer has less control of the system (i.e. is dependent on the
silicon manufacture to make the proper decisions during design of
the chip). When an interrupt occurs, the hardware interrupt
handler automatically branches to a specific address depending on
what interrupt occurred. This is much faster than the
non-vectored approach described above, however the system designer
does not get to decide what peripheral gets checked first.
Example (on a National Semiconductor COP888CG):
Rank Source Description Vector Address
------------------------------------------------------------------
1 (highest) Software INTR Instruction 01FE - 01FF
2 External Pin G0 Edge 01FA - 01FB
3 Timer T0 Underflow 01F8 - 01F9
4 Timer T1 T1A / Underflow 01F6 - 01F7
5 Timer T1 T1B 01F4 - 01F5
6 MICROWIRE/PLUS BUSY Goes Low 01F2 - 01F3
7 UART Receive 01EE - 01EF
8 UART Transmit 01EC - 01ED
9 Timer T2 T2A / Underflow 01EA - 01EB
10 Timer T2 T2B 01E8 - 01E9
11 Timer T3 T3A / Underflow 01E6 - 01E7
12 Timer T3 T3B 01E4 - 01E5
13 Port L / MIWU Port L Edge 01E2 - 01E3
14 (lowest) Default VIS Interaction 01E0 - 01E1
In ROM location 01F8 - 01F9 (2bytes x 8 bits = 16bit address) the
system designer enters the ROM location of where they want the
service routine (of the Timer T0 underflow) to be. And so on for
the rest of the addresses.
Interrupt arbitration and priority
Interrupt arbitration and priority - These are two of the most
misused words when it comes to microcontrollers (microprocessors
too for that matter) and it's generally because no one knows the
difference between them. Priority is not Arbitration.
Arbitration is not Priority. Lets see if we can sort out the
differences.
Arbitration - If you look at the above chart of the COP888CG, you
may think the interrupts are prioritized because they have some
ranking. They do have rank, but they are not prioritized. What
happens is that (in an arbitration scheme) when an interrupt
occurs, the GIE (Global Interrupt Enable) is cleared. This
effectively means that all future interrupts will be delayed until
the GIE is set. The GIE becomes set only if the system designer
sets it in a service routines, or on a RETI (Return from
Interrupt).
Quick Example 1 - Timer 1 underflows - the hardware clears the
GIE, looks at ROM locations 01F6 and 01F7 and jumps to the ROM
location pointed to by those addresses. The program does a couple
things, and then sets the GIE (because the user wants to recognize
an external interrupt during this service routine). However while
in the service routine, Timer 3 underflows. Although a timer 3
underflow is lower in rank than a timer 1 underflow, the interrupt
handler does not care - it simply looks at the GIE, and because it
is set - handles the interrupt (now we have nested interrupts).
The Timer 1 underflow service routine will not be completed until
the Timer 3 underflow is complete.
Quick Example 2 - Timer 3 underflows at the same time as an
External interrupt occur. The one to be handled first is the
External Interrupt. If the user sets the GIE, the interrupt
handler will jump down to the Timer 3 underflow handler. If the
user does not set the GIE, the microcontroller handles the
External interrupt, does a RETI, and the Timer 3 underflow can now
be handled.
Priority - In a priority scheme, things are prioritized (well,
what'd you expect?). If Timer T0 underflows, the only thing that
can interrupt that is an external or software interrupt. If a
external or software interrupt occurs, the interrupt handler will
branch to these service routines. When they are complete, it will
return to the Timer T0 underflow.
Quick Example - In the below timing diagram, the following
happens:
1) Timer T0 underflows
2) Timer T2 underflows
3) An External Interrupt occurs.
In a priority scheme, the following would happen:
External Interrupt |---------|
| |
Timer T0 Underflow |-------| |------|
| |
Timer T2 Underflow | |------|
| |
Normal Execution ---| |-------
^ ^ ^ ^ ^ ^
| | | | | |
Time -> | | | | | \-T2 Done
| | | | \-------- T0 Done
| | | \-------------- Ext Done
| | \------------------------ Ext Edge
| \----------------------- T2 Underflows
\--------------------------- T0 Underflows
This is what RTOS (Real Timer Operating Systems) do - prioritize
and handle interrupts.
Watchdog timer
A watchdog timer provides a means of graceful recovery from a
system problem. This could be a program that goes into an endless
loop, or a hardware problem that prevents the program from
operating correctly. If the program fails to reset the watchdog
at some predetermined interval, a hardware reset will be
initiated. The bug may still exist, but at least the system has a
way to recover. This is especially useful for unattended systems.
Digital Signal Processors (DSP)
Microcontrollers react to and control events - DSPs execute
repetitive math-intensive algorithms. Today many embedded
applications require both types of processors, and semiconductor
manufacturers have responded by introducing microcontrollers with
on-chip DSP capability and DSPs with on-chip microcontrollers.
The most basic thing a DSP will do is a MACC (Multiply and
ACCumulate). The number of data bits a DSP can Multiply and
ACCumulate will determine the dynamic range (and therefore the
application).
Bits Fixed/Floating Dynamic Range Typical Application
8 Fixed 48 dB Telephone-quality voice
16 Fixed 96 dB Compact disk (marginal)
24 Fixed 144 dB Compact disk
(room for error)
Clock Monitor
A clock monitor can shut the microcontroller down (by holding the
microcontroller in reset) if the input clock is too slow. This
can usually be turned on or off under software control.
Resident program loader
Loads a program by Initializing program/data memory from either a
serial or parallel port. Convenient for prototyping or trying out
new features, eliminates the erase/burn/program cycle typical with
EPROMs, and allows convenient updating of a system even from an
offsite location.
Monitor
A monitor is a program installed in the microcontroller which
provides basic development and debug capabilities. Typical
capabilities of a microcontroller monitor include: loading object
files into system RAM, executing programs, examining and modifying
memory and registers, code disassembly, setting breakpoints, and
single-stepping through code. Some simple monitors only allow
basic functions such as memory inspection, and the more
sophisticated monitors are capable of a full range of debug
functions.
Monitors can either communicate with a dumb terminal or with a
host computer such as a PC. Much of the work of the monitor (such
as user interface) can be offloaded to the host PC running a
program designed to work with the monitor. This makes it possible
to reduce the size and complexity of the code that must be
installed in the target system.
MIL transducer
An MIL transducer is a sophisticated and expensive device that
detects the presence of your mother-in-law. Sensitivity settings
are possible for a full range of stimuli such as: snarling,
stomping, nasty faces, and others. Techno-Wimp (address withheld
upon request), the sole manufacturer of the MIL transducer, has
recently announced a major new version which is sensitive enough
to detect less-tangible stimuli. This breakthrough product is
dubbed the MIL-WOMF ("Whoa, outta my face!") transducer. Both the
original MIL and the new MIL-WOMF transducers are programmable and
easy to interface to most microcontrollers.