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I2C Driver in Pseudocode

It is written in PseudoCode which is an imaginary programming language that any programmer should be capable of porting to his/her favorite language.

First we will define a set of basic interface routines. All text between / / is considered as remark.

Following variables are used :

• n,x = a general purpose BYTE
• SIZE = a byte holding the maximum number of transferred data at a time
• DATA(SIZE) = an array holding up to SIZE number of bytes. This will contain the data we want to transmit and will store the received data.
• BUFFER = a byte value holding immediate received or transmit data.

1. / $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ /
2. / **** I2C Driver V1.1 Written by V.Himpe. Released as Public Domain **** /
3. / $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ /
4. DECLARE N,SIZE,BUFFER,X Byte
5. DECLARE DATA() Array of SIZE elements
6. SUBroutine I2C_INIT / call this immediately after power-on /
7.       SDA=1
8.       SCK=0
9.       FOR n = 0 to 3
10.             CALL STOP
11.       NEXT n
12. ENDsub
13. SUBroutine START
14.       SCK=1 / BUGFIX !/
15.       SDA=1 / Improvement /
16.       SDA=0
17.       SCK=0
18.       SDA=1
19. ENDsub
20. SUBroutine STOP
21.       SDA=0
22.       SCK=1
23.       SDA=1
24. ENDsub
25. SUBroutine PUTBYTE(BUFFER)
26.       FOR n = 7 TO 0
27.             SDA= BIT(n) of BUFFER
28.             SCK=1
29.             SCK=0
30.       NEXT n
31.       SDA=1
32. ENDsub
33. SUBroutine GETBYTE
34.       FOR n = 7 to 0
35.             SCK=1
36.             BIT(n) OF BUFFER = SDA
37.             SCK=0
38.       NEXT n
39.       SDA=1
40. ENDsub
41. SUBroutine GIVEACK
42.       SDA=0
43.       SCK=1
44.       SCK=0
45.       SDA=1
46. ENDsub
47. SUBroutine GETACK
48.       SDA=1
49.       SCK=1
50.       WAITFOR SDA=0
51.       SCK=0
52. ENDSUB
53. / this concludes the low-level set of instructions for the I2C driver
54. The next functions will handle the telegram formatting on a higher level /
55. SUBroutine READ(Device_address,Number_of_bytes)
56.       Device_adress=Device_adress OR (0000.0001)b /This sets the READ FLAG/
57.       CALL START
58.       CALL PUTBYTE(Device_adress)
59.       CALL GETACK
60.       FOR x = 0 to Number_of_bytes
61.             CALL GETBYTE DATA(x)=BUFFER /Copy received BYTE to DATA array /
62.             IF X< Number_of_bytes THEN /Not ack the last byte/
63.                   CALL GIVEACK
64.             END IF
65.       NEXT x
66.       CALL STOP
67. ENDsub
68. SUBroutine WRITE(Device_address,Number_of_bytes)
69.       Device_adress=Device_adress AND (1111.1110)b / This clears READ flag /
70.       CALL START
71.       CALL PUTBYTE(Device_adress)
72.       CALL GETACK
73.       FOR x = 0 to Number_of_bytes
74.             CALL PUTBYTE (DATA(x))
75.             CALL GETACK
76.       NEXT x
77.       CALL STOP
78. ENDsub
79. SUBroutine RANDOMREAD(Device_adress,Start_adress,Number_of_bytes)
80.       Device_adress=Device_adress AND (1111.1110)b / This clears READ flag /
81.       CALL START
82.       CALL PUTBYTE(Device_adress)
83.       CALL GETACK
84.       CALL PUTBYTE(Start_adress)
85.       CALL GETACK
86.       CALL START /create a repeated start condition/
87.       Device_adress=Device_adress OR (0000.0001)b /This sets the READ FLAG/
88.       CALL PUTBYTE(Device_adress)
89.       CALL GETACK
90.       FOR x = 0 to Number_of_bytes
91.             CALL GETBYTE
92.             DATA(x)=BUFFER
93.             CALL GIVEACK
94.       NEXT x
95.       CALL STOP
96. ENDsub
97. SUBroutine RANDOMWRITE(Device_adress,Start_adress,Number_of_bytes)
98.       Device_adress=Device_adress AND (1111.1110)b / This clears READ flag /
99.       CALL START
100.       CALL PUTBYTE(Device_adress)
101.       CALL GETACK
102.       CALL PUTBYTE(Start_adress)
103.       CALL GETACK
104.       FOR x = 0 to Number_of_bytes
105.             CALL PUTBYTE (DATA(x))
106.             CALL GETACK
107.       NEXT x
108.       CALL STOP
109. ENDsub
110. / $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ /
111. / **** I2C Driver . (c)95-97 V.Himpe . Public Domain release *** /
112. / $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ /

Some notes about the high level routines.

The READ and WRITE routine read / write one or more byte(s) from / to a slave device. Generally this will be used only with Number_of_bytes set to 1. An example:

1. PCD8574=(0100.0000)b
2. CALL READ(PCD8574,1)
3. result = DATA(0)
4. / will read the status of the 8 bit input port of a PCD8574. /
5. DATA(0)=(0110.01010)b
6. CALL WRITE(PCD8574,1)
7. / will write 0110.0101 to the 8 bit port of the PCD8574 /

When do you need a multi-read ? Consider a PCF8582 EEPROM. You want to read its contents in one time.

1. PCF8582=(1010.0000)b
2. CALL READ(PCF8582,255)

You can do the same with WRITE for the EEPROM with the restriction that Number_of_bytes is not larger than 4 AND that you write on page boundaries (multiple of 4 for offset).

You will have to check the components datasheets.

The most useful instructions are RANDOMREAD and RANDOMWRITE.

Write 4 bytes of data to location 20h of the EEPROM:

1. DATA(0)=(1010.0011)b
2. DATA(1)=(1110.0000)b
3. DATA(2)=(0000.1100)b
4. DATA(3)=(1111.0000)b
5. CALL RANDOMWRITE (PCF8582,(20)h,3)

The same goes for reading 16 bytes from the EEPROM starting at address 42h:

1. CALL RANDOMREAD(PCF8582,(42)h,15)

The results are stored in DATA. All you have to do is read them out of the array for processing. When you give the devices address to these routines you don't have to care about the R/W flag. It will be automatically set to the right state inside the routines.

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