High-Speed I2C (HS-Mode)
High-speed mode (Hs-mode) devices offer a quantum leap in I2C-bus transfer speeds. Hs-mode devices can transfer information at bit rates of up to 3.4 Mbit/s, yet they remain fully downward compatible with Fast- or Standard-mode (F/S-mode) devices for bi-directional communication in a mixed-speed bus system. With the exception that arbitration and clock synchronization is not performed during the Hs-mode transfer, the same serial bus protocol and data format is maintained as with the F/S-mode system. Depending on the application, new devices may have a Fast or Hs-mode I2C-bus interface, although Hs-mode devices are preferred as they can be designed-in to a greater number of applications.
High speed transfer
To achieve a bit transfer of up to 3.4 Mbit/s the following improvements have been made to the regular I 2 C-bus specification:
- Hs-mode master devices have an open-drain output buffer for the SDAH signal and a combination of an open-drain pull-down and current-source pull-up circuit on the SCLH output (1) . This current-source circuit shortens the rise time of the SCLH signal. Only the current-source of one master is enabled at any one time, and only during Hs-mode.
- No arbitration or clock synchronization is performed during Hs-mode transfer in multi-master systems, which speeds-up bit handling capabilities. The arbitration procedure always finishes after a preceding master code transmission in F/S-mode.
- Hs-mode master devices generate a serial clock signal with a HIGH to LOW ratio of 1 to 2. This relieves the timing requirements for set-up and hold times.
- As an option, Hs-mode master devices can have a built-in bridge (1) . During Hs-mode transfer, the high speed data (SDAH) and high-speed serial clock (SCLH) lines of Hs-mode devices are separated by this bridge from the SDA and SCL lines of F/S-mode devices. This reduces the capacitive load of the SDAH and SCLH lines resulting in faster rise and fall times.
- The only difference between Hs-mode slave devices and F/S-mode slave devices is the speed at which they operate. Hs-mode slaves have open-drain output buffers on the SCLH and SDAH outputs. Optional pull-down transistors on the SCLH pin can be used to stretch the LOW level of the SCLH signal, although this is only allowed after the acknowledge bit in Hs-mode transfers.
- The inputs of Hs-mode devices incorporate spike suppression and a Schmitt trigger at the SDAH and SCLH inputs.
- The output buffers of Hs-mode devices incorporate slope control of the falling edges of the SDAH and SCLH signals.
The figure below shows the physical I 2 C-bus configuration in a system with only Hs-mode devices. Pins SDA and SCL on the master devices are only used in mixed-speed bus systems and are not connected in an Hs-mode only system. In such cases, these pins can be used for other functions.
Optional series resistors Rs protect the I/O stages of the I2C-bus devices from high-voltage spikes on the bus lines and minimize ringing and interference. Pull-up resistors Rp maintain the SDAH and SCLH lines at a HIGH level when the bus is free and ensure the signals are pulled up from a LOW to a HIGH level within the required rise time. For higher capacitive bus-line loads (>100 pF), the resistor Rp can be replaced by external current source pull-ups to meet the rise time requirements. Unless proceeded by an acknowledge bit, the rise time of the SCLH clock pulses in Hs-mode transfers is shortened by the internal current-source pull-up circuit MCS of the active master.