What is I²C

Rather than use an 8 bit parallel data bus, the obvious solution was to use a serial data link. After all, speeds of 1000 bytes/second would be trivial to obtain, and most people do not change channel that often. A pair of signal lines, Clock and Data would suffice to link each control system to the central microcontroller. This was nothing original, however Philips had the idea of a pair of signal lines that linked all the control systems to the central microcontroller. Then, a protocol was developed which allowed not only data transmission, but also the addressing of individual control systems, still only using these 2 lines. In fact, protocol even allows for multiple controlling microprocessors, but this will not be covered here.

The 2 signal lines, are, as suggested above, Clock and Data. They are conventionally called SCL and SDA respectively. Both are bi-directional signals, implemented in the following way : Each device on the I²C bus can monitor the voltage, or logic level, on both signals. Also, a device may connect the line to the system ground rail through an electronic switch, or it may leave it floating (For those who've done a bit of digital electronics, this is known as an `open collector' output). The first feature provides an input into the device, and the second an output. External resistors (typically about 4.7k ) are connected between each of the 2 signal lines and the +5V power supply rail, so that if no device on the bus is connecting a line to ground, then that line appears to be in a logic 1 state. Before the protocol can be described, it's important to understand 4 terms :

Transmitter

  Receiver

  Master

  Slave

A Transmitter is simply a device that outputs data onto the bus, while a Receiver is one that accepts data from the bus. Each communication occurs between exactly one Transmitter and one Receiver.

A Master is the device which controls the SCL line, starts and stops the data transfer, and controls the addressing of the other devices on the bus.

A Slave is simply the device that has been selected by the Master, and is being controlled by it.

Again, each communication occurs between exactly one Master and one Slave. There is, however, no connection between the functions of Master and Slave on the one hand and Transmitter and Receiver on the other. The I²C driver for IBM PC's and clones described here is always the Master, but can act as either a Transmitter or a Receiver, and can therefore communicate in both directions with Slave devices.

Data Transfer on the I²C Bus

The source code for a Turbo Pascal Unit for an IBM PC to drive the I²C bus is given in Appendix C. This program should be consulted while reading this description, even if the final method of implementing the I²C bus is different.

The most important thing to realise about data transfer on the I²C bus is that the state of the SDA line can only change when the SCL line is in the logic low state. The reason for this requirement will be explained later.

The 2 cases of data transfer, either from the Master to the Slave, or from the Slave to the Master, will now be considered separately.

Transfer from Master to Slave

8-bit bytes are transferred from the Master Transmitter to the Slave Receiver by the procedure syndi2c in Appendix C. These bytes are sent most-significant bit first, by first copying the 128s bit of the word into the variable this bit and then setting the SDA line to the state of this bit. Then the SCL line is pulsed high and low again, to clock that bit into the Slave device. The low 7 bits of the 8-bit value are now shifted left, so that each time, the next bit is placed in the 128s bit position. These operations are repeated for the 8 bits of the byte.

After the 8 bits are transferred, an extra clock pulse is generated, and during this clock pulse, the state of the SDA line is copied into the ack variable. If the Slave has accepted the byte, then it will pull the SDA line low during this clock pulse, but if it hasn't then the SDA line will remain in the logic 1 state. Therefore, if ack is non-zero, an error has occurred, and the program halts.

Transfer from Slave to Master

8-bit bytes are transferred, most significant bit first, from the Slave Transmitter and are received at the Master Receiver by the procedure readi2c. This procedure first sets SDA high (i.e. it turns off the electronic switch connecting SDA to ground), so that the slave device can control the SDA signal. It then pulses SCL high, shifts the received bits on the SDA line into the thisbyte variable, and pulls SCL low again. This process is repeated for the 8 data bits. Finally, a ninth clock pulse is generated for the acknowledge signal, and if the acknowledge parameter is set to true, then the SDA line is forced into the logic 0 state, so as to send an acknowledge bit. The I²C protocol requires that all bytes except for the last are acknowledged. The reason that the last byte is not acknowledged is to indicate to the Slave that the transfer has finished and that the Master now requires control of the bus to send a Stop condition.

I²C Bus Control and Addressing

The transfer methods that have been described so far are a relatively standard serial communication link. The special feature of the I²C bus lies in the way data transfers over the bus are controlled, and devices are addressed Start and Stop Conditions. Earlier it was stated that during a data transfer, the SDA signal could only change state while SCL was low. The reason for this is that the protocol defines 2 special conditions to start and stop communications over the bus as follows :

A Start condition is defined as a change of SDA from logic 1 to logic 0 while SCL is high. This is performed by the procedure starti2c in Appendix C. This procedure simply ensure that the SCL line is high (as it would be when the bus is idle), and then pulls SDA low. Finally SCL is pulled low so that SDA can change state without causing a special condition.

A Stop condition is defined as a change of SDA from logic 0 to logic 1 while SCL is high. In appendix C, procedure stopi2c performs this operation, by controlling the signal lines in the obvious way.

Addressing

Devices on the I²C bus are selected by an 8-bit address which is sent over the bus in the same way as data bytes. The least significant bit of this address acts as a read/write control signal, and is set to 0 to make the Slave a Receiver and 1 to make the Slave a Transmitter.

The address byte is the first byte transmitted after a Start condition. It is always transmitted by the Master. By convention, if the Slave is a Receiver and the Slave contains several registers, then the next byte transmitted after the address is an internal register address for the device. However, this is not required by the I²C specification.

The address of a particular slave device is often determined when the I²C is manufactured. For example, the SAA5246 Teletext IC that is described below is always at address 34. Some other devices have a limited number of address inputs which are compared with certain bits of the received address before the device is selected. The PCD8574 I/O port is one such device. It has 3 address inputs, while the top 4 bits of the address are fixed at 0100 binary. Therefore this device can respond to addresses between 64 and 78

A slave device that is selected by a particular address must acknowledge the receipt of the address byte, while one that is not selected must not. Therefore, it is possible for the master to determine whether or not a particular slave is present on the I²C bus, which can be useful if certain devices are optional fitments.

Implementing the I²C Bus

The I²C bus is patented by Philips. However, they grant certain rights under that patent to purchasers of their devices, or other manufacturers' devices licensed by Philips. It is normally accepted that provided one device in every transfer is licensed by Philips, then the other device may be, for example, a software implementation of the protocol. In other words, using the driver program in Appendix C to control an Philips SAA5246 Teletext IC is within the terms of the licence, while implementing the slave functions in software and then using the I²C bus to communicate between 2 PCs would not be.

Microcontrollers with built-in I²C ports

Certain Philips microcontrollers, for example the PCB80C552, have the necessary hardware on-chip to implement the I²C bus. The program on the microcontroller simply sends 8-bit bytes to the control and data registers of the interface hardware, much like using the UART chip of a PC's COM port. While this solution is sensible if a complete control system is being designed, it is not a useful method to add the I²C bus to a personal computer system.

The PCD8584 device

The Philips PCD8584 is a device that can be connected to the 8 bit data bus of a microprocessor, together with the bus control lines, and which provides a similar hardware interface to the I²C bus as that built-in to the microcontrollers described above. Although this IC is a powerful and versatile means of providing I²C communications for a personal computer system, it is not totally trivial to use.

Parallel Port I²C Implementation

There is nothing that requires that the voltage sensor on each signal line and the electronic switch to connect the line to ground are part of the same device, provided that they are both accessible to the control processor. On an IBM PC or clone, the printer port makes an ideal place to obtain these signals. With a special cable it is then possible to receive and generate I²C signals.