Issue 138 January 2002
WHAT GOOD IS IrD,Eh?
PART 1: CORDLESS PROTOCOL

byJeff Bachiochi

Start • Standards Make It Happen • IrPHY • The Range • UART To IrDA • IR Encoder/Decoder • Gimme More • Sources & PDF

UART to IrDA

Data transmission via a UART needs to be only slightly modified to be considered IrDA-compatible. As stated earlier, to reduce the IrDA power requirement to a minimum, the actual IR on time is reduced to a minimum of 1.62 µs (or a maximum of 0.1875 bit time) during each 0 bit time. Some additional circuitry is required between the UART and IR transmitter/receiver to encode/decode serial bit timing and IR pulses.

This is usually accomplished by providing a 16× (data rate) clock to a circuit, which will divide each bit time into 16 time slots. Some systems can provide this clock because their UART may already have a 16× clock. The Texas Instruments TIR1000 in Figure 2 is a standalone IrDA encoder/decoder (TL16C550C UART has a data out 16× clock).

Although the TX serial data of the UART is high, no IR pulses are created. When the TX serial data goes low (start bit), the TIR1000 starts counting 16× clock cycles. At the end of the seventh cycle, the IRTX output is driven high. The IRTX is driven low after three 16× clock cycles. This IR pulse generation (equal to 0.1875 bit time) is repeated if the TX serial data is sending another 0 bit after the sixteenth 16× clock cycle.

IR reception is done similarly. Because the IR pulse is shorter than a bit time, the circuitry must lengthen the IR pulse into standard bit timing for the UART. So, whenever an IR pulse is seen at the IRRX input, the TIR1000 drops its RX output for 16 16× clock cycles (1 bit time). A lack of IRRX input pulses keeps the RX output high. Notice that with this scheme (transmitter/receiver), the received data is delayed (in this case, 0.5 bit time) from the transmitted data but does not cause problems because the data timing is based on the start bit.

When a 16× clock is not available, there are other options. The TOIM4232 from Vishay or MCP2120 from Microchip are similar devices that contain an optional clock generator (see Figure 3). Data rate selection is by jumper or via TX serial data (while a mode pin is active). This means that a UART can change data rate (and the 16× clock), but this device will not know there is a change unless a software change routine is used to update the device.

You must take this into consideration if it will be necessary to change data rates within your application. The default (initial) connection data rate for IrDA is 9600 bps, so you must at least run at 9600 bps to be IrDA-compatible. Changing the data rate is an option and not a requirement.