TL16PIR552 Datasheet, PDF (38/52 Pages) Texas Instruments – DUAL UART WITH DUAL IrDA AND 1284 PARALLEL PORT

TL16PIR552
DUAL UART WITH DUAL IrDA AND
1284 PARALLEL PORT
SLLS222A – DECEMBER 1995 – REVISED AUGUST 1996
PRINCIPLES OF OPERATION
IR encoder and decoder
The IR encoder and decoder circuitry work with the UART to change the serial bit stream into a series of pulses
and back again. For every zero bit in the serial stream, a pulse is sent at the middle of the bit with a duration
of 3/16 of a bit time. If a one or series of ones is sent, the encode does not send a pulse. The decoding process
consists of receiving a pulse and sending a stretched version of the pulse to the UART. The stretched version
must be at least three-fourths of a bit time to be correctly decoded by most UARTs. Because the serial stream
can occur at any baud rate, some means of changing the encoding and decoding baud rate is needed. The
easiest way to accomplish this is to clock the encoder and decoder circuits with the UART baud rate 16x clock.
A block diagram of a design is shown in Figure 27.
TX _{IR Transmit}
Formatter
IRTX
UART
UART CLK
RX IR Receive
Decoder
IRRX
Figure 27. Conventional UART Connection to the External IR Encoder and Decoder
The encoder uses a 4-bit synchronous counter to transmit the pulses. It uses a delay of one-half of a bit time
to insure that a true zero bit has been sent. It sends a pulse for three UART clocks in the middle of the zero bit.
When two zeros are sent in a row, the counter simply overruns at the beginning of the next zero bit and starts
the process over. When a 1 is sent, the counter is reset.
The decoder also uses a 4-bit synchronous counter including a synchronous reset. The rising edge of an
incoming pulse sets the input flip-flop. This causes the counter to begin counting and a zero to be sent out at
the RX signal output. When the counter gets to 16, the input flip-flop is reset to wait for another input pulse. If
another pulse occurs before the counter expires, the counter is synchronously reset and the RX signal output
remains 0. When the counter expires, the RX signal output returns to 1, and the circuit waits for another input
pulse.
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