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17.3 Operation
17.3.1 Asynchronous Mode
The following steps explain how to set up Serial Ports A – D for asynchronous operation.
The serial ports can be used by polling the status byte, but their performance will be better
with an interrupt. These instructions also apply to the asynchronous operation of Serial
Ports E – F.
1. Write the interrupt vector for the interrupt service routine to the internal interrupt table.
2. Set up the desired transmit pin by writing to the appropriate parallel port function register
(PCFR or PDFR).
3. Select the appropriate mode by writing to SxCR (receive input port and 7 or 8 bits).
Also select the interrupt priority.
4. Select additional options by writing to SxER (parity, RZI encoding, clock polarity, and
behavior during break).
5. Write the desired divider value to TATxR for the appropriate serial port.
A sample asynchronous serial interrupt handler is shown below for Serial Port A.
async_sera_isr::
push af
ioi ld a, (SASR)
bit 7,a
push af
jr z, check_for_tx
rx_ready:
ioi ld a, (SADR)
;
;
;
;
save used registers
get status
check if byte ready in RX buffer
save status for next check
; read byte and clear interrupt
; do something with byte here
check_for_tx:
pop af
bit 3,a
jr nz, done
; check if TX buffer was emptied
; get next byte to be transmitted into A here
ioi ld (SADR), a
done:
pop af
ipres
ret
; load next byte into TX buffer and clear interrupt
; restore used registers
To transmit with an address (1) bit appended, write the data to SxAR instead of SxDR; to
append a long stop (0) bit write to SxLR instead.
Chapter 17 Serial Ports A – D
137