;*******************************************************************************
; Duplex UART Routines for the 8xC751 and 8xC752 Microcontrollers
;*******************************************************************************
; This is a demo program showing a way to perform simultaneous RS-232
; transmit and receive using only one hardware timer.
; The transmit and receive routines divide each bit time into 4 slices to
; allow synchronizing to incoming data that may be out of synch with outgoing
; data.
; The main program loop in this demo processes received data and sends it
; back to the transmitter in hexadecimal format. This insures that we can
; always fill up the receiver buffer (since the returned data is longer than
; the received data) for testing purposes. Example: if the letter "A" is
; received, we will echo "A41 ".
;*******************************************************************************
$Title(Duplex UART Routines for the 751/752)
$Date(8/20/92)
$MOD751
;*******************************************************************************
; Definitions
;*******************************************************************************
; Miscellaneous
TxBitLen EQU -4 + 1 ; Timer slices per serial bit transmit.
RxBitLen EQU -4 + 1 ; Timer slices per serial bit receive.
RxHalfBit EQU (RxBitLen / 4) + 1 ; Timer slices for a partial bit time.
; Used to adjust the input sampling
; time point.
; Note: TxBitLen and RxBitLen are kept separate in order to facilitate the
; possibility of having different transmit and receive baud rates. The timer
; would be set up to give four slices for the fastest baud rate, and the
; BitLen for the slower channel would be set longer for the slower baud rate.
; BitLen = -4 + 1 gives four timer interrupts per bit. BitLen = -8 + 1 would
; give 8 slices, BitLen = -16 + 1 would give 16 slices, etc.
TxPin BIT P1.0 ; RS-232 transmit pin (output).
RxPin BIT P1.5 ; RS-232 receive pin (input).
RTS BIT P1.3 ; RS-232 request to send pin (output).
CTS BIT P1.6 ; RS-232 clear to send pin (input).
; Note: P1.1 and P1.2 are used to input the baud rate selection.
; RAM Locations
Flags DATA 20h ; Miscellaneous bit flags (see below).
TxOn BIT Flags.0 ; Indicates transmitter is on (busy).
RxOn BIT Flags.1 ; Indicates receiver is on (busy).
TxFull BIT Flags.2 ; Transmit buffer (1 byte only) is full.
RxFull BIT Flags.3 ; Receiver buffer is full.
RxAvail BIT Flags.4 ; RX buffer is not empty.
OverrunErr BIT Flags.6 ; Overrun error flag.
FramingErr BIT Flags.7 ; Framing error flag.
BaudHigh DATA 21h ; High byte timer value for baud rate.
BaudLow DATA 22h ; Low byte timer value for baud rate.
TxCnt DATA 23h ; RS-232 byte transmit bit counter.
TxTime DATA 24h ; RS-232 transmit time slice count.
TxShift DATA 25h ; Transmitter shift register.
TxDat DATA 26h ; Transmitter holding register.
RxCnt DATA 27h ; RS-232 byte receive bit counter.
RxTime DATA 28h ; RS-232 receive time slice count.
RxShift DATA 29h ; Receiver shift register.
RxDatCnt DATA 2Ah ; Received byte count.
RxBuf DATA 2Bh ; Receive buffer (3 bytes long).
Temp DATA 2Fh ; Temporary holding register.
;*******************************************************************************
; Interrupt Vectors
;*******************************************************************************
ORG 00h ; Reset vector.
AJMP RESET
ORG 03h ; External interrupt 0
AJMP Intr0 ; (received RS-232 start bit).
ORG 0Bh ; Timer 0 overflow interrupt.
AJMP Timer0 ; (4X the RS-232 bit rate).
ORG 13h ; External interrupt 1.
RETI ; (not used).
ORG 1Bh ; Timer I interrupt.
RETI ; (not used).
ORG 23h ; I2C interrupt.
RETI ; (not used).
;*******************************************************************************
; Interrupt Handlers
;*******************************************************************************
; External Interrupt Int0.
; RS-232 start bit transition.
Intr0: PUSH ACC ; Save accumulator,
PUSH PSW ; and status.
CLR IE.0 ; Disable more RX interrupts.
SETB RxOn ; Set receive active flag.
MOV RxCnt,#11h ; Set bit counter to expect a start.
MOV RxTime,#RxHalfBit ; First sample is at a partial bit time.
JB TxOn,I0TimerOn ; If TX active then timer is on.
MOV RTH,BaudHigh ; Set up timer for selected baud rate.
MOV RTL,BaudLow
MOV TH,BaudHigh
MOV TL,BaudLow
SETB TR ; Start timer 0.
I0TimerOn: MOV A,RxDatCnt ; Check for buffer about to be full:
CJNE A,#2,Int0Ex ; one space left and a byte starting.
SETB RTS ; If so, tell whoever is on the
; other end to wait.
Int0Ex: POP PSW ; Restore status,
POP ACC ; and accumulator.
RETI
; Timer 0 Interrupt
; This is used to generate time slices for both serial transmit and receive
; functions.
Timer0: PUSH ACC ; Save accumulator,
PUSH PSW ; and status.
JNB TxTime.7,RS232TX ; Is this an active time slice
; for an RS-232 transmit?
JNB TxOn,CheckRx ; If transmit is active,
INC TxTime ; increment the time slice count.
CheckRx: JNB RxTime.7,RS232RX ; Is this an active time slice
; for an RS-232 receive?
JNB RxOn,T0Ex ; If receive is active, increment
INC RxTime ; the time slice count.
T0Ex: POP PSW ; Restore status,
POP ACC ; and accumulator.
MOV P3,Flags ; For demo purposes, output status
; on an extra port.
RETI
;*******************************************************************************
; RS-232 Transmit Routine
;*******************************************************************************
RS232TX: JNB TxCnt.4,TxData ; Go if data bit.
JNB TxCnt.0,TxStop ; Go if stop bit.
; Send start bit and do buffer housekeeping.
TxStart: JB CTS,TxEx1 ; Is CTS asserted (low) so can we send?
; If not, try again after 1 bit time.
CLR TxPin ; Set start bit.
MOV TxShift,TxDat ; Get byte to transmit from buffer.
CLR TxFull
MOV TxCnt,#08h ; Init bit count for 8 bits of data.
; (note: counts UP).
TxEx1: MOV TxTime,#TxBitLen ; Reset time slice count.
SJMP CheckRx ; Restore state and exit.
; Send Next Data Bit.
TxData: MOV A,TxShift ; Get un-transmitted bits.
RRC A ; Shift next TX bit to carry.
MOV TxPin,C ; Move carry out to the TXD pin.
MOV TxShift,A ; Save bits still to be TX'd.
INC TxCnt ; Increment TX bit counter
MOV TxTime,#TxBitLen ; Reset time slice count.
SJMP CheckRx ; Restore state and exit.
; Send Stop Bit and Check for More to Send.
TxStop: SETB TxPin ; Send stop bit.
JB TxFull,TxEx2 ; More data to transmit?
CLR TxOn ; If not, turn off TX active flag, and
CLR RTS ; make sure that whoever is on the
; other end knows it's OK to send.
JB RxOn,TxEx2 ; If receive active, timer stays on,
CLR TR ; otherwise turn off timer.
TxEx2: MOV TxCnt,#11h ; Set TX bit counter for a start.
MOV TxTime,#TxBitLen-1 ; Reset time slice count, stop bit
; > 1 bit time for synch.
SJMP CheckRx ; Restore state and exit.
;*******************************************************************************
; RS-232 Receive Routine
;*******************************************************************************
RS232RX: MOV C,RxPin ; Get current serial bit value.
JNB RxCnt.4,RxData ; Go if data bit.
JNB RxCnt.0,RxStop ; Go if stop bit.
;Verify start bit.
RxStart: JC RxErr ; If bit=1, then not a valid start.
MOV RxCnt,#08h ; Init counter to expect data.
MOV RxTime,#RxBitLen ; Reset time slice count.
SJMP T0Ex ; Restore state and exit.
; Get Next Data Bit.
RxData: MOV A,RxShift ; Get partial received byte.
RRC A ; Shift in new received bit.
MOV RxShift,A ; Store partial result in buffer.
INC RxCnt ; Increment received bit count.
MOV RxTime,#RxBitLen ; Reset time slice count.
SJMP T0Ex ; Restore state and exit.
; Store Data Byte, "push"ing it into the FIFO buffer.
RxStop: CLR EA ; Don't interrupt the following.
MOV A,RxBuf ; "PUSH" the receive buffer.
XCH A,RxBuf+1
XCH A,RxBuf+2
MOV RxBuf,RxShift ; Add just completed data to buffer.
INC RxDatCnt ; Increment the received byte count.
SETB EA ; Re-enable interrupts.
SETB RxAvail ; There is data in the RX buffer.
PUSH PSW ; Save Carry (received bit)for later.
MOV A,RxDatCnt ; Check receiver buffer status.
CJNE A,#4,RxChk1 ; Is RX buffer overrun?
SETB OverrunErr ; Set status reg overrun error flag.
MOV RxDatCnt,#3 ; Re-set buffer counter to "full".
RxChk1: CJNE A,#3,RxChk2 ; Is RX buffer full?
SETB RxFull ; Set buffer full status.
RxChk2: POP PSW ; Retrieve last received bit in Carry.
JC RxEx ; If bit=0, then not a valid stop.
RxErr: SETB FramingErr ; Remember bad start or stop status.
RxEx: JB TxOn,RxTimerOn ; If transmit active, timer stays on,
CLR TR ; otherwise turn timer off.
RxTimerOn: CLR RxOn ; Turn off receive active.
SETB RxTime.7 ; Set bit for no service to
; RX Time Slice Branches.
SETB IE.0 ; Re-enable RS-232 receive interrupts.
AJMP T0Ex ; Restore state and exit.
;*******************************************************************************
; Subroutines
;*******************************************************************************
; BaudRate - Determine and set the baud rate from switches.
; Note: if the baud rate is altered, the actual change will only occur when
; a transmit or receive is begun while the timer was not already running
; (i.e.: not already busy transmitting or receiving).
BaudRate: MOV DPTR,#BaudTable ; Set pointer to baud rate table.
ANL A,#03h ; Limit displacement for lookup.
RL A ; Double the table index since these
; are 2 byte entries.
PUSH ACC ; Save the table index for second byte.
MOVC A,@A+DPTR ; Get first byte, and save as the high
MOV BaudHigh,A ; byte of the baud rate timer value.
POP ACC ; Get back the table index.
INC A ; Advance to next table entry.
MOVC A,@A+DPTR ; Get second byte, and save as the low
MOV BaudLow,A ; byte of the baud rate timer value.
RET
; Entries in BaudTable are for a timer setting of 1/4 of a bit time at the given
; baud rate. The two values per entry are the high and low bytes of the value
; respectively.
; Values are calculated as follows:
; Osc Frequency
; 1/4 Bit cell time (in machine cycles) = -----------------
; Baud Rate * 48
; Example for 9600 baud with a 16MHz crystal:
; 16,000,000 / 9600 * 48 = 34.7222... machine cycles per quarter bit time.
; Rounded, this is 35. The hexadecimal value for 35 is 23.
; 10000 hex - 23 hex (truncated to 16 bits) = FFDD. Thus, the BaudTable entry
; for 9600 baud is FF, DD hex.
BaudTable: DB 0FEh,0EAh ; 1200 baud.
DB 0FFh,75h ; 2400 baud.
DB 0FFh,0BBh ; 4800 baud.
DB 0FFh,0DDh ; 9600 baud.
; TxSend - Initiate RS-232 Transmit.
TxSend: JB TxFull,$ ; Make sure TX buffer is free.
SETB TxFull ; Reserve the buffer for our use.
MOV TxDat,A ; Put character in buffer.
JB TxOn,TSTimerOn ; Exit if transmitter already running.
SETB TxOn ; Transmit active flag set.
MOV TxCnt,#11h ; Init bit counter to expect a start.
MOV TxTime,#TxBitLen ; Reset time slice count.
JB RxOn,TSTimerOn ; Exit if receiver already active.
MOV RTH,BaudHigh ; Set up timer for selected baud rate.
MOV RTL,BaudLow
MOV TH,BaudHigh
MOV TL,BaudLow
SETB TR ; Start up the bit timer.
TSTimerOn: RET
; PrByte - Output a byte as ASCII hexadecimal format.
PrByte: PUSH ACC ; Print ACC contents as ASCII hex.
SWAP A
ACALL HexAsc ; Print upper nibble.
ACALL TxSend
POP ACC
ACALL HexAsc ; Print lower nibble.
ACALL TxSend
RET
; HexAsc - Convert a hexadecimal nibble to its ASCII character equivalent.
HexAsc: ANL A,#0Fh ; Make sure we're working with only
; one nibble.
CJNE A,#0Ah,HA1 ; Test value range.
HA1: JC HAVal09 ; Value is 0 to 9.
ADD A,#7 ; Value is A to F, needs pre-adjustment.
HAVal09: ADD A,#'0' ; Adjust value to ASCII hex.
RET
; GetRx - Retrieve a byte from the receive buffer, and return it in A.
GetRx: CLR EA ; Make sure this isn't interrupted.
DEC RxDatCnt ; Decrement the buffer count.
MOV A,RxDatCnt ; Get buffer count.
JNZ GRX1 ; Test for empty receive buffer.
CLR RxAvail ; If empty, clear data available status.
GRX1: ADD A,#RxBuf ; Create a pointer to end of buffer.
MOV Temp,R0 ; Save R0.
MOV R0,A ; Put pointer where we can indirect.
MOV A,@R0 ; Get last buffer data.
MOV R0,Temp ; Restore R0.
CLR RxFull ; Buffer can't be full anymore.
SETB EA ; Re-enable interrupts.
RET
;*******************************************************************************
; Reset
;*******************************************************************************
Reset: MOV SP,#2Fh ; Initialize stack start.
MOV TCON,#0 ; Set timer off, INT0 to level trigger.
MOV P3,#0 ; Turn off all status outputs.
; For this demo, we only set up the baud rate once at reset:
MOV A, P1 ; Read baudrate bits from P1.
RR A ; The switches are on bits 2 and 1.
ACALL BaudRate ; Set up the selected baud rate.
MOV FLAGS,#0 ; Init all status flags.
MOV RxDatCnt,#0 ; Clear buffer count.
MOV IE,#93h ; Turn on timer 0 interrupt and
; external interrupt 0.
CLR RTS ; Assert RTS so we can receive.
; The main program loop processes received data and sends it back to the
; transmitter in hexadecimal format. This insures that we can always fill
; up the receiver buffer (since the returned data is longer than the
; received data) for testing purposes. Example: if the letter "A" is
; received, we will echo "A41 ".
MainLoop: JNB RxAvail,$ ; Make sure an input byte is available.
ACALL GetRx ; Get data from the receiver buffer.
ACALL TxSend ; Echo original character.
ACALL PrByte ; Output the char in hexadecimal format,
MOV A,#20h ; followed by a space.
ACALL TxSend
SJMP MainLoop ; Repeat.
END
