;*******************************************************************************

;        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.