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