; ---------------------------------------------------------------------------- ; ; tx_hex tx_nib tx_bcd tx_chr -- Transmit byte in W via RS232. ; ; - Writes to GPIO,tx_pin using RS232 8N1 at CMOS/TTL levels. ; - Polarity determined by GPIO,ttl_pin (0=rs232, 1=ttl). ; - Note 19200 baud is 52.083 us per bit = 0.521 ms per byte. ; - At 10 MHz, 130 instructions = 52.0 us = 19230.77 baud. ; - Bit symmetric isochronous code. ; - RS232 signal level terminology: ; ; break | space | +12V | 5V true | 0V inverted TTL | start bit | data 0 ; idle | mark | -12V | 0V true | 5V inverted TTL | stop bit | data 1 ; TX_Tcy equ d'130' ; nominal bit width (Tcy units) ; tx_hex -- Send 8-bit byte as 2-character hex. tx_hex: movwf tx_swap ; swapf tx_swap,W ; get upper nibble call tx_nib ; movf tx_swap,W ; get lower nibble ; (fall into) ; tx_nib -- Send 4-bit nibble as 1-character hex. tx_nib: andlw 0x0F ; isolate nibble addlw -0xA ; skpnc ; if nibble >= 10 addlw 'A'-0xA-'0' ; convert 10...15 into A...F addlw 0xA ; undo ; (fall into) ; tx_bcd -- Send one BCD digit. tx_bcd: addlw '0' ; convert to ascii digit ; (fall into) ; tx_chr -- Send single character. tx_chr: movwf tx_hold ; bcf STATUS,C ; make start bit (data 0) call _tx_out ; movlw TX_Tcy - d'14' ; delay after start bit call DelayW1 ; movlw 8 ; data bit count (8N1) movwf tx_bits ; _tx1 rrf tx_hold,F ; shift next LSB into carry flag call _tx_out ; movlw TX_Tcy - d'15' ; delay after data bit call DelayW1 ; decfsz tx_bits,F ; done? goto _tx1 ; no, do next bit nop ; (timing) bsf STATUS,C ; make stop bit (data 1) call _tx_out ; movlw TX_Tcy - d'16' ; delay after stop bit goto DelayW1 ; (goto/return) ; tx_idle -- Set pin to RS232 idle state. tx_idle: bsf STATUS,C ; set idle state (stop bit = data 1) ; (fall into) ; Set GPIO output (data bit is in carry flag). _tx_out movlw 0 ; assume transmit pin low skpc ; if carry flag is 0, then xorlw 1<