Interrupt Driven Software UART for Tiny15 (1200bps)
NOTE: See AVR305 & AVR304 for a faster & smaller software UART.. (I have a port of AVR305 to the tiny15 I've attached as another file to this project.)
Having a bit of time on my hands, I ported Mr Hristea's Half duplex software UART to the Tiny15.
At 117 words (with sample app), its significantly shorter than his code. Also, it is written to require NO SRAM, and only 3 high registers (6 other registers). It has a 3 byte receive buffer.
If you are trying to go higher than 1200 baud, I expect you'll need to deal with 2 things. The first is the hard coded magic numbers from Mr. Hristea's code (56 & 80). I assume that it'll require an oscilloscope to correct them. The second is what I believe is a bit of a bug (which if I fix, the code no longer works at 1200 baud, and without an oscilloscope, Im at a loss). On a receive the code waits for a full start bit length before sampling the RX pin. I believe that means that its sampling at near the same time the data is coming in. So it would become unstable at high speeds. Any input on these two issues would be appreciated. I did note that the original C code appeared to TX the start bit for the wrong length of time (refer to his code to see what I mean).
#defines exist to strip the transmitter or receiver, as well as reduce the stack usage to just the one entry required by the interrupts.
The sample main: routine contains 2 ifdefs. Id take them out if you want the code to be more readable. I didnt because I think the functionality they demo is important.
NOTE: See AVR305 & AVR304 for a faster & smaller software UART.. (I have a port of AVR305 to the tiny15 I've attached as another file to this project.)
Having a bit of time on my hands, I ported Mr Hristea's Half duplex software UART to the Tiny15.
At 117 words (with sample app), its significantly shorter than his code. Also, it is written to require NO SRAM, and only 3 high registers (6 other registers). It has a 3 byte receive buffer.
If you are trying to go higher than 1200 baud, I expect you'll need to deal with 2 things. The first is the hard coded magic numbers from Mr. Hristea's code (56 & 80). I assume that it'll require an oscilloscope to correct them. The second is what I believe is a bit of a bug (which if I fix, the code no longer works at 1200 baud, and without an oscilloscope, Im at a loss). On a receive the code waits for a full start bit length before sampling the RX pin. I believe that means that its sampling at near the same time the data is coming in. So it would become unstable at high speeds. Any input on these two issues would be appreciated. I did note that the original C code appeared to TX the start bit for the wrong length of time (refer to his code to see what I mean).
#defines exist to strip the transmitter or receiver, as well as reduce the stack usage to just the one entry required by the interrupts.
The sample main: routine contains 2 ifdefs. Id take them out if you want the code to be more readable. I didnt because I think the functionality they demo is important.
| CODE |
| #include <avr/io.h> #define RX_PORT _SFR_IO_ADDR(PORTB) #define RX_BIT PB2 #define RX_PIN _SFR_IO_ADDR(PINB) #define TX_PORT _SFR_IO_ADDR(PORTB) #define TX_BIT PB4 #define TX_DDR _SFR_IO_ADDR(DDRB) #define DATA_BITS 8 #define STOP_BITS 1 #define DATA_BITS_MASK (0xFF >> (8-DATA_BITS)) #define CLK 1600000 #define BAUD 19200 #define DELAY_C ( CLK / BAUD ) #define DELAY_COUNT ( (DELAY_C - 23) / 6 ) #if DELAY_COUNT <= 0 #error Baud rate too high for this clock speed #elif DELAY_COUNT > 255 #error Baud rate too low for this clock speed #else #define ERRORCYCLES (CLK - (6*DELAY_COUNT + 23)*BAUD) #if ERROR < 0 #define ABSERR (-ERRORCYCLES) #else #define ABSERR ERRORCYCLES #endif #if (ABSERR*100) > (3*CLK) #warning Greater than 3% error is bad... #endif #define ERROR ABSERR*1600 / CLK #endif #define bitcount r16 #define temp r17 #define txByte r18 #define rxByte r19 ;*************************************************************************** ;* ;* "putchar" ;* ;* This subroutine transmits the byte stored in the "Txbyte" register ;* The number of stop bits used is set with the sb constant ;* ;* Number of words :14 including return ;* Number of cycles :Depens on bit rate ;* Low registers used :None ;* High registers used :2 (bitcount, Txbyte) ;* Pointers used :None ;* ;*************************************************************************** putchar: ldi bitcount, 1+DATA_BITS+STOP_BITS;start, data, stop com txByte ;Inverte everything #if DATA_BITS < 8 andi txByte, DATA_BITS_MASK #endif sec ;Start bit putchar0: brcc putchar1 ;If carry set cbi TX_PORT, TX_BIT ; send a '0' rjmp putchar2 ;else putchar1: sbi TX_PORT, TX_BIT ; send a '1' nop putchar2: rcall UART_delay ;One bit delay rcall UART_delay lsr txByte ;Get next bit dec bitcount ;If not all bit sent brne putchar0 ; send next ;else ret ; return ;*************************************************************************** ;* ;* "getchar" ;* ;* This subroutine receives one byte and returns it in the "Rxbyte" register ;* ;* Number of words :14 including return ;* Number of cycles :Depens on when data arrives ;* Low registers used :None ;* High registers used :2 (bitcount, Rxbyte) ;* Pointers used :None ;* ;*************************************************************************** getchar: ldi bitcount, DATA_BITS+1;8 data bit + 1 stop bit getchar1: sbic RX_PIN, RX_BIT ;Wait for start bit rjmp getchar1 rcall UART_delay ;0.5 bit delay getchar2: rcall UART_delay ;1 bit delay rcall UART_delay clc ;clear carry sbic RX_PIN, RX_BIT ;if RX pin high sec ; set carry dec bitcount ;If bit is stop bit breq getchar3 ; return ;else ror rxByte ; shift bit into Rxbyte rjmp getchar2 ; go get next getchar3: #if DATA_BITS < 8 lsr rxByte #endif #if DATA_BITS < 7 #warning < 7 data bits didnt work for me... good luck... lsr rxByte #endif #if DATA_BITS < 6 lsr rxByte #endif ret ;*************************************************************************** ;* ;* "UART_delay" ;* ;* This delay subroutine generates the required delay between the bits when ;* transmitting and receiving bytes. The total execution time is set by the ;* constant "DELAY_COUNT": ;* ;* 3·DELAY_COUNT + 7 cycles (including rcall and ret) ;* ;* Number of words :4 including return ;* Low registers used :None ;* High registers used :1 (temp) ;* Pointers used :None ;* ;*************************************************************************** UART_delay: ldi temp, DELAY_COUNT UART_delay1: dec temp brne UART_delay1 ret ;***** Program Execution Starts Here ;***** Test program .global main main: ; calibrate ldi temp, 0x60 out _SFR_IO_ADDR(OSCCAL),temp sbi TX_PORT, TX_BIT ;Init port pins sbi TX_DDR, TX_BIT ldi txByte, 12 ;Clear terminal rcall putchar ; ERROR is fixed point percent error as 4.4 ldi txByte, (ERROR >> 4) rcall outhexnibble ldi txByte, (ERROR & 0xF) rcall outhexnibble ldi txByte, (DELAY_COUNT >> 4) rcall outhexnibble ldi txByte, (DELAY_COUNT & 0xF) rcall outhexnibble forever: rcall getchar mov txByte, rxByte rcall putchar ;Echo received char rjmp forever outhexnibble: andi txByte, 0x0F cpi txByte, 0x0A brlt ADD_TX_0 ldi temp, ('A'-0x0A) add txByte, temp rjmp putchar ADD_TX_0: ldi temp, '0' add txByte, temp rjmp putchar |