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- /*
- * WARNING: be careful changing this code, it is very timing dependent
- */
-
- #ifndef F_CPU
- #define F_CPU 16000000
- #endif
-
- #include <avr/io.h>
- #include <avr/interrupt.h>
- #include <util/delay.h>
- #include <stdbool.h>
-
- #include "serial.h"
-
- // Serial pulse period in microseconds. Its probably a bad idea to lower this
- // value.
- #define SERIAL_DELAY 24
-
- uint8_t volatile serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH] = {0};
- uint8_t volatile serial_master_buffer[SERIAL_MASTER_BUFFER_LENGTH] = {0};
-
- #define SLAVE_DATA_CORRUPT (1<<0)
- volatile uint8_t status = 0;
-
- inline static
- void serial_delay(void) {
- _delay_us(SERIAL_DELAY);
- }
-
- inline static
- void serial_output(void) {
- SERIAL_PIN_DDR |= SERIAL_PIN_MASK;
- }
-
- // make the serial pin an input with pull-up resistor
- inline static
- void serial_input(void) {
- SERIAL_PIN_DDR &= ~SERIAL_PIN_MASK;
- SERIAL_PIN_PORT |= SERIAL_PIN_MASK;
- }
-
- inline static
- uint8_t serial_read_pin(void) {
- return !!(SERIAL_PIN_INPUT & SERIAL_PIN_MASK);
- }
-
- inline static
- void serial_low(void) {
- SERIAL_PIN_PORT &= ~SERIAL_PIN_MASK;
- }
-
- inline static
- void serial_high(void) {
- SERIAL_PIN_PORT |= SERIAL_PIN_MASK;
- }
-
- void serial_master_init(void) {
- serial_output();
- serial_high();
- }
-
- void serial_slave_init(void) {
- serial_input();
-
- // Enable INT0
- EIMSK |= _BV(INT0);
- // Trigger on falling edge of INT0
- EICRA &= ~(_BV(ISC00) | _BV(ISC01));
- }
-
- // Used by the master to synchronize timing with the slave.
- static
- void sync_recv(void) {
- serial_input();
- // This shouldn't hang if the slave disconnects because the
- // serial line will float to high if the slave does disconnect.
- while (!serial_read_pin());
- serial_delay();
- }
-
- // Used by the slave to send a synchronization signal to the master.
- static
- void sync_send(void) {
- serial_output();
-
- serial_low();
- serial_delay();
-
- serial_high();
- }
-
- // Reads a byte from the serial line
- static
- uint8_t serial_read_byte(void) {
- uint8_t byte = 0;
- serial_input();
- for ( uint8_t i = 0; i < 8; ++i) {
- byte = (byte << 1) | serial_read_pin();
- serial_delay();
- _delay_us(1);
- }
-
- return byte;
- }
-
- // Sends a byte with MSB ordering
- static
- void serial_write_byte(uint8_t data) {
- uint8_t b = 8;
- serial_output();
- while( b-- ) {
- if(data & (1 << b)) {
- serial_high();
- } else {
- serial_low();
- }
- serial_delay();
- }
- }
-
- // interrupt handle to be used by the slave device
- ISR(SERIAL_PIN_INTERRUPT) {
- sync_send();
-
- uint8_t checksum = 0;
- for (int i = 0; i < SERIAL_SLAVE_BUFFER_LENGTH; ++i) {
- serial_write_byte(serial_slave_buffer[i]);
- sync_send();
- checksum += serial_slave_buffer[i];
- }
- serial_write_byte(checksum);
- sync_send();
-
- // wait for the sync to finish sending
- serial_delay();
-
- // read the middle of pulses
- _delay_us(SERIAL_DELAY/2);
-
- uint8_t checksum_computed = 0;
- for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
- serial_master_buffer[i] = serial_read_byte();
- sync_send();
- checksum_computed += serial_master_buffer[i];
- }
- uint8_t checksum_received = serial_read_byte();
- sync_send();
-
- serial_input(); // end transaction
-
- if ( checksum_computed != checksum_received ) {
- status |= SLAVE_DATA_CORRUPT;
- } else {
- status &= ~SLAVE_DATA_CORRUPT;
- }
- }
-
- inline
- bool serial_slave_DATA_CORRUPT(void) {
- return status & SLAVE_DATA_CORRUPT;
- }
-
- // Copies the serial_slave_buffer to the master and sends the
- // serial_master_buffer to the slave.
- //
- // Returns:
- // 0 => no error
- // 1 => slave did not respond
- int serial_update_buffers(void) {
- // this code is very time dependent, so we need to disable interrupts
- cli();
-
- // signal to the slave that we want to start a transaction
- serial_output();
- serial_low();
- _delay_us(1);
-
- // wait for the slaves response
- serial_input();
- serial_high();
- _delay_us(SERIAL_DELAY);
-
- // check if the slave is present
- if (serial_read_pin()) {
- // slave failed to pull the line low, assume not present
- sei();
- return 1;
- }
-
- // if the slave is present syncronize with it
- sync_recv();
-
- uint8_t checksum_computed = 0;
- // receive data from the slave
- for (int i = 0; i < SERIAL_SLAVE_BUFFER_LENGTH; ++i) {
- serial_slave_buffer[i] = serial_read_byte();
- sync_recv();
- checksum_computed += serial_slave_buffer[i];
- }
- uint8_t checksum_received = serial_read_byte();
- sync_recv();
-
- if (checksum_computed != checksum_received) {
- sei();
- return 1;
- }
-
- uint8_t checksum = 0;
- // send data to the slave
- for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
- serial_write_byte(serial_master_buffer[i]);
- sync_recv();
- checksum += serial_master_buffer[i];
- }
- serial_write_byte(checksum);
- sync_recv();
-
- // always, release the line when not in use
- serial_output();
- serial_high();
-
- sei();
- return 0;
- }
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