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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"
#ifndef USE_I2C
// 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;
}
#endif
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