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@@ -0,0 +1,295 @@
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+/*
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+ * WARNING: be careful changing this code, it is very timing dependent
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+ */
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+
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+#ifndef F_CPU
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+#define F_CPU 16000000
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+#endif
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+
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+#include <avr/io.h>
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+#include <avr/interrupt.h>
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+#include <util/delay.h>
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+#include <stdbool.h>
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+#include "serial.h"
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+
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+#ifdef USE_SERIAL
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+
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+#define _delay_sub_us(x) __builtin_avr_delay_cycles(x)
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+
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+// Serial pulse period in microseconds.
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+#define SELECT_SERIAL_SPEED 1
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+#if SELECT_SERIAL_SPEED == 0
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+ // Very High speed
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+ #define SERIAL_DELAY 4 // micro sec
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+ #define READ_WRITE_START_ADJUST 30 // cycles
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+ #define READ_WRITE_WIDTH_ADJUST 10 // cycles
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+#elif SELECT_SERIAL_SPEED == 1
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+ // High speed
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+ #define SERIAL_DELAY 6 // micro sec
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+ #define READ_WRITE_START_ADJUST 23 // cycles
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+ #define READ_WRITE_WIDTH_ADJUST 10 // cycles
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+#elif SELECT_SERIAL_SPEED == 2
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+ // Middle speed
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+ #define SERIAL_DELAY 12 // micro sec
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+ #define READ_WRITE_START_ADJUST 25 // cycles
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+ #define READ_WRITE_WIDTH_ADJUST 10 // cycles
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+#elif SELECT_SERIAL_SPEED == 3
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+ // Low speed
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+ #define SERIAL_DELAY 24 // micro sec
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+ #define READ_WRITE_START_ADJUST 25 // cycles
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+ #define READ_WRITE_WIDTH_ADJUST 10 // cycles
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+#elif SELECT_SERIAL_SPEED == 4
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+ // Very Low speed
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+ #define SERIAL_DELAY 50 // micro sec
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+ #define READ_WRITE_START_ADJUST 25 // cycles
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+ #define READ_WRITE_WIDTH_ADJUST 10 // cycles
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+#else
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+#error Illegal Serial Speed
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+#endif
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+
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+
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+#define SERIAL_DELAY_HALF1 (SERIAL_DELAY/2)
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+#define SERIAL_DELAY_HALF2 (SERIAL_DELAY - SERIAL_DELAY/2)
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+
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+#define SLAVE_INT_WIDTH 1
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+#define SLAVE_INT_RESPONSE_TIME SERIAL_DELAY
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+
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+uint8_t volatile serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH] = {0};
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+uint8_t volatile serial_master_buffer[SERIAL_MASTER_BUFFER_LENGTH] = {0};
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+
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+#define SLAVE_DATA_CORRUPT (1<<0)
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+volatile uint8_t status = 0;
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+
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+inline static
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+void serial_delay(void) {
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+ _delay_us(SERIAL_DELAY);
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+}
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+
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+inline static
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+void serial_delay_half1(void) {
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+ _delay_us(SERIAL_DELAY_HALF1);
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+}
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+
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+inline static
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+void serial_delay_half2(void) {
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+ _delay_us(SERIAL_DELAY_HALF2);
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+}
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+
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+inline static
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+void serial_output(void) {
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+ SERIAL_PIN_DDR |= SERIAL_PIN_MASK;
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+}
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+
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+// make the serial pin an input with pull-up resistor
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+inline static
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+void serial_input_with_pullup(void) {
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+ SERIAL_PIN_DDR &= ~SERIAL_PIN_MASK;
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+ SERIAL_PIN_PORT |= SERIAL_PIN_MASK;
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+}
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+
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+inline static
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+uint8_t serial_read_pin(void) {
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+ return !!(SERIAL_PIN_INPUT & SERIAL_PIN_MASK);
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+}
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+
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+inline static
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+void serial_low(void) {
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+ SERIAL_PIN_PORT &= ~SERIAL_PIN_MASK;
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+}
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+
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+inline static
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+void serial_high(void) {
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+ SERIAL_PIN_PORT |= SERIAL_PIN_MASK;
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+}
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+
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+void serial_master_init(void) {
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+ serial_output();
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+ serial_high();
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+}
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+
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+void serial_slave_init(void) {
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+ serial_input_with_pullup();
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+
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+#if SERIAL_PIN_MASK == _BV(PD0)
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+ // Enable INT0
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+ EIMSK |= _BV(INT0);
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+ // Trigger on falling edge of INT0
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+ EICRA &= ~(_BV(ISC00) | _BV(ISC01));
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+#elif SERIAL_PIN_MASK == _BV(PD2)
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+ // Enable INT2
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+ EIMSK |= _BV(INT2);
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+ // Trigger on falling edge of INT2
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+ EICRA &= ~(_BV(ISC20) | _BV(ISC21));
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+#else
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+ #error unknown SERIAL_PIN_MASK value
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+#endif
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+}
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+
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+// Used by the sender to synchronize timing with the reciver.
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+static
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+void sync_recv(void) {
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+ for (int i = 0; i < SERIAL_DELAY*5 && serial_read_pin(); i++ ) {
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+ }
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+ // This shouldn't hang if the slave disconnects because the
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+ // serial line will float to high if the slave does disconnect.
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+ while (!serial_read_pin());
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+}
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+
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+// Used by the reciver to send a synchronization signal to the sender.
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+static
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+void sync_send(void) {
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+ serial_low();
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+ serial_delay();
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+ serial_high();
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+}
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+
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+// Reads a byte from the serial line
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+static
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+uint8_t serial_read_byte(void) {
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+ uint8_t byte = 0;
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+ _delay_sub_us(READ_WRITE_START_ADJUST);
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+ for ( uint8_t i = 0; i < 8; ++i) {
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+ serial_delay_half1(); // read the middle of pulses
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+ byte = (byte << 1) | serial_read_pin();
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+ _delay_sub_us(READ_WRITE_WIDTH_ADJUST);
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+ serial_delay_half2();
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+ }
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+ return byte;
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+}
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+
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+// Sends a byte with MSB ordering
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+static
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+void serial_write_byte(uint8_t data) {
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+ uint8_t b = 1<<7;
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+ while( b ) {
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+ if(data & b) {
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+ serial_high();
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+ } else {
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+ serial_low();
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+ }
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+ b >>= 1;
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+ serial_delay();
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+ }
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+ serial_low(); // sync_send() / senc_recv() need raise edge
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+}
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+
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+// interrupt handle to be used by the slave device
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+ISR(SERIAL_PIN_INTERRUPT) {
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+ serial_output();
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+
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+ // slave send phase
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+ uint8_t checksum = 0;
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+ for (int i = 0; i < SERIAL_SLAVE_BUFFER_LENGTH; ++i) {
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+ sync_send();
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+ serial_write_byte(serial_slave_buffer[i]);
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+ checksum += serial_slave_buffer[i];
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+ }
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+ sync_send();
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+ serial_write_byte(checksum);
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+
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+ // slave switch to input
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+ sync_send(); //0
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+ serial_delay_half1(); //1
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+ serial_low(); //2
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+ serial_input_with_pullup(); //2
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+ serial_delay_half1(); //3
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+
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+ // slave recive phase
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+ uint8_t checksum_computed = 0;
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+ for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
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+ sync_recv();
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+ serial_master_buffer[i] = serial_read_byte();
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+ checksum_computed += serial_master_buffer[i];
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+ }
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+ sync_recv();
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+ uint8_t checksum_received = serial_read_byte();
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+
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+ if ( checksum_computed != checksum_received ) {
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+ status |= SLAVE_DATA_CORRUPT;
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+ } else {
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+ status &= ~SLAVE_DATA_CORRUPT;
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+ }
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+
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+ sync_recv(); //weit master output to high
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+}
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+
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+inline
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+bool serial_slave_DATA_CORRUPT(void) {
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+ return status & SLAVE_DATA_CORRUPT;
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+}
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+
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+// Copies the serial_slave_buffer to the master and sends the
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+// serial_master_buffer to the slave.
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+//
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+// Returns:
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+// 0 => no error
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+// 1 => slave did not respond
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+// 2 => checksum error
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+int serial_update_buffers(void) {
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+ // this code is very time dependent, so we need to disable interrupts
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+ cli();
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+
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+ // signal to the slave that we want to start a transaction
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+ serial_output();
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+ serial_low();
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+ _delay_us(SLAVE_INT_WIDTH);
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+
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+ // wait for the slaves response
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+ serial_input_with_pullup();
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+ _delay_us(SLAVE_INT_RESPONSE_TIME);
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+
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+ // check if the slave is present
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+ if (serial_read_pin()) {
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+ // slave failed to pull the line low, assume not present
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+ serial_output();
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+ serial_high();
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+ sei();
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+ return 1;
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+ }
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+
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+ // master recive phase
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+ // if the slave is present syncronize with it
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+
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+ uint8_t checksum_computed = 0;
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+ // receive data from the slave
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+ for (int i = 0; i < SERIAL_SLAVE_BUFFER_LENGTH; ++i) {
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+ sync_recv();
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+ serial_slave_buffer[i] = serial_read_byte();
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+ checksum_computed += serial_slave_buffer[i];
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+ }
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+ sync_recv();
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+ uint8_t checksum_received = serial_read_byte();
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+
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+ if (checksum_computed != checksum_received) {
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+ serial_output();
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+ serial_high();
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+ sei();
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+ return 2;
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+ }
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+
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+ // master switch to output
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+ sync_recv(); //0
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+ serial_delay(); //1
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+ serial_low(); //3
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+ serial_output(); // 3
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+ serial_delay_half1(); //4
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+
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+ // master send phase
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+ uint8_t checksum = 0;
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+
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+ for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
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+ sync_send();
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+ serial_write_byte(serial_master_buffer[i]);
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+ checksum += serial_master_buffer[i];
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+ }
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+ sync_send();
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+ serial_write_byte(checksum);
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+
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+ // always, release the line when not in use
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+ sync_send();
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+
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+ sei();
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+ return 0;
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+}
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+
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+#endif
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