Helix serial.c master to slave communication bugfix & speed up

keyboards/helix/serial.c

@@ -14,9 +14,45 @@
 
 #ifdef USE_SERIAL
 
-// Serial pulse period in microseconds. Its probably a bad idea to lower this
-// value.
-#define SERIAL_DELAY 30
+#define _delay_sub_us(x)    __builtin_avr_delay_cycles(x)
+
+// Serial pulse period in microseconds.
+#define SELECT_SERIAL_SPEED 1
+#if SELECT_SERIAL_SPEED == 0
+  // Very High speed
+  #define SERIAL_DELAY 4             // micro sec
+  #define READ_WRITE_START_ADJUST 30 // cycles
+  #define READ_WRITE_WIDTH_ADJUST 10 // cycles
+#elif SELECT_SERIAL_SPEED == 1
+  // High speed
+  #define SERIAL_DELAY 6             // micro sec
+  #define READ_WRITE_START_ADJUST 23 // cycles
+  #define READ_WRITE_WIDTH_ADJUST 10 // cycles
+#elif SELECT_SERIAL_SPEED == 2
+  // Middle speed
+  #define SERIAL_DELAY 12            // micro sec
+  #define READ_WRITE_START_ADJUST 25 // cycles
+  #define READ_WRITE_WIDTH_ADJUST 10 // cycles
+#elif SELECT_SERIAL_SPEED == 3
+  // Low speed
+  #define SERIAL_DELAY 24            // micro sec
+  #define READ_WRITE_START_ADJUST 25 // cycles
+  #define READ_WRITE_WIDTH_ADJUST 10 // cycles
+#elif SELECT_SERIAL_SPEED == 4
+  // Very Low speed
+  #define SERIAL_DELAY 50            // micro sec
+  #define READ_WRITE_START_ADJUST 25 // cycles
+  #define READ_WRITE_WIDTH_ADJUST 10 // cycles
+#else
+#error Illegal Serial Speed
+#endif
+
+
+#define SERIAL_DELAY_HALF1 (SERIAL_DELAY/2)
+#define SERIAL_DELAY_HALF2 (SERIAL_DELAY - SERIAL_DELAY/2)
+
+#define SLAVE_INT_WIDTH 1
+#define SLAVE_INT_RESPONSE_TIME SERIAL_DELAY
 
 uint8_t volatile serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH] = {0};
 uint8_t volatile serial_master_buffer[SERIAL_MASTER_BUFFER_LENGTH] = {0};
@@ -28,17 +64,25 @@ inline static
 void serial_delay(void) {
   _delay_us(SERIAL_DELAY);
 }
-void serial_delay_short(void) {
-  _delay_us(SERIAL_DELAY-1);
+
+inline static
+void serial_delay_half1(void) {
+  _delay_us(SERIAL_DELAY_HALF1);
+}
+
+inline static
+void serial_delay_half2(void) {
+  _delay_us(SERIAL_DELAY_HALF2);
 }
+
 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) {
+void serial_input_with_pullup(void) {
   SERIAL_PIN_DDR  &= ~SERIAL_PIN_MASK;
   SERIAL_PIN_PORT |= SERIAL_PIN_MASK;
 }
@@ -64,7 +108,7 @@ void serial_master_init(void) {
 }
 
 void serial_slave_init(void) {
-  serial_input();
+  serial_input_with_pullup();
 
 #ifndef USE_SERIAL_PD2
   // Enable INT0
@@ -79,92 +123,92 @@ void serial_slave_init(void) {
 #endif
 }
 
-// Used by the master to synchronize timing with the slave.
+// Used by the sender to synchronize timing with the reciver.
 static
 void sync_recv(void) {
-  serial_input();
+  for (int i = 0; i < SERIAL_DELAY*5 && serial_read_pin(); i++ ) {
+  }
   // 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();
-  _delay_us(SERIAL_DELAY-5);
 }
 
-// Used by the slave to send a synchronization signal to the master.
+// Used by the reciver to send a synchronization signal to the sender.
 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();
+  _delay_sub_us(READ_WRITE_START_ADJUST);
   for ( uint8_t i = 0; i < 8; ++i) {
+    serial_delay_half1();   // read the middle of pulses
     byte = (byte << 1) | serial_read_pin();
-    serial_delay();
-    _delay_us(1);
+    _delay_sub_us(READ_WRITE_WIDTH_ADJUST);
+    serial_delay_half2();
   }
-
   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)) {
+  uint8_t b = 1<<7;
+  while( b ) {
+    if(data & b) {
       serial_high();
     } else {
       serial_low();
     }
+    b >>= 1;
     serial_delay();
   }
+  serial_low(); // sync_send() / senc_recv() need raise edge
 }
 
 // interrupt handle to be used by the slave device
 ISR(SERIAL_PIN_INTERRUPT) {
-  sync_send();
+  serial_output();
 
+  // slave send phase
   uint8_t checksum = 0;
   for (int i = 0; i < SERIAL_SLAVE_BUFFER_LENGTH; ++i) {
-    serial_write_byte(serial_slave_buffer[i]);
     sync_send();
+    serial_write_byte(serial_slave_buffer[i]);
     checksum += serial_slave_buffer[i];
   }
-  serial_write_byte(checksum);
   sync_send();
+  serial_write_byte(checksum);
 
-  // wait for the sync to finish sending
-  serial_delay();
-
-  // read the middle of pulses
-  _delay_us(SERIAL_DELAY/2);
+  // slave switch to input
+  sync_send(); //0
+  serial_delay_half1(); //1
+  serial_low();         //2
+  serial_input_with_pullup(); //2
+  serial_delay_half1(); //3
 
+  // slave recive phase
   uint8_t checksum_computed = 0;
   for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
+    sync_recv();
     serial_master_buffer[i] = serial_read_byte();
-    sync_send();
     checksum_computed += serial_master_buffer[i];
   }
+  sync_recv();
   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;
   }
+
+  sync_recv(); //weit master output to high
 }
 
 inline
@@ -178,58 +222,69 @@ bool serial_slave_DATA_CORRUPT(void) {
 // Returns:
 // 0 => no error
 // 1 => slave did not respond
+// 2 => checksum error
 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);
+  _delay_us(SLAVE_INT_WIDTH);
 
   // wait for the slaves response
-  serial_input();
-  serial_high();
-  _delay_us(SERIAL_DELAY);
+  serial_input_with_pullup();
+  _delay_us(SLAVE_INT_RESPONSE_TIME);
 
   // check if the slave is present
   if (serial_read_pin()) {
     // slave failed to pull the line low, assume not present
+    serial_output();
+    serial_high();
     sei();
     return 1;
   }
 
+  // master recive phase
   // 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();
+    serial_slave_buffer[i] = serial_read_byte();
     checksum_computed += serial_slave_buffer[i];
   }
-  uint8_t checksum_received = serial_read_byte();
   sync_recv();
+  uint8_t checksum_received = serial_read_byte();
 
   if (checksum_computed != checksum_received) {
+    serial_output();
+    serial_high();
     sei();
     return 2;
   }
 
+  // master switch to output
+  sync_recv(); //0
+  serial_delay();  //1
+  serial_low();    //3
+  serial_output(); // 3
+  serial_delay_half1(); //4
+
+  // master send phase
   uint8_t checksum = 0;
-  // send data to the slave
+
   for (int i = 0; i < SERIAL_MASTER_BUFFER_LENGTH; ++i) {
+    sync_send();
     serial_write_byte(serial_master_buffer[i]);
-    sync_recv();
     checksum += serial_master_buffer[i];
   }
+  sync_send();
   serial_write_byte(checksum);
-  sync_recv();
 
   // always, release the line when not in use
-  serial_output();
-  serial_high();
+  sync_send();
 
   sei();
   return 0;