#include "SoftUART.h"
-static uint8_t TX_BitsRemaining, TX_Data;
-static uint8_t RX_BitMask, RX_Data;
+/** Total number of bits remaining to be sent in the current frame */
+static uint8_t TX_BitsRemaining;
+/** Temporary data variable to hold the byte being transmitted as it is shifted out */
+static uint8_t TX_Data;
+
+/** Current bit mask of the bit being shifted into the received data byte */
+static uint8_t RX_BitMask;
+
+/** Temporary data variable to hold the byte being received as it is shifted in */
+static uint8_t RX_Data;
+
+/** Initializes the software UART, ready for data transmission and reception into the global ring buffers. */
void SoftUART_Init(void)
{
- OCR1B = TCNT1 + 1; // force first compare
- TCCR1B = (1 << CS10); // CLK/1, T1 mode 0
- TCCR1C = (1 << FOC1B);
- TIMSK1 = (1 << OCIE1B); // enable tx and wait for start
- EICRA = (1 << ISC01); // -ve edge
- EIMSK = (1 << INT0); // enable INT0 interrupt
-
- STXPORT |= (1 << STX); // TX output
- STXDDR |= (1 << STX); // TX output
- SRXPORT |= (1 << SRX); // pullup on INT0
+ /* Set TX pin to output high, enable RX pull-up */
+ STXPORT |= (1 << STX);
+ STXDDR |= (1 << STX);
+ SRXPORT |= (1 << SRX);
+
+ /* Enable INT0 for the detection of incoming start bits that signal the start of a byte */
+ EICRA = (1 << ISC01);
+ EIMSK = (1 << INT0);
+
+ /* Start software UART transmission and reception timers */
+ TIMSK3 = (1 << OCIE3A);
+ TCCR3B = (1 << CS30);
+ TCCR1B = (1 << CS10);
}
-/* ISR to detect the start of a bit being sent to the software UART. */
-ISR(INT0_vect)
+/** ISR to detect the start of a bit being sent to the software UART. */
+ISR(INT0_vect, ISR_BLOCK)
{
- OCR1A = TCNT1 + ((BIT_TIME * 3) / 2) - 1; // scan 1.5 bits after start
+ /* Set reception channel to fire 1.5 bits past the beginning of the start bit */
+ OCR1A = TCNT1 + (BIT_TIME + (BIT_TIME / 2));
- RX_Data = 0; // clear bit storage
- RX_BitMask = (1 << 0); // bit mask
+ /* Clear the received data temporary variable, reset the current received bit position mask */
+ RX_Data = 0;
+ RX_BitMask = (1 << 0);
- TIFR1 = (1 << OCF1A); // clear pending interrupt
+ /* Clear reception channel ISR flag and enable the bit reception ISR */
+ TIFR1 = (1 << OCF1A);
+ TIMSK1 = (1 << OCIE1A);
- if (!(SRXPIN & (1 << SRX))) // still low
- {
- TIMSK1 = (1 << OCIE1A) | (1 << OCIE1B); // wait for first bit
- EIMSK &= ~(1 << INT0);
- }
+ /* Disable start bit detection ISR while the next byte is received */
+ EIMSK &= ~(1 << INT0);
}
-/* ISR to manage the reception of bits to the software UART. */
-ISR(TIMER1_COMPA_vect)
+/** ISR to manage the reception of bits to the software UART. */
+ISR(TIMER1_COMPA_vect, ISR_BLOCK)
{
+ /* Move the reception ISR compare position one bit ahead */
+ OCR1A += BIT_TIME;
+
+ /* Check if reception has finished */
if (RX_BitMask)
{
+ /* Store next bit into the received data variable */
if (SRXPIN & (1 << SRX))
RX_Data |= RX_BitMask;
+ /* Shift the current received bit mask to the next bit position */
RX_BitMask <<= 1;
-
- OCR1A += BIT_TIME; // next bit slice
}
else
{
+ /* Reception complete, store the received byte */
RingBuffer_Insert(&UARTtoUSB_Buffer, RX_Data);
-
- TIMSK1 = (1 << OCIE1B); // enable tx and wait for start
- EIMSK |= (1 << INT0); // enable START irq
- EIFR = (1 << INTF0); // clear any pending
+
+ /* Disable the reception ISR as all data has now been received, re-enable start bit detection ISR */
+ TIMSK1 = 0;
+ EIFR = (1 << INTF0);
+ EIMSK = (1 << INT0);
}
}
-/* ISR to manage the transmission of bits via the software UART. */
-ISR(TIMER1_COMPB_vect)
+/** ISR to manage the transmission of bits via the software UART. */
+ISR(TIMER3_COMPA_vect, ISR_NOBLOCK)
{
- OCR1B += BIT_TIME; // next bit slice
+ /* Move the transmission ISR compare position one bit ahead */
+ OCR3A += BIT_TIME;
+ /* Check if transmission has finished */
if (TX_BitsRemaining)
{
- if (--TX_BitsRemaining != 9) // no start bit
+ /* Check if we are sending a data bit, or the start bit */
+ if (--TX_BitsRemaining != 9)
{
- if (TX_Data & (1 << 0)) // test inverted data
+ /* Set the TX line to the value of the next bit in the byte to send */
+ if (TX_Data & (1 << 0))
STXPORT &= ~(1 << STX);
else
- STXPORT = (1 << STX);
+ STXPORT |= (1 << STX);
- TX_Data >>= 1; // shift zero in from left
+ /* Shift the transmission byte to move the next bit into position */
+ TX_Data >>= 1;
}
else
{
+ /* Start bit - keep TX line low */
STXPORT &= ~(1 << STX);
}
}
else if (USBtoUART_Buffer.Count)
{
+ /* Transmission complete, get the next byte to send (if available) */
TX_Data = ~RingBuffer_Remove(&USBtoUART_Buffer);
TX_BitsRemaining = 10;
}
projects / pub / USBasp.git / blobdiff