Commit bb9fb105 authored by Dean Camera's avatar Dean Camera
Browse files

Add better documentation to the Software UART code in the XPLAINBridge project.

parent 1c407b96
......@@ -39,85 +39,121 @@
#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
/* Set TX pin to output high, enable RX pullup */
STXPORT |= (1 << STX);
STXDDR |= (1 << STX);
SRXPORT |= (1 << SRX);
/* Enable INT0 for the detection of incomming start bits that signal the start of a byte */
EICRA = (1 << ISC01);
EIMSK = (1 << INT0);
/* Ensure that when the timer is enabled the transmission ISR runs immediately */
OCR1B = TCNT1 + 1;
/* Start timer 1 with transmission channel force-enabled so that it will immediatly fire */
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
TIMSK1 = (1 << OCIE1B);
TCCR1B = (1 << CS10);
}
/* ISR to detect the start of a bit being sent to the software UART. */
/** ISR to detect the start of a bit being sent to the software UART. */
ISR(INT0_vect)
{
OCR1A = TCNT1 + ((BIT_TIME * 3) / 2) - 1; // scan 1.5 bits after start
RX_Data = 0; // clear bit storage
RX_BitMask = (1 << 0); // bit mask
TIFR1 = (1 << OCF1A); // clear pending interrupt
if (!(SRXPIN & (1 << SRX))) // still low
{
TIMSK1 = (1 << OCIE1A) | (1 << OCIE1B); // wait for first bit
/* Set reception channel to fire 1.5 bits past the beginning of the start bit */
OCR1A = TCNT1 + ((BIT_TIME * 3) / 2) - 1;
/* Clear the received data temporary variable, reset the current received bit position mask */
RX_Data = 0;
RX_BitMask = (1 << 0);
/* Clear reception channel ISR flag in case it is pending */
TIFR1 = (1 << OCF1A);
/* Check that the start bit is still low to prevent noise from triggering a reception */
if (!(SRXPIN & (1 << SRX)))
{
/* Still low, enable both send and receive channels */
TIMSK1 = (1 << OCIE1A) | (1 << OCIE1B);
/* Clear the start bit detection ISR flag if it is pending */
EIMSK &= ~(1 << INT0);
}
}
/* ISR to manage the reception of bits to the software UART. */
/** ISR to manage the reception of bits to the software UART. */
ISR(TIMER1_COMPA_vect)
{
/* 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 = (1 << OCIE1B);
EIMSK |= (1 << INT0);
EIFR = (1 << INTF0);
}
}
/* ISR to manage the transmission of bits via the software UART. */
/** ISR to manage the transmission of bits via the software UART. */
ISR(TIMER1_COMPB_vect)
{
OCR1B += BIT_TIME; // next bit slice
/* Move the transmission ISR compare position one bit ahead */
OCR1B += 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;
}
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment