From bb9fb1051cc68bc81589b1dbffe1b2e614e43737 Mon Sep 17 00:00:00 2001 From: Dean Camera Date: Wed, 26 May 2010 08:10:35 +0000 Subject: Add better documentation to the Software UART code in the XPLAINBridge project. --- Projects/XPLAINBridge/Lib/SoftUART.c | 106 +++++++++++++++++++++++------------ 1 file changed, 71 insertions(+), 35 deletions(-) (limited to 'Projects/XPLAINBridge/Lib/SoftUART.c') diff --git a/Projects/XPLAINBridge/Lib/SoftUART.c b/Projects/XPLAINBridge/Lib/SoftUART.c index c001c0787..5e8e03e5d 100644 --- a/Projects/XPLAINBridge/Lib/SoftUART.c +++ b/Projects/XPLAINBridge/Lib/SoftUART.c @@ -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; } -- cgit v1.2.3