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Slightly speed up software USART in the AVRISP project - faster parity computation, ensure received data is byte aligned when receive is complete by throwing away the start bit during reception.

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This commit is contained in:
Dean Camera 2009-12-16 08:03:04 +00:00
parent 108a22a66a
commit f0b4d79629
3 changed files with 25 additions and 9 deletions
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20
Projects/AVRISP/Lib/PDITarget.c
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@ -70,8 +70,10 @@ ISR(TIMER1_COMPA_vect, ISR_BLOCK)
if ((SoftUSART_BitCount == BITS_IN_FRAME) && (BITBANG_PDIDATA_PIN & BITBANG_PDIDATA_MASK))
return;
/* Shift in the bit one less than the frame size in position, so that the start bit will eventually
* be discarded leaving the data to be byte-aligned for quick access */
if (BITBANG_PDIDATA_PIN & BITBANG_PDIDATA_MASK)
SoftUSART_Data |= (1 << BITS_IN_FRAME);
SoftUSART_Data |= (1 << (BITS_IN_FRAME - 1));
SoftUSART_Data >>= 1;
SoftUSART_BitCount--;
@ -82,6 +84,7 @@ ISR(TIMER1_COMPA_vect, ISR_BLOCK)
if (!IsSending)
return;
/* Set the data line to the next bit value */
if (SoftUSART_Data & 0x01)
BITBANG_PDIDATA_PORT |= BITBANG_PDIDATA_MASK;
else
@ -196,17 +199,20 @@ void PDITarget_SendByte(uint8_t Byte)
bool EvenParityBit = false;
uint8_t ParityData = Byte;
/* Compute Even parity bit */
for (uint8_t i = 0; i < 8; i++)
/* Compute Even parity - while a bit is still set, chop off lowest bit and toggle parity bit */
while (ParityData)
{
EvenParityBit ^= ParityData & 0x01;
ParityData >>= 1;
EvenParityBit ^= true;
ParityData &= (ParityData - 1);
}
/* Calculate the new USART frame data here while while we wait for a previous byte (if any) to finish sending */
uint16_t NewUSARTData = ((1 << 11) | (1 << 10) | ((uint16_t)EvenParityBit << 9) | ((uint16_t)Byte << 1) | (0 << 0));
while (SoftUSART_BitCount);
/* Data shifted out LSB first, START DATA PARITY STOP STOP */
SoftUSART_Data = ((uint16_t)EvenParityBit << 9) | ((uint16_t)Byte << 1) | (1 << 10) | (1 << 11);
SoftUSART_Data = NewUSARTData;
SoftUSART_BitCount = BITS_IN_FRAME;
#endif
}
@ -253,7 +259,7 @@ uint8_t PDITarget_ReceiveByte(void)
while (SoftUSART_BitCount);
/* Throw away the start, parity and stop bits to leave only the data */
return (uint8_t)(SoftUSART_Data >> 1);
return (uint8_t)SoftUSART_Data;
#endif
}