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.

1 files changed, 13 insertions, 7 deletions

diff --git a/Projects/AVRISP/Lib/PDITarget.c b/Projects/AVRISP/Lib/PDITarget.c
index a8fe21e9b..e21942a1f 100644
--- a/Projects/AVRISP/Lib/PDITarget.c
+++ b/Projects/AVRISP/Lib/PDITarget.c
@@ -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

 }