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Insane SPI optimization 

Wait a specific amount of time after each SPI transfer instead of
polling status. It works, and it saves about 21 seconds in total on an
8 MB SIMM!
This commit is contained in:
Doug Brown 2020-11-25 23:20:33 -08:00 committed by Doug Brown
parent 4394533d88
commit 9e586339dd
1 changed files with 16 additions and 2 deletions
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18
hal/at90usb646/parallel_bus.c
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@ -679,7 +679,14 @@ void ParallelBus_Read(uint32_t startAddress, uint32_t *buf, uint16_t len)
static ALWAYS_INLINE uint8_t SPITransfer(uint8_t byte)
{
SPDR = byte;
while (!(SPSR & (1 << SPIF)));
// Crazy optimization. Instead of waiting for the status register
// (see the commented-out "while" statement below), wait for 17 clock
// cycles instead. We know that our SPI bit rate is half the CPU clock.
// After 17 clock cycles, the entire byte has been written out.
__asm__ __volatile__ ("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\n");
__asm__ __volatile__ ("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\n");
__asm__ __volatile__ ("nop\n");
//while (!(SPSR & (1 << SPIF)));
return SPDR;
}
@ -691,7 +698,14 @@ static ALWAYS_INLINE uint8_t SPITransfer(uint8_t byte)
static ALWAYS_INLINE void SPITransferNoRead(uint8_t byte)
{
SPDR = byte;
while (!(SPSR & (1 << SPIF)));
// Crazy optimization. Instead of waiting for the status register
// (see the commented-out "while" statement below), wait for 17 clock
// cycles instead. We know that our SPI bit rate is half the CPU clock.
// After 17 clock cycles, the entire byte has been written out.
__asm__ __volatile__ ("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\n");
__asm__ __volatile__ ("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\n");
__asm__ __volatile__ ("nop\n");
//while (!(SPSR & (1 << SPIF)));
}
/** Asserts a control pin