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[avr] Improved SPI speed on 16bit transfer.

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From https://github.com/arduino/Arduino/pull/2376#issuecomment-59671152

Quoting Andrew Kroll:

   [..this commit..] introduces a small delay that can prevent the wait
   loop form iterating when running at the maximum speed. This gives
   you a little more speed, even if it seems counter-intuitive. At
   lower speeds, it is unnoticed. Watch the output on an oscilloscope
   when running full SPI speed, and you should see closer back-to-back
   writes.

Quoting Paul Stoffregen:

   I did quite a bit of experimenting with the NOP addition. The one
   that's in my copy gives about a 10% speedup on AVR.
This commit is contained in:
Cristian Maglie 2014-10-20 19:46:08 +02:00
parent 6eef453944
commit 53e25d8b55
1 changed files with 10 additions and 0 deletions
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10
hardware/arduino/avr/libraries/SPI/SPI.h
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@ -184,6 +184,12 @@ public:
// Write to the SPI bus (MOSI pin) and also receive (MISO pin)
inline static uint8_t transfer(uint8_t data) {
SPDR = data;
/*
* The following NOP introduces a small delay that can prevent the wait
* loop form iterating when running at the maximum speed. This gives
* about 10% more speed, even if it seems counter-intuitive. At lower
* speeds it is unnoticed.
*/
asm volatile("nop");
while (!(SPSR & _BV(SPIF))) ; // wait
return SPDR;
@ -193,16 +199,20 @@ public:
in.val = data;
if (!(SPCR & _BV(DORD))) {
SPDR = in.msb;
asm volatile("nop"); // See transfer(uint8_t) function
while (!(SPSR & _BV(SPIF))) ;
out.msb = SPDR;
SPDR = in.lsb;
asm volatile("nop");
while (!(SPSR & _BV(SPIF))) ;
out.lsb = SPDR;
} else {
SPDR = in.lsb;
asm volatile("nop");
while (!(SPSR & _BV(SPIF))) ;
out.lsb = SPDR;
SPDR = in.msb;
asm volatile("nop");
while (!(SPSR & _BV(SPIF))) ;
out.msb = SPDR;
}