8000 [AVR] SPI atomicity fix avr (part 1/2 of #2376), second try by cmaglie · Pull Request #2449 · arduino/Arduino · GitHub
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[AVR] SPI atomicity fix avr (part 1/2 of #2376), second try #2449

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Merged
merged 5 commits into from
Dec 2, 2014
Merged

[AVR] SPI atomicity fix avr (part 1/2 of #2376), second try #2449

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53e25d8
[avr] Improved SPI speed on 16bit transfer.
cmaglie Oct 20, 2014
8344812
[avr] Made SPI.begin() and SPI.end() synchronized (Andrew Kroll)
cmaglie Oct 21, 2014
d9537cb
[avr] Added SPI.notUsingInterrupt() (Andrew Kroll)
cmaglie Oct 21, 2014
84b6cc2
[avr] Made SPI.usingInterrupt() synchronized (Andrew Kroll)
cmaglie Nov 18, 2014
a9735bf
Fix atomicity issues in SPI::beginTransaction and SPI::endTransaction…
cmaglie Nov 18, 2014
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[avr] Improved SPI speed on 16bit transfer. 
From #2376 (comment)

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.
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@cmaglie
cmaglie committed Nov 25, 2014
commit 53e25d8b5534c842a89eb62da1ab2558463c077b
10 changes: 10 additions & 0 deletions hardware/arduino/avr/libraries/SPI/SPI.h
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Original file line number Diff line number Diff line change
Expand Up @@ -184,6 +184,12 @@ class SPIClass {
// 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;
Expand All @@ -193,16 +199,20 @@ class SPIClass {
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;
}
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