From a552fae34e7ca3e0c4e95fd53498c97b0b1c0d3b Mon Sep 17 00:00:00 2001
From: lady ada <limor@ladyada.net>
Date: Sat, 18 Jul 2020 15:57:15 -0400
Subject: [PATCH] only write MOSI if its changed

---
 Adafruit_SPIDevice.cpp | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/Adafruit_SPIDevice.cpp b/Adafruit_SPIDevice.cpp
index 7f49546..6bccd9d 100644
--- a/Adafruit_SPIDevice.cpp
+++ b/Adafruit_SPIDevice.cpp
@@ -123,6 +123,8 @@ void Adafruit_SPIDevice::transfer(uint8_t *buffer, size_t len) {
     startbit = 0x80;
   }
 
+  bool towrite, lastmosi = ! (buffer[0] & startbit);
+
   // for softSPI we'll do it by hand
   for (size_t i = 0; i < len; i++) {
     // software SPI
@@ -138,15 +140,17 @@ void Adafruit_SPIDevice::transfer(uint8_t *buffer, size_t len) {
     // Serial.print(send, HEX);
     for (uint8_t b = startbit; b != 0; b = (_dataOrder == SPI_BITORDER_LSBFIRST) ? b << 1 : b >> 1) {
       if (_dataMode == SPI_MODE0 || _dataMode == SPI_MODE2) {
-        if (_mosi != -1) {
+        towrite = send & b;
+        if ((_mosi != -1) && (lastmosi != towrite)) {
 #ifdef BUSIO_USE_FAST_PINIO
-          if (send & b)
+          if (towrite)
             *mosiPort |= mosiPinMask;
           else
             *mosiPort &= ~mosiPinMask;
 #else
-          digitalWrite(_mosi, send & b);
+          digitalWrite(_mosi, towrite);
 #endif
+          lastmosi = towrite;
         }
 
 #ifdef BUSIO_USE_FAST_PINIO