From eca1341c7b6fef097ff07b51020a18eac88a9353 Mon Sep 17 00:00:00 2001
From: Cristian Maglie <c.maglie@arduino.cc>
Date: Wed, 8 Oct 2014 11:17:24 +0200
Subject: [PATCH] Limit SPI max clock speed to 12Mhz
SAMD21G18A doesn't operate correctly with clock dividers lower than 4
---
libraries/SPI/SPI.cpp | 6 +++++-
libraries/SPI/SPI.h | 14 ++++++++++++--
2 files changed, 17 insertions(+), 3 deletions(-)
diff --git a/libraries/SPI/SPI.cpp b/libraries/SPI/SPI.cpp
index 4f520a27..29ecbcb6 100644
--- a/libraries/SPI/SPI.cpp
+++ b/libraries/SPI/SPI.cpp
@@ -97,7 +97,11 @@ void SPIClass::setDataMode(uint8_t mode)
void SPIClass::setClockDivider(uint8_t div)
{
- _p_sercom->setBaudrateSPI(div);
+ if (div < SPI_MIN_CLOCK_DIVIDER) {
+ _p_sercom->setBaudrateSPI(SPI_MIN_CLOCK_DIVIDER);
+ } else {
+ _p_sercom->setBaudrateSPI(div);
+ }
}
byte SPIClass::transfer(uint8_t data)
diff --git a/libraries/SPI/SPI.h b/libraries/SPI/SPI.h
index 02ac74f3..6473ab23 100644
--- a/libraries/SPI/SPI.h
+++ b/libraries/SPI/SPI.h
@@ -18,6 +18,16 @@
#define SPI_MODE2 0x03
#define SPI_MODE3 0x01
+#if defined(__SAMD21G18A__)
+ // Even if not specified on the datasheet, the SAMD21G18A MCU
+ // doesn't operate correctly with clock dividers lower than 4.
+ // This allows a theoretical maximum SPI clock speed of 12Mhz
+ #define SPI_MIN_CLOCK_DIVIDER 4
+ // Other SAMD21xxxxx MCU may be affected as well
+#else
+ #define SPI_MIN_CLOCK_DIVIDER 2
+#endif
+
class SPISettings {
public:
SPISettings(uint32_t clock, BitOrder bitOrder, uint8_t dataMode) {
@@ -36,8 +46,8 @@ class SPISettings {
uint8_t div;
if (clock < (F_CPU / 255)) {
div = 255;
- } else if (clock >= (F_CPU / 2)) {
- div = 2;
+ } else if (clock >= (F_CPU / SPI_MIN_CLOCK_DIVIDER)) {
+ div = SPI_MIN_CLOCK_DIVIDER;
} else {
div = (F_CPU / (clock + 1)) + 1;
}
--
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