From 4d350040ba8298e284112692826f8d80c0ba58e1 Mon Sep 17 00:00:00 2001
From: Neil McKechnie <neilmck999@gmail.com>
Date: Tue, 31 Jan 2023 12:28:51 +0000
Subject: [PATCH] I2CManager_SAMD.h - avoid bus hangs on speed changes

The speed change is deferred until the next transmission is about to start to avoid issues with the I2C module being disabled and enabled during a transmission.
---
 I2CManager_SAMD.h | 64 +++++++++++++++++++++++++++++++----------------
 1 file changed, 43 insertions(+), 21 deletions(-)

diff --git a/I2CManager_SAMD.h b/I2CManager_SAMD.h
index 38ee996..63efcd6 100644
--- a/I2CManager_SAMD.h
+++ b/I2CManager_SAMD.h
@@ -29,6 +29,9 @@
 //#include <avr/interrupt.h>
 #include <wiring_private.h>
 
+// Storage for new baud rate.  Zero means no change pending
+static uint32_t pendingBaudRate = 0;
+
 /***************************************************************************
  *  Interrupt handler.
  *  IRQ handler for SERCOM3 which is the default I2C definition for Arduino Zero
@@ -65,22 +68,37 @@ void I2CManagerClass::I2C_setClock(uint32_t i2cClockSpeed) {
     i2cClockSpeed = 100000L;
     t_rise = 1000;
   }
-
-  // Disable the I2C master mode and wait for sync
-  s->I2CM.CTRLA.bit.ENABLE = 0 ;
-  while (s->I2CM.SYNCBUSY.bit.ENABLE != 0);
-
   // Calculate baudrate - using a rise time appropriate for the speed
-  s->I2CM.BAUD.bit.BAUD = SystemCoreClock / (2 * i2cClockSpeed) - 5 - (((SystemCoreClock / 1000000) * t_rise) / (2 * 1000));
+  pendingBaudRate = SystemCoreClock / (2 * i2cClockSpeed) - 5 - (((SystemCoreClock / 1000000) * t_rise) / (2 * 1000));
+}
 
-  // Enable the I2C master mode and wait for sync
-  s->I2CM.CTRLA.bit.ENABLE = 1 ;
-  while (s->I2CM.SYNCBUSY.bit.ENABLE != 0);
+/***************************************************************************
+ * Internal function to actually change the baud rate register, executed from 
+ * interrupt code to avoid in-progress I2C transactions.
+ ***************************************************************************/
+static void checkForPendingClockSpeedChange() {
+  if (pendingBaudRate > 0) {
+    // Wait while the bus is busy
+    while (s->I2CM.STATUS.bit.BUSSTATE != 0x1);
 
-  // Setting bus idle mode and wait for sync
-  s->I2CM.STATUS.bit.BUSSTATE = 1 ;
-  while (s->I2CM.SYNCBUSY.bit.SYSOP != 0);
+    // Disable the I2C master mode and wait for sync
+    s->I2CM.CTRLA.bit.ENABLE = 0 ;
+    while (s->I2CM.SYNCBUSY.bit.ENABLE != 0);
 
+    // Update baudrate
+    s->I2CM.BAUD.bit.BAUD = pendingBaudRate;
+
+    // Enable the I2C master mode and wait for sync
+    s->I2CM.CTRLA.bit.ENABLE = 1 ;
+    while (s->I2CM.SYNCBUSY.bit.ENABLE != 0);
+
+    // Setting bus idle mode and wait for sync
+    s->I2CM.STATUS.bit.BUSSTATE = 1 ;
+    while (s->I2CM.SYNCBUSY.bit.SYSOP != 0);
+
+    // Clear pending rate now it's been implemented.
+    pendingBaudRate = 0;
+  }
   return;
 }
 
@@ -148,25 +166,29 @@ void I2CManagerClass::I2C_init()
  *  Initiate a start bit for transmission.
  ***************************************************************************/
 void I2CManagerClass::I2C_sendStart() {
+  // Check if the clock is to be changed, if so do it now.  It doesn't matter
+  // what else is going on over the I2C bus as the clock change only affects
+  // this master.
+  checkForPendingClockSpeedChange();
+
+  // Set counters here in case this is a retry.
   bytesToSend = currentRequest->writeLen;
   bytesToReceive = currentRequest->readLen;
 
-  // We may have initiated a stop bit before this without waiting for it.
-  //  However, the state machine ensures that the start bit isn't sent
-  //  until the stop bit is complete.
-  //while (s->I2CM.STATUS.bit.BUSSTATE == 0x2);
+  // On a single-master I2C bus, the start bit won't be sent until the bus 
+  // state goes to IDLE so we can request it without waiting.  On a 
+  // multi-master bus, the bus may be BUSY under control of another master, 
+  // in which case we can avoid some arbitration failures by waiting until
+  // the bus state is IDLE.  We don't do that here.
 
   // If anything to send, initiate write.  Otherwise initiate read.
   if (operation == OPERATION_READ || ((operation == OPERATION_REQUEST) && !bytesToSend))
   {
-    // Wait while the I2C bus is BUSY
-    //while (s->I2CM.STATUS.bit.BUSSTATE != 0x1);
-    // Send start and address with read/write flag or'd in
+    // Send start and address with read flag (1) or'd in
     s->I2CM.ADDR.bit.ADDR = (currentRequest->i2cAddress << 1) | 1;
   }
   else {
-    // Wait while the I2C bus is BUSY
-    //while (s->I2CM.STATUS.bit.BUSSTATE != 0x1);
+    // Send start and address with write flag (0) or'd in
     s->I2CM.ADDR.bit.ADDR = (currentRequest->i2cAddress << 1ul) | 0;
   }
 }