resets I2C pins back to original state, not blindly to input

Author: MihaMarkic

Raspberry.IO.InterIntegratedCircuit/I2cDriver.cs

@@ -21,6 +21,8 @@ public class I2cDriver : IDisposable
 
         private readonly ProcessorPin sdaPin;
         private readonly ProcessorPin sclPin;
+        private readonly bool wasSdaPinSet;
+        private readonly bool wasSclPinSet;
 
         private readonly IntPtr gpioAddress;
         private readonly IntPtr bscAddress;
@@ -72,8 +74,9 @@ public I2cDriver(ProcessorPin sdaPin, ProcessorPin sclPin)
                 throw new InvalidOperationException("Unable to access device memory");
 
             // Set the I2C pins to the Alt 0 function to enable I2C access on them
-            SetPinMode((uint) (int) sdaPin, Interop.BCM2835_GPIO_FSEL_ALT0); // SDA
-            SetPinMode((uint) (int) sclPin, Interop.BCM2835_GPIO_FSEL_ALT0); // SCL
+            // remembers if the values were actually changed to clear them or not upon dispose
+            wasSdaPinSet = SetPinMode((uint)(int)sdaPin, Interop.BCM2835_GPIO_FSEL_ALT0); // SDA
+            wasSclPinSet = SetPinMode((uint) (int) sclPin, Interop.BCM2835_GPIO_FSEL_ALT0); // SCL
 
             // Read the clock divider register
             var dividerAddress = bscAddress + (int) Interop.BCM2835_BSC_DIV;
@@ -89,9 +92,15 @@ public I2cDriver(ProcessorPin sdaPin, ProcessorPin sclPin)
         /// </summary>
         public void Dispose()
         {
-            // Set all the I2C/BSC1 pins back to input
-            SetPinMode((uint) (int) sdaPin, Interop.BCM2835_GPIO_FSEL_INPT); // SDA
-            SetPinMode((uint) (int) sclPin, Interop.BCM2835_GPIO_FSEL_INPT); // SCL
+            // Set all the I2C/BSC1 pins back to original values if changed
+            if (wasSdaPinSet)
+            {
+                SetPinMode((uint)(int)sdaPin, Interop.BCM2835_GPIO_FSEL_INPT); // SDA
+            }
+            if (wasSclPinSet)
+            {
+                SetPinMode((uint)(int)sclPin, Interop.BCM2835_GPIO_FSEL_INPT); // SCL
+            }
 
             Interop.munmap(gpioAddress, Interop.BCM2835_BLOCK_SIZE);
             Interop.munmap(bscAddress, Interop.BCM2835_BLOCK_SIZE);
@@ -369,15 +378,31 @@ private static uint GetBscBase(ProcessorPin sdaPin, ProcessorPin sclPin)
             }
         }
 
-        private void SetPinMode(uint pin, uint mode)
+        /// <summary>
+        /// 
+        /// </summary>
+        /// <param name="pin"></param>
+        /// <param name="mode"></param>
+        /// <returns>True when value was changed, false otherwise.</returns>
+        private bool SetPinMode(uint pin, uint mode)
         {
             // Function selects are 10 pins per 32 bit word, 3 bits per pin
             var paddr = gpioAddress + (int) (Interop.BCM2835_GPFSEL0 + 4*(pin/10));
             var shift = (pin%10)*3;
             var mask = Interop.BCM2835_GPIO_FSEL_MASK << (int) shift;
             var value = mode << (int) shift;
 
-            WriteUInt32Mask(paddr, value, mask);
+            var existing = ReadUInt32(paddr) & mask;
+            if (existing != value)
+            {
+                //Console.WriteLine($"existing is {x} masked:{x & mask} vs mask:{mask} value:{value}");
+                WriteUInt32Mask(paddr, value, mask);
+                return true;
+            }
+            else
+            {
+                return false;
+            }
         }
 
         private static void WriteUInt32Mask(IntPtr address, uint value, uint mask)