diff --git a/libraries/AP_HAL_AVR/UARTDriver.cpp b/libraries/AP_HAL_AVR/UARTDriver.cpp
index 5a1889efa3..3c6410789b 100644
--- a/libraries/AP_HAL_AVR/UARTDriver.cpp
+++ b/libraries/AP_HAL_AVR/UARTDriver.cpp
@@ -61,8 +61,8 @@ void AVRUARTDriver::begin(uint32_t baud, uint16_t rxSpace, uint16_t txSpace) {
 		if (0 == txSpace)
 			txSpace = _txBuffer->mask + 1;
 
-		if (rxSpace == (_rxBuffer->mask + 1) && 
-			txSpace == (_txBuffer->mask + 1)) {
+		if (rxSpace == (_rxBuffer->mask + 1U) && 
+			txSpace == (_txBuffer->mask + 1U)) {
 			// avoid re-allocating the buffers if possible
 			need_allocate = false;
 			*_ucsrb &= ~(_portEnableBits | _portTxBits);
@@ -179,7 +179,7 @@ void AVRUARTDriver::flush(void) {
 }
 
 size_t AVRUARTDriver::write(uint8_t c) {
-	uint16_t i;
+	uint8_t i;
 
 	if (!_open) // drop bytes if not open
 		return 0;
@@ -212,7 +212,7 @@ size_t AVRUARTDriver::write(uint8_t c) {
 
 bool AVRUARTDriver::_allocBuffer(Buffer *buffer, uint16_t size)
 {
-	uint16_t mask;
+	uint8_t mask;
 	uint8_t	 shift;
 
 	// init buffer state
@@ -224,7 +224,7 @@ bool AVRUARTDriver::_allocBuffer(Buffer *buffer, uint16_t size)
 	// Note that we ignore requests for more than BUFFER_MAX space.
 	for (shift = 1; (1U << shift) < min(_max_buffer_size, size); shift++)
 		;
-	mask = (1 << shift) - 1;
+	mask = (1U << shift) - 1;
 
 	// If the descriptor already has a buffer allocated we need to take
 	// care of it.
diff --git a/libraries/AP_HAL_AVR/UARTDriver.h b/libraries/AP_HAL_AVR/UARTDriver.h
index c153bf1e4c..b97f0769d1 100644
--- a/libraries/AP_HAL_AVR/UARTDriver.h
+++ b/libraries/AP_HAL_AVR/UARTDriver.h
@@ -64,8 +64,8 @@ public:
 	///
 	/// Public so the interrupt handlers can see it
 	struct Buffer {
-		volatile uint16_t head, tail;	///< head and tail pointers
-		uint16_t mask;					///< buffer size mask for pointer wrap
+		volatile uint8_t head, tail;	///< head and tail pointers
+		uint8_t mask;					///< buffer size mask for pointer wrap
 		uint8_t *bytes;					///< pointer to allocated buffer
 	};
 private:
@@ -119,7 +119,7 @@ private:
 	/// @note if we could bring the max size down to 256, the mask and head/tail
 	///       pointers in the buffer could become uint8_t.
 	///
-	static const uint16_t _max_buffer_size = 512;
+	static const uint16_t _max_buffer_size = 256;
 };
 
 extern AP_HAL_AVR::AVRUARTDriver::Buffer __AVRUARTDriver__rxBuffer[];
@@ -131,7 +131,7 @@ extern AP_HAL_AVR::AVRUARTDriver::Buffer __AVRUARTDriver__txBuffer[];
 ISR(_RXVECTOR, ISR_BLOCK)                                               \
 {                                                                       \
         uint8_t c;                                                      \
-        uint16_t i;                                                     \
+        uint8_t i;                                                     \
                                                                         \
         /* read the byte as quickly as possible */                      \
         c = _UDR;                                                       \