From a50a181b862b422d25616c9eb2be97f3ebf6deb2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?King=20K=C3=A9vin?= <kingkevin@cuvoodoo.info>
Date: Tue, 26 Mar 2019 18:12:21 +0100
Subject: [PATCH] I2C: fix read NACK

NACKing in receive mode is different when 1, 2, or more bytes are
read.
see the reference manual for the cases and how to handle them.
---
 lib/i2c_master.c | 19 +++++++++++++++++--
 lib/i2c_master.h |  1 +
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/lib/i2c_master.c b/lib/i2c_master.c
index 8d2cf1f..93bf078 100644
--- a/lib/i2c_master.c
+++ b/lib/i2c_master.c
@@ -417,7 +417,18 @@ enum i2c_master_rc i2c_master_read(uint32_t i2c, uint8_t* data, size_t data_size
 	// sanity check
 	if (data==NULL || data_size==0) { // no data to read
 		return I2C_MASTER_RC_NONE;
-	}	
+	}
+	if (1 == data_size) {
+		i2c_nack_current(i2c); // [N]ACK current byte
+		i2c_disable_ack(i2c); // NACK after first byte
+	} else if (2 == data_size) {
+		i2c_nack_next(i2c); // [N]ACK next byte
+		i2c_enable_ack(i2c); // NACK first byte
+	} else {
+		i2c_nack_current(i2c); // ACK current byte
+		i2c_enable_ack(i2c); // NAK after next byte
+	}
+	// reading SR2 will also also clear ADDR and start the transaction
 	if (!(I2C_SR2(i2c) & I2C_SR2_MSL)) { // I2C device is not master
 		return I2C_MASTER_RC_NOT_MASTER;
 	}
@@ -430,7 +441,11 @@ enum i2c_master_rc i2c_master_read(uint32_t i2c, uint8_t* data, size_t data_size
 
 	// read data
 	for (size_t i=0; i<data_size; i++) { // read bytes
-		if (i==data_size-1) { // prepare to sent NACK for last byte
+		if (2 == data_size - i) { // prepare to sent NACK for second last byte
+			i2c_nack_next(i2c); // NACK next byte
+			i2c_disable_ack(i2c); // NACK received to stop slave transmission
+		} else if (1 == data_size - i) { // prepare to sent NACK for last byte
+			i2c_nack_current(i2c); // ACK current byte
 			i2c_disable_ack(i2c); // NACK received to stop slave transmission
 		} else {
 			i2c_enable_ack(i2c); // ACK received byte to continue slave transmission
diff --git a/lib/i2c_master.h b/lib/i2c_master.h
index 878788f..562fb89 100644
--- a/lib/i2c_master.h
+++ b/lib/i2c_master.h
@@ -73,6 +73,7 @@ enum i2c_master_rc i2c_master_select_slave(uint32_t i2c, uint16_t slave, bool ad
  *  @param[in] i2c I2C base address
  *  @param[out] data array to store bytes read
  *  @param[in] data_size number of bytes to read
+ *  @note the last read byte is NACKed, but the I2C peripheral will clock the read for at least 2 more bytes after the NACK
  *  @return I2C return code
  */
 enum i2c_master_rc i2c_master_read(uint32_t i2c, uint8_t* data, size_t data_size);