From 9b6c86454b485ca342300a0d663270281b9a242c Mon Sep 17 00:00:00 2001
From: dooku <mj370@protonmail.com>
Date: Wed, 9 Jun 2021 18:33:13 +0200
Subject: [PATCH 1/2] Added CRC8 generating function

---
 fw/Core/Inc/crc8.h | 22 ++++++++++++++++++++++
 fw/Core/Src/crc8.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 fw/Core/Inc/crc8.h
 create mode 100644 fw/Core/Src/crc8.c

diff --git a/fw/Core/Inc/crc8.h b/fw/Core/Inc/crc8.h
new file mode 100644
index 0000000..e799f7e
--- /dev/null
+++ b/fw/Core/Inc/crc8.h
@@ -0,0 +1,22 @@
+/*
+ * crc.h
+ *
+ *  Created on: Jun 9, 2021
+ *      Author: user
+ */
+
+#ifndef INC_CRC8_H_
+#define INC_CRC8_H_
+
+#include "stdint.h"
+
+/*
+ * Definitions & macros
+ */
+
+#define CRC8_POLYNOMIAL 0x31
+#define CRC8_INIT 0xFF
+
+uint8_t crc8_calculate(const uint8_t *data, uint16_t count);
+
+#endif /* INC_CRC8_H_ */
diff --git a/fw/Core/Src/crc8.c b/fw/Core/Src/crc8.c
new file mode 100644
index 0000000..312cdf4
--- /dev/null
+++ b/fw/Core/Src/crc8.c
@@ -0,0 +1,28 @@
+/*
+ * crc.c
+ *
+ *  Created on: Jun 9, 2021
+ *      Author: user
+ */
+
+#include "crc8.h"
+
+/* Stolen from Sensirion SCD4x datasheet, section 3.11 */
+uint8_t crc8_calculate(const uint8_t *data, uint16_t count)
+{
+	uint16_t current_byte;
+	uint8_t crc = CRC8_INIT;
+	uint8_t crc_bit;
+	/* calculates 8-Bit checksum with given polynomial */
+	for (current_byte = 0; current_byte < count; ++current_byte) {
+		crc ^= (data[current_byte]);
+		for(crc_bit = 8; crc_bit > 0; --crc_bit) {
+			if (crc & 0x80) {
+				crc =(crc << 1) ^ CRC8_POLYNOMIAL;
+			} else {
+				crc = (crc << 1);
+			}
+		}
+	}
+	return crc;
+}

From 6b8e2f5903994147f161a8113c87f2fed58f9fff Mon Sep 17 00:00:00 2001
From: dooku <mj370@protonmail.com>
Date: Wed, 9 Jun 2021 18:52:44 +0200
Subject: [PATCH 2/2] Fixed warning for implicit declaration of LL_mDelay

---
 fw/Core/Inc/sht4x.h | 3 +++
 fw/Core/Src/sht4x.c | 1 +
 2 files changed, 4 insertions(+)

diff --git a/fw/Core/Inc/sht4x.h b/fw/Core/Inc/sht4x.h
index 0e133f3..b21b67b 100644
--- a/fw/Core/Inc/sht4x.h
+++ b/fw/Core/Inc/sht4x.h
@@ -10,7 +10,9 @@
 
 #include "stdint.h"
 #include "stm32l0xx_ll_i2c.h"
+#include "stm32l0xx_ll_utils.h"
 #include "i2c.h"
+#include "crc8.h"
 
 /*
  * Defines & macros
@@ -24,6 +26,7 @@
 
 #define SHT4X_OK 0
 #define SHT4X_ERROR -1 // generic error
+#define SHT4X_CRC8_ERROR -2 // checksum failed
 
 /*
  * Data types
diff --git a/fw/Core/Src/sht4x.c b/fw/Core/Src/sht4x.c
index 50521da..59aced5 100644
--- a/fw/Core/Src/sht4x.c
+++ b/fw/Core/Src/sht4x.c
@@ -34,6 +34,7 @@ int sht4x_measure(int *temperature, int *relative_humidity)
 	if (result != I2C_OK) {
 		return SHT4X_ERROR;
 	}
+	// TODO checksum
 	// Convert to T and RH; taken directly from pseudocode in SHT4x datasheet, page 3
 	uint32_t t_ticks = (buffer[0] << 8) + buffer[1];
 	uint32_t rh_ticks = (buffer[3] << 8) + buffer[4];