From bb00e90e2499cd44d65fe581c8d806ffe3227189 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ond=C5=99ej=20Hru=C5=A1ka?= <ondra@ondrovo.com>
Date: Thu, 1 Feb 2018 11:38:41 +0100
Subject: [PATCH] added checksum support

---
 main.c      |  5 +++--
 ow_search.c | 45 +++++++++++++++++++++++++++++++++++++++++----
 ow_search.h | 17 ++++++++++++++++-
 3 files changed, 60 insertions(+), 7 deletions(-)

diff --git a/main.c b/main.c
index b08d029..a52cd37 100644
--- a/main.c
+++ b/main.c
@@ -16,7 +16,7 @@ int main(void)
     // than could fit in the address buffer.
 
     struct ow_search_state search_state;
-    ow_search_init(&search_state, 0xF0); // SEARCH_ROM
+    ow_search_init(&search_state, 0xF0 /* SEARCH_ROM */, false /* checksums */); 
 
     // Buffer for the found addresses - can have up to 65536 rows
     // Keep in mind that each address uses 8 bytes
@@ -57,7 +57,7 @@ struct owunit {
     int state;
 };
 
-#define UNITS_COUNT 12
+#define UNITS_COUNT 13
 struct owunit units[UNITS_COUNT] = {
     {.romcode = {0b00000000}},//00
     {.romcode = {0b00000001}},//01
@@ -71,6 +71,7 @@ struct owunit units[UNITS_COUNT] = {
     {.romcode = {0b11110111}},//f7
     {.romcode = {0xFF,0x00,0xFF,0x00,0x55,0x00,0xAA,0x00}},
     {.romcode = {0x12,0x34,0x56,0x78,0x9a,0xbc,0xde,0xf0}},
+    {.romcode = {0x10,0x76,0xc7,0x04,0x01,0x08,0x00,0x71}}, // this is from a real 1-wire thermometer - checksum should be OK
 };
 
 void rom_device_reset(struct owunit *device)
diff --git a/ow_search.c b/ow_search.c
index aca62be..b2d606a 100644
--- a/ow_search.c
+++ b/ow_search.c
@@ -10,13 +10,47 @@
 
 #include "ow_search.h"
 
-void ow_search_init(struct ow_search_state *state, uint8_t command)
+// -------- CHECKSUM --------
+
+static inline uint8_t crc8_bits(uint8_t data)
+{
+    uint8_t crc = 0;
+    if(data & 1)     crc ^= 0x5e;
+    if(data & 2)     crc ^= 0xbc;
+    if(data & 4)     crc ^= 0x61;
+    if(data & 8)     crc ^= 0xc2;
+    if(data & 0x10)  crc ^= 0x9d;
+    if(data & 0x20)  crc ^= 0x23;
+    if(data & 0x40)  crc ^= 0x46;
+    if(data & 0x80)  crc ^= 0x8c;
+    return crc;
+}
+
+static uint8_t crc8_add(uint8_t cksum, uint8_t byte)
+{
+    return crc8_bits(byte ^ cksum);
+}
+
+uint8_t ow_checksum(const uint8_t *buff, uint16_t len)
+{
+    uint8_t cksum = 0;
+    for(uint16_t i = 0; i < len; i++) {
+        cksum = crc8_add(cksum, buff[i]);
+    }
+    return cksum;
+}
+
+// -----------------------------
+
+
+void ow_search_init(struct ow_search_state *state, uint8_t command, bool test_checksums)
 {
     state->prev_last_fork = 64;
     memset(state->prev_code, 0, 8);
     state->status = OW_SEARCH_MORE;
     state->command = command;
     state->first = true;
+    state->test_checksums = test_checksums;
 }
 
 uint16_t ow_search_run(struct ow_search_state *state, ow_romcode_t *codes, uint16_t capacity)
@@ -77,10 +111,13 @@ uint16_t ow_search_run(struct ow_search_state *state, ow_romcode_t *codes, uint1
             }
         }
 
-        // Record a found address
         memcpy(state->prev_code, code, 8);
-        memcpy(codes[found_devices], code, 8);
-        found_devices++;
+
+        if (!state->test_checksums || 0 == ow_checksum(code, 8)) {
+            // Record a found address
+            memcpy(codes[found_devices], code, 8);
+            found_devices++;
+        }
 
         // Stop condition
         if (last_fork == -1) {
diff --git a/ow_search.h b/ow_search.h
index a706428..efdbfb6 100644
--- a/ow_search.h
+++ b/ow_search.h
@@ -75,6 +75,7 @@ struct ow_search_state {
     uint8_t command;
     enum ow_search_result status;
     bool first;
+    bool test_checksums;
 };
 
 /**
@@ -82,8 +83,9 @@ struct ow_search_state {
  *
  * @param[out] state - inited struct
  * @param[in] command - command to send for requesting the search (e.g. SEARCH_ROM)
+ * @param[in] test_checksums - verify checksums of all read romcodes
  */
-void ow_search_init(struct ow_search_state *state, uint8_t command);
+void ow_search_init(struct ow_search_state *state, uint8_t command, bool test_checksums);
 
 /**
  * Perform a search of the 1-wire bus, with a state struct pre-inited
@@ -101,4 +103,17 @@ void ow_search_init(struct ow_search_state *state, uint8_t command);
  */
 uint16_t ow_search_run(struct ow_search_state *state, ow_romcode_t *codes, uint16_t capacity);
 
+/**
+ * Compute a 1-wire type checksum.
+ * If the buffer includes the checksum, the result should be 0.
+ *
+ * (this function may be used externally, or you can delete the implementation
+ * from the c file if another implementation is already available)
+ *
+ * @param[in] buf - buffer of bytes to verify
+ * @param[in] len - buffer length
+ * @return checksum
+ */
+uint8_t ow_checksum(const uint8_t *buf, uint16_t len);
+
 #endif //OW_SEARCH_H