From dd7e55d7a6c7d2ae82fef2e3dcf063101f3c850e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Krac=C3=ADk?= <petrkr@petrkr.net>
Date: Wed, 12 Jun 2019 01:04:16 +0200
Subject: [PATCH] BCC Checksum calculation

---
 ZPA_elektromer_UARTtest.ino | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/ZPA_elektromer_UARTtest.ino b/ZPA_elektromer_UARTtest.ino
index 7fc083d..f4fc101 100644
--- a/ZPA_elektromer_UARTtest.ino
+++ b/ZPA_elektromer_UARTtest.ino
@@ -78,8 +78,17 @@ void setup() {
 
 }
 
+// BCC is calculated as follows (source https://github.com/lvzon/dsmr-p1-parser/blob/master/doc/IEC-62056-21-notes.md):
+// The BCC is calculated over the bytes after STX up to and including the ETX byte
+// To calculate this BCC, take the first byte XOR 0xff
+// XOR this value with the second byte, and so forth up to and including the last byte (include 0x03 ETX !!),
+// and XOR the final value with 0xff.
+
+bool firstBCCbyte = false;
+uint8_t localBCC = 0;
+
 void loop() {
-  // put your main code here, to run repeatedly:
+
   ArduinoOTA.handle();
 
 
@@ -88,19 +97,34 @@ void loop() {
 
     if (r == 0x02) {
       Serial.print("<STX>");
+      firstBCCbyte = true;
+      uint8_t localBCC = 0;
       return;
     }
 
     if (r == 0x03) {
       Serial.println("<ETX>");
       Serial.print("Checksum <BCC>: ");
-      Serial.println(Serial2.read());
+      Serial.print(Serial2.read());
+      Serial.println();
+      Serial.print("Calculated <BCC>: ");
+      localBCC = localBCC ^ 0x03;
+      localBCC = localBCC ^ 0xFF;
+      Serial.print(localBCC);
+      Serial.println();
       Serial.println();
       Serial.println("Changing back baudrate to 300");
       Serial2.updateBaudRate(300);
       return;
     }
 
+    // Calculate BCC checksum
+    if (firstBCCbyte) {
+      firstBCCbyte = false;
+      localBCC = r ^ 0xFF;
+    } else {
+      localBCC = localBCC ^ r;
+    }
     Serial.write(r);
   }