CAN bus data conversion issue

Projects Programming
1

I am getting an encoder Angle information via a CAN message. The first two bytes hold the information. I am using the following code to extract same :

unsigned int axle1_RawAngle = (rxBuf[1] << 8) | rxBuf[0];
      float aX1G = (axle1_RawAngle / 256) - 125; 
      Serial.println(axle1_RawAngle, HEX);
      Serial.println(aX1G);

With the above conversion I expect to read as below :

0x5F00 should read -30 Deg

0X7D00 should read 0 Deg

0x9B00 should read +30 Deg

Values above 0X7D00 work as expected . But values below 0x7D00 result in 65535. Possibly there is a error in the bit shift operation and storing in INT … the rXBuf[8] is declared as char.

Where is this going wrong ?

2

===>

    float aX1G = (axle1_RawAngle / 256.0) - 125.0;
3

what microcontroller are you using?

with problems like this it is a good idea to implement a simple program to test the algorithm, e.g.

void decode(uint8_t* rxBuf) {
  int axle1_RawAngle = (rxBuf[1] << 8) | rxBuf[0];  // << change to signed int
  float aX1G = (axle1_RawAngle / 256) - 125;
  Serial.println(axle1_RawAngle, HEX);
  Serial.println(aX1G);
}

void setup() {
  Serial.begin(115200);
  delay(2000);
  uint8_t d[] = { 0x00, 0x7d };
  decode(d);
  uint8_t d1[] = { 0x00, 0x5f };
  decode(d1);
  uint8_t d2[] = { 0x00, 0x9b };
  decode(d2);
}

void loop() {}

when run on an ESP32 displays

7D00
0.00
5F00
-30.00
9B00
30.00
1 Like
5

Perfect …. the data type definition was the issue. The moment I changed from Char to uint8_t all things fell in place !! I am using a Mega 2560 mcu

unsigned int axle1_RawAngle = (rxBuf[1] << 8) | rxBuf[0];   // Axle 1 Angle
aX1G = (float(axle1_RawAngle) / 256) - 125;

Thanks for the quick help !!

1 Like