Pololu TReX Jr - Manual (Page 20)

// generate a table value for all 256 possible byte values
for (i = 0; i < 256; i++)
{
CRCTable[i] = (i & 0x80) ? i ^ CRCPoly : i;
for (j = 1; j < 8; j++)
{
CRCTable[i] <<= 1;
if (CRCTable[i] & 0x80)
CRCTable[i] ^= CRCPoly;
}
}
}
// adds a message byte to the current CRC-7 to get a the new CRC-7
unsigned char CRCAdd(unsigned char CRC, unsigned char message_byte)
{
return CRCTable[(CRC << 1) ^ message_byte];
}
// returns the CRC-7 for a message of "length" bytes
unsigned char getCRC(unsigned char message[], int length)
{
int i;
unsigned char CRC = 0;
for (i = 0; i < length; i++)
CRC = CRCAdd(CRC, message[i]);
return CRC;
}
5.g. Resetting the Serial Communication Parameters
It is possible to reset the TReX Jr to its default serial settings using the following jumper sequence:
1. Power off your TReX Jr.
2. Attach the channel-mix jumper.
3. Attach the learning-mode/firmware-update jumper (i.e. short SO (Serial Out ) pin to G (Ground)).
4. Remove the mode-selection jumper (i.e. set the TReX Jr for Serial Mode).
5. Restore power to your TReX Jr. You should see the red and green status LEDs alternating around once per
second.
6. Remove the channel-mix jumper. You should now see the red and green status LEDs alternating around
four times per second.
7. Power off your TReX Jr.
When you next power up your TReX Jr, its serial settings will be back to their default values: 19.2 kbps baud, 1
stop bit, no error detection
Pololu TReX Jr User's Guide © 2001–2009 Pololu Corporation
5. The Serial Interface Page 20 of 23