Moog Crossbow NAV440 Series - Manual (Page 112)

Page 112
NAV440 User Manual
7430‐0131‐01 Rev. F
* CRC-CCITT 16-bit standard maintained by the ITU
* (International Telecommunications Union).
* ARGUMENTS: queue_ptr is pointer to queue holding area to be CRCed
* startIndex is offset into buffer where to begin CRC calculation
* num is offset into buffer where to stop CRC calculation
* RETURNS: 2-byte CRC
*******************************************************************************/
unsigned short calcCRC(QUEUE_TYPE *queue_ptr, unsigned int startIndex, unsigned int num) {
unsigned int i=0, j=0;
unsigned short crc=0x1D0F; //non-augmented inital value equivalent to augmented initial
value 0xFFFF
for (i=0; i<num; i+=1) {
crc ^= peekByte(queue_ptr, startIndex+i) << 8;
for(j=0;j<8;j+=1) {
if(crc & 0x8000) crc = (crc << 1) ^ 0x1021;
else crc = crc << 1;
}
}
return crc;
}
/*******************************************************************************
* FUNCTION: Initialize - initialize the queue
* ARGUMENTS: queue_ptr is pointer to the queue
*******************************************************************************/
void Initialize(QUEUE_TYPE *queue_ptr)
{
queue_ptr->count = 0;
queue_ptr->front = 0;
queue_ptr->rear = -1;
}
/*******************************************************************************
* FUNCTION: AddQueue - add item in front of queue
* ARGUMENTS: item holds item to be added to queue
* queue_ptr is pointer to the queue
* RETURNS: returns 0 if queue is full. 1 if successful
*******************************************************************************/
int AddQueue(char item, QUEUE_TYPE *queue_ptr)
{
int retval = 0;
if(queue_ptr->count >= MAXQUEUE)
{
retval = 0; /* queue is full */
}
else
{
queue_ptr->count++;
queue_ptr->rear = (queue_ptr->rear + 1) % MAXQUEUE;
queue_ptr->entry(queue_ptr->rear) = item;