User guide

110 DC 900-1338I
Protocol Software Toolkit Programmer Guide
no longer transfers characters to the SCC or IUSC), a transmit underrun is generated.
The SCC or IUSC transmits the two-byte CRC followed by a closing flag to terminate
the frame.
To receive, the DMA transfer count is set to the maximum block size and will not nor-
mally reach terminal count. The SCC or IUSC automatically calculates CRC during the
received frame and generates an end-of-frame (special receive condition) interrupt
when the closing flag is detected. The interrupt service routine reads an SCC register to
determine whether the CRC that the SCC calculated matched the CRC bytes received at
the end of the frame. The IUSC posts the status of the reception in the linked list header
record (located at the end of the data buffer) which is then examined by the ISR.
The following interrupts are processed in HDLC/SDLC mode:
SCC DMA Receive Terminal Count or IUSC End of Buffer If terminal count is
reached before end-of-frame, the received message is too long (receiving more
data would overrun the receive buffer). In this case, the interrupt service routine
increments an error count and restarts the receiver using the current receive
buffer.
SCC Special Receive Condition or IUSC RDMA Complete This interrupt is generated
at the end of a received frame. If the SCC or IUSC indicates a CRC error, an error
count is incremented, and the receiver is restarted using the current buffer. If the
CRC is good, the receiver is restarted using the next buffer in the link-to-board
queue.
SCC Transmit Buffer Empty or IUSC End of Buffer This interrupt is enabled only by
the external or transmit status interrupt service routine when a transmit under-
run occurs while the transmit buffer is not yet empty. The end of the transmission
is processed.