Motorola MPC823e - Page 740

Communication Processor Module
16-286 MPC823e REFERENCE MANUAL MOTOROLA
COMMUNICATION
16
PROCESSOR MODULE
SCCs
16.9.19.12.5 Differences Between HDLC and ASYNC HDLC. There are four main
differences between the HDLC and ASYNC HDLC modes of operation:
• There is no maximum received frame length counter in the ASYNC HDLC controller.
Therefore, the controller receives all characters between opening and closing flags and
there is no way to stop the controller from writing to memory. This in no way affects the
number of bytes received into a specific buffer descriptor. It just means that a frame is
received into memory in its entirety.
• If an error prevents a frame from being received, the character received at the moment
the error occurred is not written into memory. For example, if a CD
lost error occurred,
the frame is closed and the partial character is not written to memory. Thus, the octet
count only reflects the number of bytes written to memory.
• The automatic error counters in HDLC mode have not been implemented in
asynchronous HDLC mode.
• Noisy characters (those whose three samples are not the same) are not accounted for
in the ASYNC HDLC controller. It is assumed that the CRC catches any data integrity
problems.
16.9.19.12.6 SCCx ASYNC HDLC Programming Guide. The following is an initialization
sequence guide for a serial communication controller in asynchronous HDLC mode.
1. Initialize the SDCR.
2. In NMSI mode, configure the port A pins to enable RXDx and TXDx.
3. Configure a baud rate generator to generate appropriate channel clocking frequency.
4. Program the SICR to route the baud rate generator clocking to a serial communication
controller that is in asynchronous HDLC mode.
5. Select whether the channel is using the time-slot assigner or the NMSI pins in the
SICR.
6. Write RBASE and TBASE in the SCCx parameter RAM to point to the first RX and TX
buffer descriptor.
7. Issue the INIT RX AND TX PARAMS command for the serial communication
controller.
8. Program the RFCR and TFCR.
9. Write the MRBLR with the maximum receiver buffer size.
10.Write C_MASK and C_PRES with the standard values.
11.Write the ZERO register to 0x0000.
12.Program the RFTHR to the number of frames that must be received before an interrupt
is generated.
Note: The GRACEFUL STOP TRANSMIT command is not supported by the ASYNC
HDLC controller.