SCC Asynchronous HDLC Mode and IrDA
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
25-13
The data length and buffer pointer fields are described in
Section 21.3, “SCC Buffer Descriptors (BDs).”
25.16 Differences between HDLC and Asynchronous HDLC
The basic differences between HDLC and asynchronous HDLC modes are as follows:
•
Asynchronous HDLC does not support the
GRACEFUL
STOP
TRANSMIT
command.
•
Because asynchronous HDLC has no maximum received frame length counter, it receives all
characters between opening and closing flags. There is no way to keep it from writing to memory.
This does not affect the number of bytes received into a specific BD. A frame over the maximum
length is received into memory in its entirety.
•
If an error causes a frame to stop being received, the character being received at the moment the
error occurred is not written into memory. For example, if a CD lost error occurs, the frame is
closed and the partial character is not written to memory. Thus, the octet count reflects only the
number of bytes written to memory.
•
The automatic error counters in the HDLC controller are not implemented in the asynchronous
HDLC controller.
•
Noisy characters (characters for which all three samples are not identical) are not accounted for in
the asynchronous HDLC controller. It is assumed that the CRC catches any data integrity problems.
25.17 SCC Asynchronous HDLC Programming Example
The following example shows initialization for an SCC in asynchronous HDLC mode.
1. Initialize SDCR.
2. In NMSI mode, configure ports A and C to enable RXD, TXD, CTS, CD, and RTS. In other modes,
configure the TSA and its pins.
3. Configure a baud rate generator to the appropriate channel clocking frequency.
4. Program SICR. Route the BRG clocking to the SCC and select whether the channel is using the
TSA or the NMSI.
5. Point RBASE and TBASE in the SCC parameter RAM to the first RxBD and TxBD.
6. Issue the
INIT
RX
AND
TX
PARAMETERS
command for the SCC.
6
CM
Continuous mode
0 Normal operation
1 The CP does not clear R after this BD is closed, allowing its buffer to be resent when the CP next
accesses this BD. However, R is cleared if an error occurs during transmission, regardless of CM.
7–14
—
Reserved, should be cleared.
15
CT
CTS lost. In NMSI mode, CTS is lost during frame transmission. If more than one buffer has data in
the FIFO when this error occurs, CT is set in the currently open TxBD. Written by the asynchronous
HDLC controller after it finishes sending the buffer.
Table 25-11. Asynchronous HDLC TxBD Status
and Control Field Descriptions (continued)
Bits
Name
Description
Содержание PowerQUICC MPC870
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