MPC8280 datasheet(735/1386 Pages) FREESCALE

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SCC HDLC Mode

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

22-5

22.5

Programming the SCC in HDLC Mode

HDLC mode is selected for an SCC by writing GSMR_L[MODE] = 0b0000. The HDLC controller uses

the same buffer and BD data structure as other modes and supports multibuffer operation and address

comparisons. Receive errors are reported through the RxBD; transmit errors are reported through the

TxBD.

22.6

SCC HDLC Commands

The transmit and receive commands are issued to the CP command register (CPCR). Transmit commands

are described in Table 22-2.

Receive commands are described in Table 22-3.

Table 22-2. Transmit Commands

Command

Description

STOP

TRANSMIT

After a hardware or software reset and a channel is enabled in the GSMR, the transmitter starts polling

the first BD in the TxBD table every 64 Tx clocks, or immediately if TODR[TOD] = 1, and begins sending

data if TxBD[R] is set. If the SCC receives the STOP TRANSMIT command while not transmitting, the

transmitter stops polling the BDs. If the SCC receives the command during transmission, transmission

is aborted after a maximum of 64 additional bits, the Tx FIFO is flushed, and the current BD pointer

TBPTR is not advanced (no new BD is accessed). The transmitter then sends an abort sequence (0x7F)

and stops polling the BDs.

When not transmitting, the channel sends flags or idles as programmed in the GSMR.

Note that if PSMR[MFF] = 1, multiple small frames could be flushed from the Tx FIFO; a GRACEFUL STOP

TRANSMIT

command prevents this.

GRACEFUL

STOP

TRANSMIT

Stops transmission smoothly. Unlike a STOP TRANSMIT command, it stops transmission after the current

frame is finished or immediately if no frame is being sent. SCCE[GRA] is set when transmission stops.

HDLC Tx parameters and Tx BDs can then be updated. TBPTR points to the next TxBD. Transmission

begins once TxBD[R] of the next BD is set and a RESTART TRANSMIT command is issued.

RESTART

TRANSMIT

Enables frames to be sent on the transmit channel. The HDLC controller expects this command after a

STOP TRANSMIT

is issued and the channel in its GSMR is disabled, after a GRACEFUL STOP TRANSMIT

command, or after a transmitter error. The transmitter resumes from the current BD.

INIT TX

PARAMETERS

Resets the Tx parameters in the parameter RAM. Issue only when the transmitter is disabled. INIT TX

AND RX PARAMETERS

resets both Tx and Rx parameters.

Table 22-3. Receive Commands

Command

Description

ENTER HUNT

MODE

After a hardware or software reset, once an SCC is enabled in the GSMR, the receiver is automatically

enabled and uses the first BD in the RxBD table. While the SCC is looking for the beginning of a frame,

that SCC is in hunt mode. The ENTER HUNT MODE command is used to force the HDLC receiver to stop

receiving the current frame and enter hunt mode, in which the HDLC continually scans the input data

stream for a flag sequence. After receiving the command, the buffer is closed and the CRC is reset.

Further frame reception uses the next BD.

CLOSE RXBD

Should not be used in the HDLC protocol.

INIT RX

PARAMETERS

Resets the Rx parameters in the parameter RAM.; issue only when the receiver is disabled. Note that

INIT TX AND RX PARAMETERS

resets both Tx and Rx parameters.

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