Zilog Z16C30 Manual de usuario descargar pdf (Pagina 116)

5-49

Z16C30 USC

USER'S MANUAL

ZILOG

UM97USC0100

5.24 BETWEEN FRAMES, MESSAGES, OR CHARACTERS

5.24.1 Synchronous Transmission

When software issues a “Set EOF/EOM” command and

then writes data to a channel’s TDR, or when the TCC is

enabled and software or an external Transmit DMA con-

troller fetches enough data so that the TCC counts down to

zero, the channel flags the last character of the message

or frame in the TxFIFO. After this last character passes

through the TxFIFO and out onto the serial link, the Trans-

mitter terminates the frame or message. The Transmitter

also terminates a frame or message if it needs a character

from the TxFIFO but it’s empty (an “underrun” condition).

The Transmitter’s exact actions at these points depend on

the serial mode/protocol and perhaps on certain pro-

grammed options.

If the TxCRCatEnd bit in the Transmit Mode Register

(TMR8) is 1, the Transmitter sends the CRC code it has

accumulated during the frame, after a character marked

as the end of a frame or message. If the TxSubMode field

says to do so, the Transmitter sends its accumulated CRC

in an underrun situation. The CRC can be 16 or 32 bits long.

After sending a CRC for either reason, or right after the last

character from the TxFIFO if it doesn’t send the CRC,

except in 802.3 (Ethernet) mode the Transmitter sends a

closing Sync or Flag sequence as determined by the

TxMode and sometimes the TxSubMode, as follows:

TxMode Closing sequence:

Monosync (TSR15-8)

Slaved Monosync (TSR15-8)

Bisync (TSR15-8) if CMR14=0

(TSR7-0)(TSR15-8) if CMR14=1

Transparent Bisync SYN if CMR14=0

DLE-SYN if CMR14=1

(ASCII or EBCDIC per CMR12)

802.3 (Ethernet) None

HDLC/SDLC Flag (01111110)

HDLC/SDLC Loop Flag (01111110)

1a. The UnderWait bit (CCSR11) is 0 and/or the TxCtrlBlk

field (CCR15-14) is 0x, and there is at least 1 character

in the TxFIFO, or

1b. UnderWait is 1 and TxCtrlBlk is 10, and the TxFIFO is

full or a complete frame has been placed in the

TxFIFO, and

2a. Either the Wait2Send bit in the Transmit Interrupt

Control Register (TICR2) is 0, or

2b. Software has written the “Send Frame/Message” com-

mand to the TCmd field of the Transmit Command/

Status Register (TCSR15-12) since the end of the last

frame.

If these conditions aren’t met, the Transmitter sends the

“Idle line condition” specified by the TxIdle field of the

Transmit Command/Status Register (TCSR10-8). This field

also determines what the Transmitter sends between char-

acters in async modes. The Transmitter interprets TxIdle

as follows:

TxIdle Idle Line Condition

000 The idle line condition is the default for the

mode/protocol defined by TxMode:

* All ones in 802.3 and all async modes.

* Flags in HDLC/SDLC and HDLC/SDLC Loop.

* Sync sequences in Monosync, Slaved Monosync,

Bisync, and Transparent Bisync. (In the Bisync

modes these are like closing Syncs: they may be

single characters or pairs based on CMR14.)

001 Alternating zeroes and ones

010 Continuous zeroes

011 Continuous ones

100 Reserved; do not program

101 Alternating Mark and Space

110 Continuous Space (TxD low)

111 Continuous Mark (TxD high)

With choices 000-011, the Transmitter encodes the Idle

condition as specified by the TxEncode field of the Trans-

mit Mode Register (TMR15-13), while for choices 101-111

it doesn’t encode the condition. Software can use these

idle-condition options to keep Phase Locked Loop and

decoding circuits at the remote receiver “in sync” between

messages, frames, or async characters. Consider the

sections of Chapter 4 that deal with data encoding and the

DPLL, and whatever standards or specifications apply to

your application, in selecting how to program TxIdle.

Then, or right after sending the CRC in 802.3 (Ethernet)

mode, the Transmitter decides whether to send another

frame or message immediately or not. In

HDLC/SDLC Loop mode only, when it sends a closing or

idle Flag the Transmitter checks whether software has

cleared the CMR13 bit to signal the end of sending activity.

If so, it returns to repeating data from RxD onto TxD. In any

other mode, and in Loop mode if CMR13 is 1, the Transmit-

ter commits to sending a new message or frame when:

UM009402-0201

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