Zilog Z16C30 Manual de usuario descargar pdf (Pagina 117)

5-50

Z16C30 USC

USER'S MANUAL

UM97USC0100

ZILOG

5.24 BETWEEN FRAMES, MESSAGES, OR CHARACTERS (Continued)

In sync modes, once the conditions to start sending a

message or frame (described above) are met, the Trans-

mitter may send a bit sequence called a Preamble. A

Preamble can be used to synchronize Phase Locked Loop

and decoding circuits at the remote receiver, or, for USCs

manufactured after June 1993, to guarantee a minimum

number of Flags between HDLC/SDLC frames. Whether

the Transmitter sends a Preamble is a function of the

TxMode and sometimes the TxSubMode, as follows:

TxMode Preamble sent?

Monosync If CMR13=1

Slaved Monosync Never

Bisync If CMR13=1

Transparent Bisync If CMR13=1

802.3 (Ethernet) Always

HDLC/SDLC If CMR13=1

HDLC/SDLC Loop Never

If the Transmitter sends a Preamble, the TxPreL and

TxPrePat fields of the Channel Control Register (CCR11-

10 and CCR9-8) control its length and content:

TxPreL Length of Preamble Sent

00 8 bits

01 16 bits

10 32 bits

11 64 bits

TxPrePat Preamble Pattern Sent

00 All zeroes

01 All ones, or Flags

10 101010...

11 010101...

For HDLC/SDLC mode, if TxPrePat is 01 and the

FlagPreamble bit in the Channel Control Register (CCR12,

see Figure 5-17) is 1, a USC manufactured after June 1993

sends 1, 2, 3, 4, or 8 Flags as the Preamble. Including the

opening and closing Flags, this guarantees a minimum of

3, 4, 6, or 10 Flags between frames respectively. This is

useful when sending to certain kinds of equipment that

can’t handle less Flags, or as a means of slowing the gross

frame rate slightly, perhaps as a “congestion manage-

ment” measure.

FlagPreamble should be 0 in all other modes. For 802.3

(Ethernet) mode, program TxPreL=11 and TxPrePat=10;

the Transmitter automatically modifies the last (64th) bit

from a 0 to a 1 to act as the “start bit”. For other modes,

consider the sections of Chapter 4 that deal with data

encoding and the DPLL, and whatever standards or speci-

fications apply to your application, in deciding whether to

use a preamble and if so what kind.

After sending the Preamble, or when the conditions for

starting a frame have been met if there is no Preamble,

except in 802.3 (Ethernet) mode the Transmitter sends an

opening Flag or Sync sequence. In the two Bisync modes

this may differ from the closing sequence:

TxMode Opening sequence

Monosync (TSR15-8)

Slaved Monosync (TSR15-8)

Bisync (TSR7-0)(TSR15-8)

Transparent Bisync DLE-SYN

(ASCII or EBCDIC per CMR12)

802.3 (Ethernet) None

HDLC/SDLC Flag (01111110)

HDLC/SDLC Loop Flag (01111110)

In the HDLC/SDLC and HDLC/SDLC Loop modes only, the

Transmitter will combine the closing and opening Flags

into a single instance if software has not selected sending

a Preamble (CMR13=0; this doesn’t apply in Loop mode),

and the conditions for starting a frame (described earlier in

this section) are met as the Flag is going out.

As described in the earlier section 'Status Reporting',

software can use four of the bits in the Transmit Command/

Status Register (TCSR) to track the progress of the Trans-

mitter through these inter-frame activities. They occur in

the time order CRCSent, then EOF/EOM Sent, IdleSent,

and finally PreSent. Chapter 7 describes how software can

enable any or all of these conditions to cause an interrupt.

5.24.2 Async Transmission

As described in the previous section, the TxIdle field of the

Transmit Command/Status Register (TCSR10-8) controls

what kind of idle line condition the Transmitter sends

between characters (or words) in asynchronous modes.

The bits in the Channel Command Register that define the

Preamble in sync modes (CCR11-8) can be used in Async

mode to “shave” the length of transmitted Stop bits.

UM009402-0201

1 2 ... 112 113 114 115 116 117 118 119 120 121 122 ... 207 208

Comentarios a estos manuales

Sin comentarios

Zilog Z16C30 Manual de usuario Pagina 117

Comentarios a estos manuales

Relacionado con productos y manuales para Sensores Zilog Z16C30