Zilog Z16C35 Manuel d'utilisateur télécharger pdf (Page 83)

ISCC

User Manual

UM011002-0808

Table 4–20. Residue Codes

As indicated in the table, these bits allow the processor todetermine those bits in the infor-

mation (and not CRC) field. This allows transparent retransmission of the received frame.

The

Residue Code bits do not go through a FIFO so they change in RR1 when the last

character of the frame is loaded into the receive data FIFO. If there are any characters

already in the receive data FIFO the Residue Code will be updated before they are read by

the processor.

As an example of how the codes are interpreted, conside

r the case of ei

ght bits per charac-

ter and a residue code of 101. The number of valid bits for the previous, second previous,

and third previo

us bytes are 0, 7, and 8 respectively. This indicates that the information

field (I-field) boundary falls on the second previous byte as shown in Figure 4-14.

Figure 4–29. Residue Code 101 Interpretation

A frame is terminated by the detection of a closing flag. Upon detection of the flag the fol-

lowing actions take place: the contents of the Receive Shift Register are transferred to the

receive data FIFO,

the Residue Code is latched, the CRC Error bit is latched and the End

of Frame upon reaching the top of the FIFO can cause a special receive condition. The

processor can then read RR1 to determine the result of the CRC calculation as well as the

Residue Code.

Only the CRC-CCITT polynomial may be used for CRC calculation in SDLC mode,

although the generator and checker may be preset to all “1s” or all “0s”. The CRC-CCITT

polynomial is selected by setting

bit D2 of WR5 to “0”, bit D7 of WR10 controls the pre-

Residue Code Bits in Previous Byte Bits in Second Previous Byte Bits in Third Previous Byte

2 1 0 8B/C 7B/C 6B/C 5B/C 8B/C 7B/C 6B/C 5B/C 8B/C 7B/C 6B/C 5B/C

100 0000 3100 8752

010 0000 4200 8763

110 0000 5310 8764

001 0000 6420 8765

101 0000 7531 8765

011 00 0 86 4 87 6

111 10 87 87

000 2 8 8

7 Bits

I-Field CRC Field

Third Previous

Byte

Second Previous

Byte

Previous Byte

Page 77 of 316

1 2 ... 78 79 80 81 82 83 84 85 86 87 88 ... 321 322

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Zilog Z16C35 Manuel d'utilisateur Page 83

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