Zilog Z16C35 User Manual
Page 69
ISCC
User Manual
UM011002-0808
63
Figure 4–23. Changing Character Length
Either of two CRC polynomials may be used in synchronous modes, selected by bit D2 in
WR5. If this bit is set to “1”, the CRC-16 polynomial is used, if this bit is set to “0”, the
CRC-CCITT polynomial is used. This bit controls the polynomial selection for both the
receiver and transmitter.
The initial state of the generator and checker is controlled by bit D7 of WR10. When this
bit is set to “1”, both the generator and checker will have an initial value of all ones, if this
bit is set to “0”, the initial values will be all “0s”. The ISCC presets the checker whenever
the receiver is in Hunt mode so a CRC reset command is not strictly necessary. However,
there is a Reset CRC Checker command in WR0. This command is encoded in bits D7 and
D6 of WR0. If CRC is to be used the CRC checker must be enabled by setting bit D0 of
WR3 to “1”.
If sync characters are being stripped from the data stream, this may be done at any time
before the first non-sync character is received. If the sync strip feature is not being used,
CRC must not be enabled until after the first data character has been transferred to the
6
5
4
3
2
1
Receive Data Buffer
7
8
11 10
9
8
7
6
12
13
19 18 17 16 15 14
20
21
27 26 25 24 23 22
28
29
32 31 30 29 28 27
33
34
37 36 35 34 33 32
38
39
Time
Change from Five to Eight
Change from Eight to Five
5 Bits
5 Bits
5 Bits
8 Bits
8 Bits
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