Z85233 Datasheet, PDF (99/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

SCC™/ESCC™ User’s Manual
Data Communication Modes
4.3 BYTE-ORIENTED SYNCHRONOUS MODE (Continued)
Time
Change from Five to Eight
Receive Data Buffer
8 7 6 5 4 3 2 1 ^{5 Bits}
13 12 11 10 9 8 7 6 ^{8 Bits}
21 20 19 18 17 16 15 14 ^{8 Bits}
Change from Eight to Five
29 28 27 26 25 24 23 22 ^{5 Bits}
34 33 32 31 30 29 28 27 ^{5 Bits}
39 38 37 36 35 34 33 32
Figure 4-13. Changing Character Length
Most bit-oriented protocols allow an arbitrary number of
bits between opening and closing flags. The SCC allows
for this by providing three bits of Residue Code in RR1.
These indicate which bits in the last three bytes transferred
from the receive data FIFO by the processor are actually
valid data bits (and not part of the frame check sequence
or CRC). Table 4-10 gives the meanings of the different
codes for the four different character length options. The
valid data bits are right-justified, meaning, if the number of
valid bits given by the table is less than the character
length, then the bits that are valid are the right-most or
least significant bits. It should also be noted that the Resi-
due Code is only valid at the time when the End of Frame
bit in RR1 is set to 1.
Residue Code
210
100
010
110
001
101
011
111
000
Table 4-10. Residue Codes
Bits in
Previous Byte
8B/C 7B/C 6B/C 5B/C
0000
0000
0000
0000
0000
000
10
2
Bits in Second
Previous Byte
8B/C 7B/C 6B/C 5B/C
3100
4200
5310
6420
7531
864
87
8
Bits in Third
Previous Byte
8B/C 7B/C 6B/C 5B/C
8752
8763
8764
8765
8765
876
87
8
As indicated in the table, these bits allow the processor to
determine those bits in the information (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 is updated before they are read by the processor.
4-24
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