Z85233 Datasheet, PDF (83/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

SCC™/ESCC™ User’s Manual
Data Communication Modes
4.3 BYTE-ORIENTED SYNCHRONOUS MODE
The SCC supports three byte-oriented synchronous proto-
cols. They are: monosynchronous, bisynchronous, and ex-
ternal synchronous.
In synchronous communications, the bit cell boundaries
are referenced to a clock signal common to both the trans-
mitter and receiver. Consequently, they operate in a fixed-
phase relationship. This eliminates the need for the receiv-
er to locate the bit cell boundaries with a clock 16, 32, or
64 times the receive data rate, allowing for higher speed
communication links. Some applications may encode (i.e.,
NRZI or FM coding) the clock information on the same line
as the data. Therefore, these applications require that the
receiver use a high speed clock to find the bit cell bound-
aries (decoding is typically done with the PLL—Phase-
Locked Loop; the SCC has on-chip Digital PLL). Data en-
coding eliminates the need to transmit the synchronous
clock on a separate wire from the data.
Synchronous data does not use start and stop bits to de-
lineate the boundaries for each character. This eliminates
the overhead associated with every character and increas-
es the line efficiency. Because of the phase relationship of
synchronous data to a clock, data is transferred in blocks
with no gaps between characters. This requires that there
be an agreement as to the location of the character
boundaries so that the characters can be properly
framed. This is normally accomplished by defining spe-
cial synchronization patterns, or Sync characters. The
synchronization pattern serves as a reference; it signals
the receiver that a character boundary occurs immediate-
ly after the last bit of the pattern. For example Monosync
Protocol usually uses 16 Hex as this special character,
and the SDLC protocol uses 0, six 1s, followed by a 0 (7E
Hex; usually referred to as Flag Pattern) to mark the be-
ginning and end of a block of data. Another way of iden-
tifying the character boundaries (i.e., achieving synchro-
nization) is with a logic signal that goes active just as the
first character is about to enter the receiver. This method
is referred to as External Synchronization.
Figure 4-4 shows the character format for synchronous
transmission. For example, bits 1-8 might be one charac-
ter and bits 9-13 part of another character; or, bit 1 might
be part of a second character, and bits 10-13 part of a third
character. This is accomplished by defining a synchroniza-
tion character, commonly called a Sync Character.
Modem Clock
Bit
Bit State
1 Bit Time
1 2 3 4 5 6 7 8 9 10 11 12 13 . . .
0 110 10 00 11 01 01 01
Data LSB
Sync Character
Data Character
Figure 4-4. Monosync Data Character Format
4.3.1 Byte-Oriented Synchronous Transmit
Once Synchronous mode has been selected, any of three
of the following sync character lengths may be selected:
s 6-bit
s 8-bit
s 16-bit
The 6-bit option sync character is selected by setting bits
4 and 5 of WR4 to zeros and bit 0 of WR10 to one. Only
the least significant six bits of WR6 are transmitted.
The 8-bit sync character is selected by setting bits 4 and 5
of WR4 to zeros and bit 0 of WR10 to zeros. With this op-
tion selected, the transmitter sends the contents of WR6
when it has no data to send.
For a 16-bit sync character, set bit D4 of WR4 to 1 and bit
D5 of WR4 and bit D0 of WR10 to 0. In this mode, the
transmitter sends the concatenation of WR6 and WR7 for
the idle line condition.
Because the receiver requires that sync characters be left-
justified in the registers, while the transmitter requires
them to be right justified, only the receiver works with a 12-
bit sync character. While the receiver is in External Sync
4-8
UM010901-0601