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Z8530 Datasheet(PDF) 83 Page - Zilog, Inc. |
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Z8530 Datasheet(HTML) 83 Page - Zilog, Inc. |
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83 / 317 page  SCC™/ESCC™ User’s Manual Data Communication Modes 4-8 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. 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 Figure 4-4. Monosync Data Character Format Modem Clock Bit Bit State Data LSB Sync Character Data Character 1 Bit Time 1 2 3 4 5 6 7 8 9 10 11 12 13 . . . 0 1 1 0 1 0 0 0 1 1 0 1 0 1 0 1 UM010901-0601 |
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