AM8530 datasheet(85/194 Pages) AMD

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Data Communication Modes Functional Description

AMD

4–33

via bit D4 in WR3. The Enter Hunt Mode bit in WR3 is a command so writing a ‘0’ to it has

no effect.

In Synchronous modes, once character synchronization has been established, Hunt

mode is terminated and must remain so until the end of message has been received. At

this point, the Enter Hunt Mode command can be re-issued for the next message. Issuing

this command prematurely can lead to false character synchronization. Thus, the SYNC/

HUNT status bit in RR0 will be set only when the Enter Hunt Mode command is issued.

The Hunt status of the Receiver is reported in the SYNC/HUNT status bit in RR0 (D4).

This status bit is one of the possible sources of External/Status interrupts, with both tran-

sitions causing an interrupt. This is true even if the SYNC/HUNT bit is set as a result of

the processor issuing the Enter Hunt Mode command.

While in Hunt mode, the receiver path used in establishing character synchronization will

depend on the mode selected. In either case, however, synchronization will be estab-

lished at the beginning of each transmission either through a two character (BISYNC) or a

single character (MONOSYNC) synchronizing pattern. When character synchronization is

established Hunt mode is terminated and the receiver stops scanning the communication

line for the synchronizing pattern. At this point data passes to the Receive Shift Register

and characters are formed by assembling the proper number of consecutive bits following

the synchronizing pattern before being transferred into the Receive Data FIFO.

4.10.1.1

SYNC Detect Output

In Synchronous modes, except External SYNC mode, if bit D7 of WR11 is set to ‘0’, the

SYNC pin will be configured as an output and the SCC will drive it Low every time a sync

character is detected in the data stream. Note, however, that the SYNC pin is activated

regardless of character boundaries so any external circuitry using it in Synchronous

modes should respond only to the SYNC pulse that occurs while the receiver is in Hunt

mode. The timing for the SYNC signal is shown in Figure 4–20.

4.10.1.1.1

MONOSYNC Mode

The message format for MONOSYNC is shown in Figure 4–21. In this mode, the incom-

ing data are clocked into the Receive Sync Register and compared with the contents of

WR7 on a bit-by-bit basis until a sync character is found. When a sync character is found,

character synchronization is established and data passes to the Receive Shift Register.

In this mode, WR6 is always used to open a message being transmitted, and as time fill

when the transmitter has nothing to send.

4.10.1.1.2

BISYNC Mode

The BISYNC message format is shown in Figure 4–22. In this mode, the synchronization

procedure is similar to that of MONOSYNC except that two sync characters are used for

character synchronization instead of one. In this mode, incoming data are shifted into the

Receive Shift Register while the next eight bits are assembled in the Receive Sync Regis-

ter. If these two characters match the programmed characters in WR6 and WR7, respec-

tively, synchronization is established and the incoming data bypasses the Receive Sync

In this mode, the concatenation of WR6 with WR7 is always used during transmit and re-

ceive operations.

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