Z80 datasheet(315/330 Pages) ZILOG

315 / 330 page

Z80 CPU Peripherals

User Manual

UM008101-0601

Serial Input/Output

295

When the SYNC input goes High again, another External/ Status interrupt

is generated that must also be cleared. The Enter Hunt Mode control bit is

set whenever character synchronization is lost or the end of message is

detected. In this case, the Z80 SIO again looks for a High-to-Low transition

of the SYNC input and the operation repeats as explained previously. This

implies the CPU should also inform the external logic that character

synchronization has been lost and that the Z80 SIO is wailing for SYNC to

become active.

In the Monosync and Bisync Receive modes, the Sync/Hunt status bit is

initially set to 1 by the Enter Hunt Mode bit. The Sync/Hunt bit is reset

when the Z80 SIO establishes character synchronization. The High-to-Low

transition of the Sync/Hunt bit causes an External/Status interrupt that must

be cleared by the CPU issuing the Reset External/ Status Interrupt

command. This enables the Z80 SIO to detect the next transition of other

External/Status bits.

When the CPU detects the end of message or loss of character synchroniza-

tion, it sets the Enter Hunt Mode control bit, which-in turn-sets the Sync/

Hunt bit to 1. The Low-to-High transition of the Sync/Hunt fit sets the

External/Status interrupt, which must also be cleared by the Reset External/

Status Interrupt command.

The SYNC pin functions as an output in this mode and goes Low when a

sync pattern is detected in the data stream.

In the SDLC mode, the Sync/Hunt bit is initially set by the Enter Hunt

mode bit, or when the receiver is disabled. It is reset to 0 when the opening

flag of the first frame is detected by the Z80 SIO. The External/Status inter-

rupt is also generated, and should be handled as discussed previously.

Unlike the Monosync and Bisync modes, when the Sync/Hunt bit is reset in

the SDLC mode, it does not need to be set when the end of message is

detected. The Z80 SIO automatically maintains synchronization. The Sync/

Hunt bit can only be set again by the Enter Hunt Mode bit or by disabling

the receiver.

Z80 Datasheet(PDF) 315 Page - Zilog, Inc.