Z80 datasheet(287/330 Pages) ZILOG

287 / 330 page

Z80 CPU Peripherals

User Manual

UM008101-0601

Serial Input/Output

267

Special Receive Condition Interrupts

The Special Receive Condition interrupt is not, as such, a separate interrupt

mode. Before the Special Receive condition can cause an interrupt, either

Interrupt On First Receive Character only or Interrupt On Every Character

must be selected. The Special Receive Condition interrupt is caused by a

Receive Overrun or End-of-Frame detection. When the Receive Overrun

status bit is latched, the error status read indicates that an error occurred in

the current word in the receive buffer in addition to any errors received after

the last Error Reset command. The Receive overrun status bit can only be

reset by the Error Reset command. The End-of-Frame status bit indicates

that a valid ending flag has been received and that the CRC Error and

Residue codes are also valid.

Character length may be changed on-the-fly. If the address and control

bytes are processed as 8-bit characters, the receiver may be switched to a

shorter character length while the first information character is being

assembled. This change must be made fast enough so that it is effective

before the number of bits specified for the character length have been

assembled. For example, if the change is to be from the 8-bit control field to

a 7-bit information field, the change must be made before the first seven

bits of the I-Field are assembled.

SDLC Receive CRC Checking

Control of the receive CRC checker is automatic. It is reset by the leading

flag and CRC is calculated up to the final flag. The byte that has the End-of-

Frame bit set is the byte that contains the result of the CRC check. If the

CRC/Framing Error bit is not set, the CRC indicates a valid message. A

special check sequence is used for the SDLC check because the transmitted

CRC check is inverted. The final check must be 0001 1101 0000 1111.

The 2-byte CRC check characters must be read by the CPU and discarded

because the Z80 SIO, while using them for CRC checking, treats them as

ordinary data.

Z80 Datasheet(PDF) 287 Page - Zilog, Inc.