Zilog Z16C30 Manual de usuario descargar pdf (Pagina 92)

5-25

Z16C30 USC

USER'S MANUAL

ZILOG

UM97USC0100

5.17 PARITY CHECKING

A USC channel can handle a Parity bit in each character in

either asynchronous or synchronous modes, although

many synchronous protocols use CRC checking only.

If the TxParEnab bit in the Transmit Mode Register (TMR5)

is 1, the Transmitter creates a parity bit as specified by the

TxParType field (TMR7-6) and sends it with each charac-

ter. Similarly, if the RxParEnab bit (RMR5) is 1, the Re-

ceiver checks a parity bit in each received character,

according to the RxParType field (RMR7-6). A channel

interprets TxParType and RxParType as follows:

xMR7-6 Type of Parity

00 Even

01 Odd

10 Zero

11 One

For unencoded data, 10/Zero is the same as “Space

parity” and 11/One is the same as “Mark parity”.

TxParEnab and TxParType are “global states” in that a

channel doesn’t carry these bits through the TxFIFO with

each character.

In asynchronous modes, the Transmitter and Receiver

handle the parity bit as an additional bit after the number

of bits specified by the TxLength and RxLength fields

(TMR4-2 and RMR4-2). In synchronous modes they handle

the parity bit as the last (most significant) bit of that

number. The Receiver includes a parity bit in the data

characters in the RxFIFO and Receive Data Register

(RDR), except in asynchronous modes with 8-bit data.

In HDLC/SDLC protocols, a USC Receiver can queue

either a Parity Error or an Abort indication through the

RxFIFO, but not both. Regardless of the protocol, in order

to have the Receiver check parity, the QAbort bit in the

Receive Mode Register (RMR8) must be 0.

If QAbort is 0, RxParEnab is 1, and the Receiver finds that

the parity bit of a received character is not as specified by

RxParType, it sets a Parity Error bit. This bit accompanies

the character through the RxFIFO, eventually appearing

as the Abort/PE bit in the Receive Command/Status Reg-

ister (RCSR2). The Abort/PE bit can represent a latched

interrupt bit, or the status at the time that an RxBound

character was read from the RxFIFO, or the status of the

oldest 1 or 2 characters that are still in the RxFIFO, as

described in the next section.

UM009402-0201

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