AM85C30 datasheet(63/194 Pages) AMD

63 / 194 page

Data Communication Modes Functional Description

AMD

4–11

The character length may be changed at any time, but the desired length must be se-

lected before the character in the transmit buffer is transferred to the the Transmit Shift

before writing the data to the transmit buffer.

4.5.2

Tx Parity

In all modes of operation bit D0 (Parity Enable) of WR4 determines whether an additional

bit will be appended to each character sent as an indication of the “oddness” or “even-

ness” of the number of ‘1’ bits transmitted in the character. If this bit is set to ‘1’ an addi-

tional bit will be sent in addition to the number of bits specified in WR4, or by the data for-

mat used when transmitting less than five bits per character.

Bit D1 of WR4 determines the even/odd sense of this additional bit when Parity is en-

abled. If this bit is set to ‘1’, the transmitter adds a bit that makes the total number of ‘1’

bits in the character being transmitted even; if set to ‘0’, a bit will be added to make the

sum of ‘1’ bits in the character being transmitted odd.

4.5.3

Break Generation

The transmitter may be programmed to send a break condition (i.e., the TxD pin is pulled

Low) in all modes of operation via bit D4 of WR5. When this bit is set to ‘1’, the transmit-

ter suspends any data being transmitted at the time and sends continuous ‘0’s from the

first transmit clock edge after this command is issued, until the first transmit clock edge

after this bit is reset, at which point the transmitter continues to send the contents of the

Transmit Shift Register. The transmit clock edges referred to here are those that define

transmitted bit cell boundaries. Note that the TxD pin will be pulled Low whether or not

the transmitter is enabled.

4.5.4

Transmit Modem Control

There are two modem control signals associated with the transmitter on the SCC. The

RTS pin is a general-purpose output that carries the inverted state of the RTS bit in WR5

(D1), and the

CTS pin is a general-purpose input to the CTS status bit in RR0 (D5). How-

ever, if the Auto Enables Mode is selected (by setting D5 of WR3 to ‘1’), CTS becomes

an enable for the transmitter. That is, if Auto Enables is on and the

CTS pin is HIGH the

transmitter will be disabled; while the

CTS pin is LOW the transmitter will be enabled.

Note, however, that in all modes of operation, the Transmitter Enable bit must be set be-

fore the

CTS pin can be used in this manner.

If the SCC channel is programmed in Asynchronous mode, and the Auto Enable bit is set

to ‘1’, RTS will remain Low until the transmitter is completely empty and the last stop bit

has left the TxD pin. In SDLC and Synchronous modes, the

RTS pin is just a general-

purpose output.

4.5.5

Auto

RTS Reset

On the CMOS SCC, if bits D0 of WR15 and D2 of WR7’ are set to ‘1’ and the channel is

in SDLC Mode, the

RTS pin may be reset early in the Tx Underrun routine and the RTS

pin will remain active until the last zero bit of the closing flag leaves the TxD pin as shown

in Figure 4–10.

Note that in order for this to function properly, bits D3 and D2 of WR10 must be set to ‘1’

and ‘0’, respectively.

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