ZiLOG Z8 Technical Manual

R247 P3M
Port 3 Mode Register
(% F7; Write Only)
o PARITY OFF
1 PARITY ON
Figure 12-9. Parity and Port J Hode Register
TO's output drives a divide-by-16 counter which in
turn generates a shift clock every 16 counts.
This counter is reset when the transmitter buffer
is written by an instruction. This reset
synchronizes the shift clock to the software. The
transmitter then outputs one bit per shift clock,
through Port 3 pin P3
7
, until a start bit, the
character written to the buffer, and two stop bits
have been transmitted. After the second stop bit
has been transmitted, the output is again forced
to a marking state. Interrupt request IRQ4 is
Transmitted Data
(No Parity)
Transmitted Data
(With Parity)
Serial I/O
generated and this notifies the processor that the
transmitter is ready to accept another character.
12.4.1 Overwrites
The user is not protected from overwr iting the
transmitter, so it is up to the software to
respond to IRQ4 appropriately. I f polling is
used, the IRQ4 bit in the Interrupt Request regis-
ter must be reset.
12.4.2 Parity
The data format supported by the transmitter has a
start bit, eight data bits, and at least two stop
bits. If parity is on, bit D7 of the data trans-
mitted will be replaced by an odd parity bit.
Figure 12-10 shows the transmitter data formats.
Parity is enabled by setting Port 3 Mode register
bit D7 to 1. If even parity is required, the
parity mode should be disabled (i.e. P3M D7 set to
0), and software must modify the data to include
even parity.
Since the transmitter can be overwritten, the user
is able to generate a break signal. This is done
by writing null characters to the transmitter buf-
fer (510, %FO) at a rate which does not allow the
stop bits to be output. Each time the 510 is
loaded, the divide-by-16 counter is re-synchro-
nized and a new start bit is output followed by
data.
L'START BIT
'--------EIGHT DATA BITS
TWO
STOP BITS
T
I
LSTART BIT
L..
___________ SEVEN DATA BITS
. ODD PARITY
TWO STOP BITS
Figure 12-10. Transmitter Data Formats
3047-028,2037-009
12-5

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English