ACT apricot - Page 61

TIMER
The system software views the three counters and the
control word register set as an array of peripheral input/
output ports. The port address locations defined by the
timer select and system address bus connections are
detailed below:
Port
Address
Data
Counter 0
58H Count state 0
Counter 1
5AH Count state 1
Counter 2
5CH Count state 2
Control word register set 5EH
Control word
Data can be written to all four address locations but can
be read only from the three counter locations. Each counter
has to be initialized with the required mode of operation,
the count state format, and the number of bytes in the count
state, using the control word, prior to loading the count
state. The counters begin counting downwards on
completion of the count state load operation. The count
state can be one or two bytes. The format of the control
word is as follows:
Control Word Format
07
DO
I sCllscoIRLI\RLo\M2\ MI \ MolBCD \
IT
~counter format
L
Mode
Count state byte operation
'------- Control word selection
Control word selection
SCI SC2
0 0
Counter 0
0
I
Counter I
I
0
Counter 2
I
I
Invalid