CHAPTER 5. COUNTER/TIMERS
The ZT 8905 includes one Intel-compatible 8254 device with a total of three programmable
counter/timers. The counter/timers are useful for software timing loops, timed interrupts, and periodic
interrupts. The major features of the counter/timers are listed below.
•
Three 16-bit counter/timers
•
Six programmable operating modes
•
Binary and BCD counting
•
Interrupt and polled operation
The counter/timer architecture is illustrated in the "Counter/Timer Architecture" figure below. In some
cases, not all counter/timers are available for application development. In an MS DOS system, for
example, counter/timer 0 generates a periodic system interrupt and should not be programmed by
the application. Please refer to the selected operating system manual for more information.
The six programmable operating modes are summarized in the "Counter/Timer Operating Modes"
table on the following page.
TIMER 0
GATE0
CLK0
OUT0
INTERRUPT IR0
SYSTEM TIMER
TICK
1.19318 MHz
LOGICAL ONE
TIMER 1
GATE1
CLK1
OUT1
REFRESH
COUNTER
1.19318 MHz
LOGICAL ONE
TIMER 2
GATE2
CLK2
OUT2
SPEAKER
FREQUENCY
1.19318 MHz
LOGICAL ONE
Counter/Timer Architecture
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