ÛZIATECH
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5. COUNTER/TIMERS
The ZT 8907 includes two Intel-compatible 8254 devices for a total of six programmable
counter/timers. The counter/timers are useful for generating timing loops, timed and
periodic interrupts, and for counting external asynchronous events. The major features
of the counter/timers are listed below.
•
Six 16-bit counter/timers
•
Six programmable operating modes
•
Binary and BCD counting
•
Interrupt and polled operation
The six programmable operating modes are summarized in the "
" table below.
Three of the counter/timers are PC-compatible and are
dedicated
for use by the system
software. The architecture for these is illustrated in the "
" figure on the following page.
The other three counter/timers are auxiliary counter/timers
available to the user
for use
by the application. The architecture for these is illustrated in the "
Counter/Timer (CTC 2) Architecture
" figure on the following page. The clock source for
these counter/timers is selectable between the backplane clock, a local 8 MHz oscillator
(default), or frontplane connector J2 by configuring cuttable traces CT4-CT6. If a
frontplane clock is used, it must meet the following specifications:
•
Operating frequency between DC and 10 MHz
•
Rise and fall times of less than 25 ns
•
Clock low and clock high times of greater than 50 ns
