IBM Series 1 User Manual page 108

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INTRODUCTION
The timer feature provides two 16-bit timers (Figure 3-1).
Each timer can be used as an interval timer, pulse counter,
or pulse duration counter with end interrupt. The timers
are packaged on one printed circuit card that plugs into a
backpanel that distributes the processor I/O channel signal
lines. Electrical power for the timer circuitry is obtained
from
the
backpanel through card-connector pins.
The timer feature also has a 16-pin connector that
allows the timers to be used with external signals. Each
timer has
two
output lines "run state and "external gate
enable" and two input lines "customer clock" and "external
gate". Drivers and receivers for these lines are TTL
compatible.
The two timers are separately
started, stopped, read, or set to any
under program control. The timers
disturbing their operation; however,
value or mode, it must
be
stopped.
addressable and are
value independently
can be read without
to set the timer's
Each timer also has a mode register that is used to
select one of four internal time bases or
an
external time
base. The
time~
value is decremented with the selected time
base. The internal time bases are 1,
5, 25,
and
50
microseconds. The external time base is provided by the
user and must be equal to or greater than 20 microseconds
when the input is filtered, 1 microsecond when not filtered.
For additional information,
see "Receivers" in the "Tiaer
Feature Electrical Characteristics" section of this chapter.
An
external gate enable bit is also contained in the mode
register. The time base and external gate enable are
program selectable.
Four program selectable running modes are available for
each timer:
• Periodic interrupts--internal
•
A
16-bit auto-load register is set to any value
by
program control. This register automatically reloads
the timer when the timer underflows, and an interrupt
is yenerated. This provides the capability of
generating periodic interrupts on
65,536
possible base
values of the timer without program intervention.
Aperiodic interrupts--internal
The timer is loaded with a value under program control,
and an interrupt occurs when the timer underflows.
After the first
int~rrupt,
the timer is .!lot reloaded
from the auto-load register and therefore counts the
full 65,536
counts before the next interrupt occurs
unless a new value is loaded under program control.
Timer Feature 3-1