Zenith Z-100 PC series Service Manual page 97

Page
4-11
Detailed Circuit Description
Programmable Interval Timer
The programmable interval timer (U240) contains three independent
programmable counters, which are all connected to the TCLK input
from the gate array. These counters are used to generate output which
may be processed as system interrupts or may cause specific functions to
occur.
For example, counter 0 causes the microprocessor to issue a memory
refresh signal, counter I is used to update the real-time clock, and
counter 2 provides frequencies for the speaker. Figure 4-6 identifies the
pinouts on the programmable interval timer.
The programmable interval timer can operate in any of several modes, as
determined by a control word. These modes are identified in Table 4-4.
Table 4-5 identifies the programming codes for this device.
Table 4-4: Programmable Interval Timer Operating Modes
MODE
NUMBER DESCRIPTION
o
A terminal value is loaded into the selected counter, which
then begins counting from O. Each time the terminal value is
reached, a logic high is sent out at the appropriate output.
This signal remains until the terminal value is re-Ioaded.
1 Same as mode 0 except that the high output is sent only for a
fixed time and then returns to low.
2 The incoming clock signal is divided by a predetermined
value to generate output pulses.
3 Same as mode 2 except that the outgoing pulse remains high
until the next count reaches 1/2 of the starting count.
4
Similar to mode 2 except that the output starts high and goes
low when the terminal count is reached.
5 A hardware-triggered strobe; the count begins when a
low-to-high transition occurs at one of the gate inputs.