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System Timers
Three programmable timer/counters are used by the system as
follows: Channel 0 is used as a general-purpose timer providing a
constant time base for implementing a time-of-day clock.
Channel 0
GATE 0
CLKINO
CLKOUTO
System Timer
Tied on
1.193182 MHz OSC
8259A IRQ 0
Channel 1 is used to time and request refresh cycles from the
DMA channel.
Channell
GATE 1
CLKIN 1
CLKOUT 1
Refresh Request Generator
Tied on
1.193182 MHz OSC
Request refresh cycle
Note:
Channel 1 is programmed as a rate generator to
produce a 15-microsecond period signal.
Channel 2 is used to support the tone generation for the audio
speaker. Each channel has a minimum timing resolution of
1.05p-s.
Channel 2
GATE 2
CLKIN2
CLKOUT2
Tone Generation for Speaker
Controlled by bit 0 of port hex 61, PPI bit
1.193182 MHz OSC
Used to drive the speaker
The 8254-2 Timer/Counter is a programmable interval
timer/ counter that system programs treat as an arrangement of
four external I/O ports. Three ports are treated as counters; the
fourth is a control register for mode programming. The following
is a system-timer block diagram.
1-10
System Board
- Quick Links:
- Section 1. System Board
- 64/256K System Board
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