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
To Next Page
Loading...
Need help?
General
