Tandy 1000 HX Technical Reference Manual page 37

. TANDY COMPUTER PRODUCTS
Timer Function
The Timer is an 8253 Timer/Counter consisting of three
independent counters. The clock for all three counters is
1.19 25 MHz. The gates for counters #0 and #1 are
permanently "on". The gate for counter #2 is controlled by
a bit of the keyboard interface (8255 Port B). The output
of counter #0 is dedicated to system interrupt #0 (8259 IRO)
for software timing functions. The output of counter #1 is
dedicated to the REFRESH function. When the optional
DMA/Memory board is installed, DMA channel #0 is used for
refreshing the RAM memory. Counter #1 sets RFSHRQ* (DRQO)
every 15 micro-seconds to initiate a single "dummy" memory
read. The output of counter #2 is routed to the sound
circuit and into the 8255 Port C for monitoring by the CPU.
See Figure 10.
Sound Function
The sound function consists of an internal and an external
sound circuit. These are directly connected to the
Headphone Output via U5. The source of the sound
frequencies is U19, a Complex Sound Generator. Internally,
U19 has three programmable tone generators and a noise
generator. The frequency and output level of each is
controlled by software. The four internal generators are
summed with an external input into a single output. The
external source is from the 8253 counter #2 (programmable
frequency and fixed amplitude). It is one of two selectable
sources for the external audio out signal. This signal is
intended as an input into an external earplug. The two
sound frequency sources are:
1. Complex sound generator U19.
2. The 8253 counter at channel 2.
The output driver for Audio Out is U5. See Figure 11.
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