Texas Instruments 99/4A - Page 82

SOUND GENERATION CONTROLLER
GENERAL DESCRIPTION
Tht± THS vSlv Sound bensrat i on Controller (SGC) is an I L coT ,n on^n
designed to provide low cost tone/noise generation capability i
microprocessor systems. The SGC is a data bus based I/O
peri pheral .
KEY FEATURES
- 3 programmable tone generators
# Programmable white noise generator
# Programmable attenuation
# 8 ohm speaker drive capability
# External audio input
$ 16 RIN package
# 1 L technology
DEVICE ARCHITECTURE
The device consists of three programmable tone generators, a pro 
grammable noise generator, a clock scaler, and an audio summer
output buffer. The SGC has a parallel 8 bit inter-Face through
which the microprocessor transfers the data which controls the
audio output.
TONE GENERATORS
Each tone generator consists of a frequency synthesis section and
an attenuation section. The frequency synthesis section requires
10 bits of information (F0-F9) to define half the period of the
desired frequency (n). This information is loaded into a 10 stage
tone counter, which is decremented at a N/16 rate where N is the
input clock frequency. When the tone counter decrements to zero,
a borrow signal is produced. This borrow signal toggles the
frequency flip-flop and also reloads the tone counter. Thus, the
period of the desired frequency is twice the value of the period
register. The frequency can be calculated by the following:
N
f = ---------
32n
The output of the frequency flip-flop will feed into a four state
attenuator. The attenuator values, along with their bit position
in the data word, are shown in table. 14 multiple attenuation
control bits may be true simultaneously. Thus, the maximum
attenuation is 30 db.
TABLE 14: ATTENUATION CONTROL
BIT POSITION
A u Al A 2 A3
0 0 0 1
Q Q 1 Q
0 1 0 0
1 0 Q 0
1111
73
WEIGHT
2 DB
4 DB
8 DB
16 DB
OFF