Functional Descriptions; Control Registers; Tone Generation - Tandy 1000 HX Technical Reference Manual

SECTION 2
FUNCTIONAL DESCRIPTIONS
2.1 Control Registers
The NCR 8496 Sound Generator has eight (8) internal
registers used to control three (3) tone generators and one
(1) noise generator. A three (3) bit data word used to
determine the destination control register is contained in
the first byte of data for all data transfers. The internal
register designations are as follows:
Address Bits
RO
0
0
0
0
1
1
1
1
Rl
0
0
1
1
0
0
1
1
R2
0
1
0
1
0
1
0
1
Register Destination
Description
Tone 1: Frequency
Tone 1: Attenuation
Tone 2: Frequency
Tone 2: Attenuation
Tone 3: Frequency
Tone 3: Attenuation
Noise : Control
Noise : Attenuation
Note: RO is the most significant address bit
2.2 Tone Generation
The NCR 8496 sound generator has three (3) programmable tone
generators, each with separate frequency synthesis and
attenuation sections. The frequency synthesis section
requires ten (10) bits of data (F0 to F9) to define half the
period of the desired frequency. This data is entered into
a ten (10) stage tone counter, which is decremented at a
rate of N/16 where N is the clock input frequency. A signal
is produced when this tone counter decrements to one, which
toggles a divide by two counter and reloads the tone
counter. Therefore, the period of the desired frequency is
twice the value of the tone generator.