SA9401
EPROM Interface
The EPROM Interface controls all aspects of the transfer of program and tone data to
the SA9401. This module comprises seven distinct functions.
The
Program Select Register
reflects the settings on pins PS2 - PS0 which are latched
into the register at the start of every frame. The mapping of the pin settings to the
selected program is by a simple binary code (PS2 = msb).
The
Cycle Register
is loaded with a 9-bit value (CYCLE) indicating the number of
samples in a Tone Block (or basic waveform). It is assumed that all basic waveforms in
a single Tone Plan will have a common size.
Successive samples in a Tone Block are addressed by the 9-bit
Sample Counter
which
cycles through from zero to Cycle-1. The current value of this register and the contents
of the Cycle Register are fed to a
Comparator
which resets the counter after it reaches
the terminal value of Cycle-1.
The required Tone Block (or waveform) is addressed by the
Tone Block Register.
The
content of the register is a function of the Program Select and Cadence Controller.
As previously described, Tone Plans are defined in terms of a basic waveform (Tone
Block) and a Program of descriptive parameters which together are stored in an exter-
nal EPROM. Consequently a Data Fetch from the EPROM may retrieve either a Pro-
gram Instruction or a Tone Sample. In either case the
Address Multiplexer
constructs
the address according to the following table.
ADDRESS LINE
A13
A12
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
Program Instruction Fetch
1
0
0
0
PSEL2
PSEL1
PSEL0
PC6
PC5
PC4
PC3
PC2
PC1
PC0
Tone Sample Fetch
0
TONE_BLOCK.3
TONE_BLOCK.2
TONE_BLOCK.1
TONE_BLOCK.0
SAMPLE_COUNT.8
SAMPLE_COUNT.7
SAMPLE_COUNT.6
SAMPLE_COUNT.5
SAMPLE_COUNT.4
SAMPLE_COUNT.3
SAMPLE_COUNT.2
SAMPLE_COUNT.1
SAMPLE_COUNT.0
The details of the Address mapping will be described later in the
External Data Stor-
age
section
6
sames
SAMES [ SAMES ]
