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SA9401

6

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

Program Instruction Fetch

Tone Sample Fetch

A13

1

0

A12

0

TONE_BLOCK.3

A11

0

TONE_BLOCK.2

A10

0

TONE_BLOCK.1

A9

PSEL2

TONE_BLOCK.0

A8

PSEL1

SAMPLE_COUNT.8

A7

PSEL0

SAMPLE_COUNT.7

A6

PC6

SAMPLE_COUNT.6

A5

PC5

SAMPLE_COUNT.5

A4

PC4

SAMPLE_COUNT.4

A3

PC3

SAMPLE_COUNT.3

A2

PC2

SAMPLE_COUNT.2

A1

PC1

SAMPLE_COUNT.1

A0

PC0

SAMPLE_COUNT.0

The details of the Address mapping will be described later in the External Data Stor-

age section