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Page 17 Sensoray Model 425 Instruction Manual

A/D Interface

This starts a conversion and sets the status register

“busy” flag. When the conversion ends — typically

within 9µs after starting — the busy flag is cleared and

the result may be read.

Timing Diagram

Reading the Conversion Result

Digitized data may be read from the A/D Data Register

any time no conversion is in progress:

The 16-bit output data format is a function of the

digitizer input mode:

Name Signal Description

START* A/D start strobe. Coincident with host CPU

write strobe to Start A/D Conversion port

BSY “A/D busy” flag as viewed by host through

Status port

A/D Data Register (base + 16, read only)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Unipolar Mode

Input Data

Output data is formatted as

unsigned integer with leading

zeros in the most-significant

nibble. Assuming unity gain, the

table to the right illustrates the

relationship between input volt-

age and output codes.

0.000V 0000

2.441mV 0001

... ...

4.998V 07FF

5.000V 0800

5.002V 0801

... ...

+9.997V 0FFF

Bipolar Mode

Input Data

Output data is formatted as

signed integer with sign exten-

sion through the most-signifi-

cant nibble. Assuming unity

gain, the table to the right illus-

trates the relationship between

input voltage and output codes.

-5.000V F800

-4.997V F801

... ...

-2.441mV FFFF

0.000V 0000

+2.441mV 0001

... ...

+4.997V 07FF

START*

BSY

10µs,max

The digitizer consists of an eight-channel differential

analog multiplexer, instrumentation amplifier, and a

12-bit analog-to-digital converter. A/D conversions are

accomplished as follows:

• Select the conversion mode: unipolar or bipolar

• Select the analog input channel to be digitized.

• Start an A/D conversion.

• Wait for the conversion to finish.

• Read the conversion result.

Mode Selection

The digitizer input mode may be configured for either

unipolar or bipolar operation. The input mode is

programmed by writing to the General Control port

with bit 10 set:

Channel Selection

A channel is selected for conversion by writing the

channel number to the A/D Channel Register:

Valid channel numbers range from 0 to 7, inclusive.

Settling time must be allowed between writing to the

Channel Register and starting a conversion. 14µs is

adequate for low impedance analog sources. If you are

digitizing high impedance sources or employing high

gain, longer settling time may be required to meet

specified performance.

Note: it is not necessary to repeatedly write to the A/D

Channel Register if your application dwells on one

channel. No settling time is required in such cases.

Starting a Conversion

Conversions are started by writing to the General

Control port with bits 15 and 12 set:

General Control Port (base + 40, write only) : Set A/D Mode

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

V 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0

V specifies input mode according

to table shown at right. Unipolar

is the default mode after a reset.

Mode

0 Unipolar

1 Bipolar

A/D Channel Register (base + 22, write only)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

0 0 0 0 0 0 0 0 0 0 0 0 0 D2 D1 D0

General Control port (base + 40, write only) : Start Conversion

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 33 34

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