A/d interface, Timing diagram reading the conversion result, Mode selection – Sensoray 425 User Manual
Page 19: Channel selection, Starting a conversion, Unipolar mode, Bipolar mode, Input data, Start* bsy, Select the conversion mode: unipolar or bipolar
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 9
µ
s 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
10
µ
s,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. 14
µ
s 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.
V
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