Datasheet

ADS1100
13
SBAS239B
www.ti.com
USING GPIO PORTS FOR I
2
C
Most microcontrollers have programmable input/output pins
that can be set in software to act as inputs or outputs. If an
I
2
C controller is not available, the ADS1100 can be con-
nected to GPIO pins, and the I
2
C bus protocol simulated, or
bit-banged, in software. An example of this for a single
ADS1100 is shown in Figure 6.
Note that no pull-up resistor is shown on the SCL line. In this
simple case, the resistor is not needed; the microcontroller
can simply leave the line on output, and set it to one or zero
as appropriate. It can do this because the ADS1100 never
drives its clock line low. This technique can also be used with
multiple devices, and has the advantage of lower current
consumption due to the absence of a resistive pull-up.
If there are any devices on the bus that may drive their clock
lines low, the above method should not be used; the SCL line
should be high-Z or zero and a pull-up resistor provided as
usual. Note also that this cannot be done on the SDA line in
any case, because the ADS1100 does drive the SDA line low
from time to time, as all I
2
C devices do.
Some microcontrollers have selectable strong pull-up circuits
built in to their GPIO ports. In some cases, these can be
switched on and used in place of an external pull-up resistor.
Weak pull-ups are also provided on some microcontrollers,
but usually these are too weak for I
2
C communication. If
there is any doubt about the matter, test the circuit before
committing it to production.
SINGLE-ENDED INPUTS
Although the ADS1100 has a fully differential input, it can
easily measure single-ended signals. A simple single-ended
connection scheme is shown in Figure 7. The ADS1100 is
configured for single-ended measurement by grounding ei-
ther of its input pins, usually V
IN
–, and applying the input
signal to V
IN
+. The single-ended signal can range from –0.2V
to V
DD
+ 0.3V. The ADS1100 loses no linearity anywhere in
its input range. Negative voltages cannot be applied to this
circuit because the ADS1100 inputs can only accept positive
voltages.
1
2
3
6
5
4
V
IN+
GND
SCL
V
IN–
V
DD
V
DD
SDA
SDA
SCL
ADS1100A0
1
2
3
6
5
4
V
IN+
GND
SCL
V
IN–
V
DD
SDA
ADS1100A1
1
2
3
6
5
4
V
IN+
GND
SCL
V
IN–
V
DD
SDA
ADS1100A2
Microcontroller or
Microprocessor
with I
2
C Port
I
2
C Pull-Up Resistors
1kΩ to 10kΩ (typ.)
NOTE: ADS1100 power
and input connections
omitted for clarity.
FIGURE 5. Connecting Multiple ADS1100s.
1
2
3
6
5
4
V
IN+
GND
SCL
V
IN–
V
DD
SDA
SDA
SCL
V
DD
Microcontroller or
Microprocessor
with I
2
C Port
NOTE: ADS1100 power
and input connections
omitted for clarity.
ADS1100
FIGURE 6. Using GPIO with a Single ADS1100.
1
2
3
6
5
4
V
IN+
GND
SCL
V
IN–
V
DD
0V - V
DD
Single-Ended
Filter Capacitor
33pF to 100pF
(typ.)
Output
Codes
0-32767
SDA
ADS1100
V
DD
FIGURE 7. Measuring Single-Ended Inputs.
Bit-banging I
2
C with GPIO pins can be done by setting the
GPIO line to zero and toggling it between input and output
modes to apply the proper bus states. To drive the line low,
the pin is set to output a zero; to let the line go high, the pin
is set to input. When the pin is set to input, the state of the
pin can be read; if another device is pulling the line low, this
will read as a zero in the port’s input register.
The ADS1100 input range is bipolar differential with respect
to the reference, i.e. ±V
DD
. The single-ended circuit shown in
Figure 7 covers only half the ADS1100 input scale because
it does not produce differentially negative inputs; therefore,
one bit of resolution is lost. The Burr-Brown DRV134 bal-
anced line driver from Texas Instruments can be employed
to regain this bit for single-ended signals.