VDD

GPIO_0

GPIO_1

Microcontroller or

Microprocessor

with GPIO Ports

5

14

1

2

3

4

CH0

CH1

CH2

CH3

CH4

SCL

15

8

7

6

SDA

CH7

CH6

CH5

ADS7828-Q1

ADS7828-Q1

SBAS456B – DECEMBER 2008 – REVISED JANUARY 2016

www.ti.com

8.1.1.2 Using GPIO Ports for Communication

Most microcontrollers have programmable input-output (I/O) pins that can be set in software to act as inputs or

outputs. If an I2C controller is not available, the ADS7828-Q1 device can be connected to GPIO pins and the I2C

bus protocol simulated, or bit-banged, in software. Figure 23 shows an example of this configuration for a single

ADS7828-Q1 device.

Bit-banging the I2C with GPIO pins occurs by setting the GPIO line to 0 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 0; 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 configuration reads as a 0 in the port input register.

Note that no pullup 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 1 or 0 as appropriate. This action is possible

because the ADS7828-Q1 never drives the clock line low. This technique can also be used with multiple devices,

and has the advantage of lower current consumption as a result of the absence of a resistive pullup.

If there are any devices on the bus that may drive the clock lines low, this method should not be used; the SCL

line should be high-Z or 0 and a pullup resistor provided as usual.

Some microcontrollers have selectable strong pullup circuits built into the GPIO ports. In some cases, these

circuits can be switched on and used in place of an external pullup resistor. Weak pullups are also provided on

some microcontrollers, but usually these are too weak for I2C communication. If there is any doubt about the

matter, test the circuit before committing it to production.

ADS7828-Q1 power and input connections omitted for clarity.

Figure 23. Using GPIO with a Single ADS7828-Q1

8.1.1.3 Single-Ended Inputs

Although the ADS7828-Q1 device has four differential inputs, the device can easily measure eight single-ended

signals. Figure 24 shows a single-ended connection scheme. The ADS7828-Q1 device is configured for single-

ended measurement by configuring the MUX to measure each channel with respect to ground. Data are then

read out of one input based on the selection on the configuration register. The single-ended signal can range

from 0 V to the supply voltage. The ADS7828-Q1 device loses no linearity anywhere within the input range.

Negative voltages cannot be applied to this circuit because the ADS7828-Q1 device can only accept positive

voltages.

The ADS7828-Q1 input range is bipolar differential with respect to the reference. The single-ended circuit shown

in Figure 24 covers only half the ADS7828-Q1 input scale because it does not produce differentially negative

inputs; therefore, one bit of resolution is lost.

24

Submit Documentation Feedback

Copyright © 2008–2016, Texas Instruments Incorporated

Product Folder Links: ADS7828-Q1