ADC:Измерение сопротивлений - технические детали/en

The LRADC channel of the processor that the Wiren Board uses to operate the ADC has an integrated current source. This allows to measure the resistance of the load connected to the outputs of the ADC, i.e., to read values of resistive sensors.

See also the description in the processor documentation, p. 1334 -Reference Manual

Resistance measurement is implemented in the wb-homa-adc driver, see ADC.

The measuring procedure in the manual mode

Make sure the wb-homa-adc daemon is not running

Disable output

(only for Wiren Board rev. 2.8)

• For terminals 2-7: close the transistor. See List of GPIOs, Low voltage load management.
• For terminal strip 9: set GPIO 2 to input mode (Managing GPIO)

Channel selection

Use wb-adc-set-mux to select a channel(ADC)

Setting output current

Setting the output current is not supported by the driver, so it is implemented using the userspace application with direct memory access.

Command lradc-set-current (https://github.com/contactless/wirenboard/tree/master/utils/adc/resistance).

root@wirenboard:~# lradc-set-current
Set output current for LRADC1 channel
USAGE: ./lradc_current <uA>|off
Available currents: 0uA, 20uA, ..., 280uA, 300uA
Use "off" to switch off current source. 0uA setting could result in some current

The maximum current is 300 µa. The current is set in increments of 20 µa.

Setting 100 µa current:

root@wirenboard:~# lradc-set-current 100

Disconnecting the current source:

root@wirenboard:~# lradc-set-current off

It is necessary to set the maximum possible value of the output current at which the voltage on the LRADC does not exceed 1.85 V, i.e. the raw result is not equal to 4095.

In the following example 40 µa is the optimum current:

root@wirenboard:~# lradc-set-current 100
root@wirenboard:~# wb-adc-get-value
4095
root@wirenboard:~# lradc-set-current 20 
root@wirenboard:~# wb-adc-get-value
1663
root@wirenboard:~# lradc-set-current 60
root@wirenboard:~# wb-adc-get-value
4095
root@wirenboard:~# lradc-set-current 40
root@wirenboard:~# wb-adc-get-value
3154

Calculating the results

The resistance is calculated by the formula:

r = 1.0/(I/V  - 1/R1) - R2

r is the measured resistance, R2 is the series resistance of the channel

R1 is parallel resistance (divider arm/leakage currents).

V is input voltage LRADC measured by the processor:

V =  N / 4095 * 1.85V

where N is the raw LRADC readings, I is the output current.

For different versions:

• Wiren Board 5 (terminals R1-R2): R2 = 3150 Ohm, R1 = 1000kOhm.
• Wiren Board 4 (terminals R1-R2): R2 = 3570 Ohm, R1 = 120kOhm.
• Wiren Board Smart Home rev. 3.5 (terminals R1-R4): R2 = 600 Ohm, R1 = 93kOhm.
• Wiren Board rev. 2.8. R2 = 12kOhm, R1 = 33.0 kOhm.