Hello,
I've developed a PCB that connects an RTD (PT100) to a Wheatstone bridge, set to 100 ohms at 0°C. The bridge terminals (as seen in the diagram) connect to AIN4 and AIN6 of the nRF52840. However, no matter what devicetree configuration I try (differential mode, single mode, gain, vref, etc), the measurements are very unstable, and I can't calculate a correct temperature value. What could be the cause of this problem?
Schematic (RTD_V1 to AIN6 and RTD_V2 to AIN4):
Device tree snippets:
channel@4 {
reg = <4>;
zephyr,gain = "ADC_GAIN_1_6";
zephyr,reference = "ADC_REF_INTERNAL";
zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
zephyr,input-positive = <NRF_SAADC_AIN4>; /* P0.03 */
zephyr,input-negative = <NRF_SAADC_AIN6>; /* P0.03 */
zephyr,resolution = <12>;
};Application snippets:
#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
!DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
#error "No suitable devicetree overlay specified"
#endif
#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
ADC_DT_SPEC_GET_BY_IDX(node_id, idx),
static const struct adc_dt_spec adc_chan4 = ADC_DT_SPEC_GET_BY_IDX(DT_PATH(zephyr_user), 1);
static const struct gpio_dt_spec rtd = GPIO_DT_SPEC_GET(DT_PATH(zephyr_user), rtd_gpios);
int main(void)
{
int err;
uint32_t count = 0;
uint16_t buf_1, buf_2;
struct adc_sequence sequence_1 =
{
.buffer = &buf_1,
/* buffer size in bytes, not number of samples */
.buffer_size = sizeof(buf_1),};
int rc;
rc = gpio_pin_configure_dt(&rtd, GPIO_OUTPUT);
if (rc < 0)
{
printf("Could not configure sw0 GPIO (%d)\n", rc);
return 0;
}
gpio_pin_set_dt(&rtd, 0);
if (!adc_is_ready_dt(&adc_chan4))
{
printk("ADC controller device %s not ready\n", adc_chan4.dev->name);
return 0;
}
err = adc_channel_setup_dt(&adc_chan4);
if (err < 0)
{
printk("Could not setup channel (%d)\n", err);
return 0;
}
;
while (1)
{
int32_t val_mv_1 = 0;
(void)adc_sequence_init_dt(&adc_chan4, &sequence_1);
err = adc_read_dt(&adc_chan4, &sequence_1);
if (err < 0)
{
printk("Could not read (%d)\n", err);
continue;
}
val_mv_1 = (int32_t)((int16_t)buf_1);
err = adc_raw_to_millivolts_dt(&adc_chan4,
&val_mv_1);
if (err < 0)
{
printk(" (value in mV not available)\n");
}
else
{
printk(" = %d mV\n", val_mv_1);
}
k_msleep(1000);
}
return 0;
}
RTT:
00> = -4 mV
00> = -9 mV
00> = 10 mV
00> = -13 mV
00> = -9 mV
00> = -4 mV
00> = -8 mV
00> = 0 mV
00> = -6 mV
00> = -4 mV
00> = -2 mV
00> = 1 mV
00> = -9 mV
00> = 10 mV
00> = -13 mV
00> = -9 mV
00> = -4 mV
00> = -8 mV
00> = 0 mV
00> = -6 mV
00> = -4 mV
00> = -2 mV
00> = 1 mV
and go on.
Thanks!!
