Simple sample for measuring coin cell battery voltage with Zephyr and nRF52840

Question

My project uses Zephyr and the nRF52840 with a CR2450 coin cell battery. All is working well, and now I am looking to add in battery level. I've downloaded and read the following samples. 
 zephyr/samples/boards/nrf/battery zephyr/samples/drivers/adc 
 These samples seem to focus on particular hardware (like the Thingy) that use lithium batteries, and so they have a /vbatt voltage divider to bring the voltage down to a range the nRF52840 can read. There are some ifdef and devicetree mechanisms that imply they could work without a voltage divider, but I'm getting confused by the samples. 
 My board is powered directly by the CR2450, and so I believe I don't need a voltage divider. My board also has P0.02/AIN0 directly connected to the coin cell (3.3V nominal) in anticipation of using this pin for reading the voltage. 
 If I could read the voltage, I'll do the math to determine battery level. 
 My questions are as follows. 
 - From parts of the samples, I'm guessing that the nRF52840 has an internal way to measure VDD. (See "zephyr,input-positive = <NRF_SAADC_VDD>;" in the nRF52840DK devicetree overlay in the "add" sample.) Is this true? If so, how do I access it? 
 - If the above is not true, could my connection between P0.02/AIN0 and the coin cell be used to measure its voltage? 
 - Is there a sample, or some guidance within an existing sample, that you can give me to make this clearer? 
 Thanks, 
 Steve

abvio · Accepted Answer

Hi Hieu, Thank you for your follow up. I'm happy to report I have it completely solved and am reading the internal VDD using the ADC. It still wasn't clear why my code wasn't compiling, but after iterating on a lot of possibilities, I did change the sample.yaml in my project to match the sample.yaml in the ADC example, which mentioned "adc". After that and a few more iterations, the code started to compile. And after some further debugging, it started to work. In case someone is looking for some simpler sample code, I reduced the devicetree to just one channel that looks like this: / { zephyr,user { io-channels = <&adc 0>; }; }; &adc { #address-cells = <1>; #size-cells = <0>; status="okay"; channel@0 { reg = <0>; zephyr,gain = "ADC_GAIN_1_6"; zephyr,reference = "ADC_REF_INTERNAL"; zephyr,acquisition-time = ; zephyr,input-positive = ; zephyr,resolution = <14>; zephyr,oversampling = <8>; }; }; The code I use to access it looks like this: #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), /* Data of ADC io-channels specified in devicetree. */ static const struct adc_dt_spec adc_channels[] = { DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels, DT_SPEC_AND_COMMA) }; K_MSGQ_DEFINE(battery_msgq, sizeof(battery_event_t), 10, 4); int32_t battery_level(void) { int err; int32_t val_mv; int16_t buf = 0; uint8_t level = 0; struct adc_sequence sequence = { .buffer = &buf, /* buffer size in bytes, not number of samples */ .buffer_size = sizeof(buf), }; if (!device_is_ready(adc_channels[0].dev)) { printk("ADC controller device not ready "); return level; } err = adc_channel_setup_dt(&adc_channels[0]); if (err < 0) { printk("Could not setup channel #%d (%d) ", 0, err); return level; } (void)adc_sequence_init_dt(&adc_channels[0], &sequence); err = adc_read(adc_channels[0].dev, &sequence); if (err < 0) { printk("Could not read (%d) ", err); return level; } val_mv = buf; err = adc_raw_to_millivolts_dt(&adc_channels[0], &val_mv); if (err < 0) { printk(" (value in mV not available) "); return level; } // Voltage to level curve for a coin cell 3V battery. #define BATTERY_FULL_MV 2950 #define BATTERY_HALF_MV 2900 #define BATTERY_QUARTER_MV 2800 #define BATTERY_DEAD_MV 2300 if (val_mv >= BATTERY_FULL_MV) { level = 100; } else if (val_mv <= BATTERY_DEAD_MV) { level = 0; } else if (val_mv >= BATTERY_HALF_MV) { level = (val_mv - BATTERY_HALF_MV) * 50 / (BATTERY_FULL_MV - BATTERY_HALF_MV) + 50; } else if (val_mv >= BATTERY_QUARTER_MV) { level = (val_mv - BATTERY_QUARTER_MV) * 25 / (BATTERY_HALF_MV - BATTERY_QUARTER_MV) + 25; } else { level = (val_mv - BATTERY_DEAD_MV) * 25 / (BATTERY_QUARTER_MV - BATTERY_DEAD_MV) + 0; } return level; // return val_mv / 100; } Thanks again for your help, Steve

Simple sample for measuring coin cell battery voltage with Zephyr and nRF52840

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