I'm using the board RAK19007 with its RAK4630 nrf52840, meant for solar and battery, where the AIN3 (P0.05) comes from a voltage divider of VBAT, where AIN3 = VBAT*(1.5M/2.5M). The Thevenin equivalent resistor should be 600KOhm.
The nrf52840 is powered in High Voltage mode, where VDDH=VBAT. I noticed that VDD=MAX(3v3,VDDH) with a multimeter. We use the default LDO in regulators; I tried using the DC/DC converters but I cannot observe any difference.
I'm telling the system to go off (sd_power_system_off()) when VBAT < 3.1V (using ADC read in AIN3), but before system off, I program LPCOMP like this:
nrf_lpcomp_config_t c;
/* We have AIN3 with a VBAT divider so AIN3 = VBAT * (1.5/2.5)
* We have the device going deep sleep under 3.1V, which is AIN3 = 1.86V
* So we can wake up when VBAT>=VDD is restored to 3.3V, where AIN3 = 1.98V
* 1.98/3.3 = 6/10, but that's close to the VBAT divider, so we
* pick 6/8VDD, which means VBAT=4.1V */
c.reference = NRF_LPCOMP_REF_SUPPLY_6_8;
c.detection = NRF_LPCOMP_DETECT_UP;
c.hyst = NRF_LPCOMP_HYST_NOHYST;
nrf_lpcomp_configure(NRF_LPCOMP, &c);
nrf_lpcomp_input_select(NRF_LPCOMP, NRF_LPCOMP_INPUT_3); // RAK4630 AIN0 = nrf52840 AIN3 = Pin 5
nrf_lpcomp_enable(NRF_LPCOMP);
/* Disable RAM retention in System OFF, otherwise the
* system does not reboot well, part of config reset */
for(int i=0; i < 9; ++i)
{
nrf_power_rampower_mask_off(NRF_POWER, i,
NRF_POWER_RAMPOWER_S0RETENTION_MASK |
NRF_POWER_RAMPOWER_S1RETENTION_MASK |
NRF_POWER_RAMPOWER_S2RETENTION_MASK |
NRF_POWER_RAMPOWER_S3RETENTION_MASK |
NRF_POWER_RAMPOWER_S4RETENTION_MASK |
NRF_POWER_RAMPOWER_S5RETENTION_MASK |
NRF_POWER_RAMPOWER_S7RETENTION_MASK |
NRF_POWER_RAMPOWER_S8RETENTION_MASK |
NRF_POWER_RAMPOWER_S9RETENTION_MASK |
NRF_POWER_RAMPOWER_S10RETENTION_MASK |
NRF_POWER_RAMPOWER_S11RETENTION_MASK |
NRF_POWER_RAMPOWER_S12RETENTION_MASK |
NRF_POWER_RAMPOWER_S13RETENTION_MASK |
NRF_POWER_RAMPOWER_S14RETENTION_MASK |
NRF_POWER_RAMPOWER_S15RETENTION_MASK
);
}
auto ok = sd_power_system_off();
I would expect that if I set VBAT=2.5V for example (with a regulable power supply), the system will go off. It does, but it reboots immediately. It reboots, then goes off, then reboots, then goes off, ...
I checked that it has to be the LPCOMP resetting the system, because if I don't program LPCOMP the system never resets.
Why LPCOMP does wake the device up immediately after SystemOff?
VBAT=VDD=2.5V, that means that AIN3 = 1.5V < 6/8*VDD = 1.875V, set in LPCOMP. The ANADETECT signal should not be triggered.
Is there a problem with the high resistance of the voltage divider? Is there something I wrote wrong?
In another test I changed the LPCOMP input to AIN7, which is Not Connected to anywhere, and it also reboots automatically.
