Founder of a tiny logistics startup, very new to hardware, so apologies if this is obvious.
We put solar-charged 12 V lead-acid batteries on trailer gate sensors. I want an Arduino Pro Mini (ATmega328P, 3.3 V, 8 MHz, power LED removed) to sleep in power-down and wake only when the battery falls below about 11.8 V, then send one message to Hermetry so someone swaps the battery. Divider is 100k/33k so 11.8 V is about 2.93 V at the pin.
I read about the analog comparator (AIN0/AIN1) and its interrupt, and wired the divider to AIN1 with a 2.5 V reference on AIN0. With the comparator interrupt set, sleep in SLEEP_MODE_PWR_DOWN never wakes. In SLEEP_MODE_IDLE it wakes fine but my meter shows about 1.3 mA, which kills the point.
Is there a register trick I’m missing to make the comparator wake it from power-down? Code is basically:
ACSR = (1 << ACIE) | (1 << ACIS1); // interrupt on falling edge
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable(); sei(); sleep_cpu();Couldn’t you wake on the watchdog every 8 s, read the ADC, and go back to sleep? That’s what our door counters do.
Tried that, it works, average around 20 µA. I was hoping for less, and it feels clumsy.
Why does the reference matter so much? Couldn’t it use the internal 1.1 V bandgap?
No register trick, I’m afraid. As far as I remember the sleep mode table in the 328P datasheet, the analog comparator is only a wake source in idle (and ADC noise reduction), not power-down. In power-down only the external interrupts, pin change, TWI address match and the watchdog wake it. Your idle current is the clocks running.
Move the comparator outside. A nanopower comparator with push-pull output into INT0 (D2) lets the 328P sit in power-down at a few µA. TLV3691 draws about 75 nA, and the reference can be a micropower 2.5 V shunt. Add hysteresis with a resistor from output to the + input, a few hundred mV at the battery side, otherwise a battery hovering at 11.8 V will wake you a hundred times a night as the load changes.
attachInterrupt(digitalPinToInterrupt(2), wake, FALLING);
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable(); sleep_cpu(); // wakes on INT0 only
// on wake: read ADC once for the real value, send it, sleep againAoife: you can use the bandgap internally, but the comparator itself still can’t wake power-down, so it doesn’t help here.
On the Hermetry side I’d still send a heartbeat once a day with the voltage. Then an offline alarm on the profile tells you a sensor died, which a wake-only design never would. Put the low-voltage alarm rule on bat_v with some hysteresis too.
Ordered TLV3691s. Breadboard version with a 2.5 V reference and about 300 mV hysteresis sits at 4.8 µA in power-down and wakes cleanly at 11.79 V on my bench supply. Daily heartbeat added, and the offline rule. Thanks Laura, and Keisha the WDT version is my fallback.
- Du musst angemeldet sein, um auf dieses Thema antworten zu können.
Mitdiskutieren
Melde dich an, um Fragen zu stellen und zu beantworten, hilfreiche Beiträge mit „Gefällt mir“ zu markieren und die Antwort zu kennzeichnen, die dein Problem gelöst hat. Jeder Beitrag bringt Punkte und Stufen.