Arduino boot up button and shutdown mode on regulator

atmegabootstrapbuttonvoltage-regulator

We've a projet with an important autonomy concern. We use an homemade arduino (atmega328), and for the moment we're using interrupts on buttons to wake up the atmega.

so we have: sleep ->(button pressed) -> wake -> (user RFID identification) -> (RF Communications) -> sleep.

We've optimized the total consumption with advanced sleep, modules isolated from current with transistors and so on…

I was asking myself if we could use a voltage regulator with a shutdown pin, and link one button and one pin of the atmega with it.

We would have this cycle:
shutdown -> (button pressed) -> shutdown pin HIGH -> atmega is powered-> atmega set shutdown pin HIGH during sketch configuration -> user RFID identification) -> (RF Communications) -> atmega set shutdown pin LOW -> shutdown

You get get the schematic here:

So to summarize: I need the button during my sketch, so when it's pressed and get the shdn pin high on the regulator, the first thing the arduino makes is maintaining this high with its pin 4. Furthermore, it restore the pull-up configuration of the button by reconnecting it to the ground through the npn transistor. When arduino is shutdown, button is linked to VIN as the npn make a NOT function with arduino state (HIGH or LOW).

Do you think it could work? If something is unclear don't hesitate to tell me 🙂

Thank's a lot!

Best Answer

Unfortunately I can not open your schematic, but I have an improvement that will definitely make this thing work.

Have a look at this:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

where:

reg_shdwn is the regulator shutdown pin
atm_wake_me is the pin you use to wake up the ATMega
atm_keep_on is the pin you would use to keep the regulator on

When SW1 is pressed all three pins go high, C1 is charged at \$V_{cc}\$ and the micro turns on. If the user releases the button before that the ATMega managed to pull atm_keep_on high the whole system does not shutdown though, since the capacitor will need some time to discharge through R1. You should size R1 and C1 to have a time constant \$\tau\$ that allows your micro the time it needs to pull the pin high.

The downside is that you dissipate some power through R1 when the system is on, but of course the resistor can be sized to minimize this power, while still being smaller than the input resistance of the arduino and the reg_shdwn pin.

Since of course \$\tau=RC\$ we can make an example calculation to aid you. I'd say that for R1 you should choose somethin in the \$100\$k\$\Omega\$ range, let's say \$R1=470\$k\$\Omega\$. \$\tau\$ can be some \$100\$us, i.e. 100 clock cycles at 1MHz. This leads to \$C\approx22\$nF, a perfectly reasonable value.

As a side note, a \$470\$k\$\Omega\$ resistor would dissipate about 23uW if \$V_{cc}=3.3\$V.

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Electronic – arduino – digitalRead sampling rate for Arduino Uno

You have two problems right now that are separate from each other.

1) Button bounce

2) Improper button wiring

First, you need to introduce a delay into your interrupt function so that it won't keep registering each rise and fall of the bounce as a separate button press. Use "delayMicroseconds()" because apparently you can't use regular "delay()" inside interrupts. Id suggest a 20,000 microsecond delay.

Second, you need to put a pull down resistor onto the digital pin that you are sampling.

The problem you are having is that the pin is in input mode and thus at high impedance (very high resistance) and thus has a tenancy to "float". If you put a voltage to it, it charges up like a capacitor and since the pin itself has very high resistance to ground, it takes a while for the charge to dissipate to the point where the Arduino will register a LOW again. A 10k resistor from the pin to ground will force the "capacitor" to discharge quickly after current from the button has stopped flowing.

Finally, the Arduino is pretty quick at it's read/write operations (I'm a mechanical engineer, ALL electronics happen "quickly" in comparison to mechanical systems, so it's relative I guess). Here's a link to some test results.

TL;DR verson:

What these values tell me, is that we can do about:

10 analog 10-bit readings per millisecond with analogRead()
128 pwm settings per millisecond with analogWrite()
220 pin reads per millisecond with digitalRead()
224 pin writes per millisecond with digitalWrite()
1056 pin reads per millisecond with direct port reads
1059 pin writes per millisecond with direct port writes

Arduino – How to get the atmega328 to run for a year on batteries

The Atmega328 provides six power saving modes, ordered from minimal to excellent (estimated current consumptions from this forum post):

SLEEP_MODE_IDLE: 15 mA
SLEEP_MODE_ADC: 6.5 mA
SLEEP_MODE_PWR_SAVE: 1.62 mA
SLEEP_MODE_EXT_STANDBY: 1.62 mA
SLEEP_MODE_STANDBY : 0.84 mA
SLEEP_MODE_PWR_DOWN : 0.36 mA

Quoting the original question:

I figured I could put sleep calls for certain times inside the loop method"

You would need to use sleep_cpu() after setting up the sleep mode you require, from the list above. The Arduino Playground has a useful post about this.

The application needs to be interrupt driven, use the above sleep modes extensively, and wake the processor up on button push, timer overflow and watchdog timer events to actually execute tasks.

Additional power savings can be obtained through the following steps:

Use the microcontroller's internal oscillator and a low clock rate (8MHz instead of 16) - but ensure that time and timing related code still works as expected. A different version of the bootloader might be needed for this.
Avoid keeping LEDs on for long if the application uses them. Using a rapid double or triple flash of short duration (0.05 second on, 0.5 second off), with gaps of seconds in between, ensures noticeable indication with minimal power consumption
Use a switching regulator instead of a linear one, if a regulator is required.
Run the microcontroller at lower voltage if supported, 3.0 Volts (e.g. CR2032 Lithium cell, no regulator needed) or 3.3 Volts instead of 5 Volts.
Follow recommendations in the datasheet for unused input and output pin settings for minimum power wastage.

Incorporating these suggestions allows for running microcontroller applications for weeks or months on a single CR2032 coin cell, and years on a LR123 type lithium cell. Of course, your mileage may vary depending on what sensors, outputs and actual processing your application requires.

Some useful references:

Best Answer

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