Electronic – Converting Open-Drain to Active-High (low voltage, low power)

logic-gateslow-powermosfetopen-drainrtc

In my design, I need an RTC (PCF85263A) to enable a boost converter (TPS61021A) after the RTC alarm triggers an interrupt. The interrupt pin on the RTC (INTA) is open-drain, but the enable pin on the boost converter (EN) is active-high. This is a problem.

When the alarm triggers the interrupt on the RTC, the INTA line is held at 0V, and when the interrupt isn't active, INTA is high-impedance. However, I need it the other way around: the EN pin needs to be held up to battery voltage when the alarm interrupt is active.

My first guess is to use an N-channel MOSFET logic inverter to achieve this. The input, of course, would have a pull-up resistor to VCC.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

However, the battery voltage VCC can go as low as 0.9V in this design. Another important requirement is that the power consumption be as low as possible while the RTC is "sleeping" – before the alarm triggers an interrupt. The RTC uses 300nA and the boost converter has a shutdown current of 500nA, so my power budget can take about 1-2μA more during sleep.

As such, I am not sure if this MOSFET inverter will be the most power-friendly option. Additionally, a MOSFET might not be the right choice for such low voltage – I assume that I must browse for VGS/VCE/VSAT choices at or below 0.9V.

What would be a power-friendly transistor choice to convert an open-drain, active-low output signal into an active-high signal for an input pin?

My design originally used the TPL5111 power gating timer instead of an RTC. It has a proper push-pull, active-high enable pin to wake the boost converter. Unfortunately, it can only "go to sleep" for up to 2 hours at a time. I thus want to migrate to an RTC so that my device can sleep for days or months instead; having the device wake up is a power-costly operation.

Edit: This PNP solution forgoes a pull-up at INTA. Credit to Tim Wescott.

schematic

^{simulate this circuit}

Best Answer

The circuit simulator isn't working for me today (grr).

PNP. Emitter to +V, base to RTC through a resistor (100k\$\Omega\$?). Collector to boost converter, with resistor to ground (same as base resistor). Size the resistor for a happy medium between turning on & off reliably, and not consuming too much current. It should consume very little current when off.

Related Solutions

Electronic – Arduino low power consumption and multiplexer

Single transistors don't really have any "quiescent current". If you don't feed them some current (or, in case of MOSFETs, voltage), they don't conduct. (A couple of nA leakage current can safely be ignored.)

As @geometrikal stated, these days there are better ICs for your application. Modern port expanders have an interrupt output, so you don't need the array of diodes which your solution would require. This particular IC has a standby current of just one µA, so you don't need to turn it off.

Electronic – arduino – Problem using DS1337 – RTC

Your connections seem fine. Also, from the RS1337 datasheet, you can power the IC with both 5V and 3.3V. Both are within the IC's described recommended DC operating conditions.

If you have an oscilloscope handy, put the probe on X1 then on X2 (I never know which is the correct one, so I try them both, one at a time) and check if the crystal is oscillating. If you get a clean 32,768Hz square wave, then it's oscillating properly.

Also, you need to double-check whether you've followed the board layout restrictions regarding ground and signals going near the crystal pins, as per datasheet. I'm copying the relevant part below:

Restrictions for board layout for DS1337

If there's a signal going near the crystal pins, the internal IC registers will get corrupted. Then you'll get all sort of weird symptoms. That may be the cause of your problem.

I got sloppy in one of my layouts with the DS1307 RTC and got similar problems. In my case, I could set the time, but the minutes register would decrease unit by unit, randomly.

If you're board doesn't follow the recommendations, one way of fixing it is to ground crystal case (like so: Is case grounding compulsory in typical 32.768kHz crystal for Real Time Clock?). That did the trick for me.

Edit: Looking again at your serial monitor printout, it looks like your code was not able to set the clock time. The time it's printing on serial is likely the time you tried to set, but failed. To me, that means that the I2C communication is not working properly. Maybe it's the device address that's wrong, or a problem with the wiring. I see that you have both pull-up resistors properly wired to SCL and SDA, so that's good. But I would double-check all that wiring.

To properly debug the I2C communication, you'll need an oscilloscope (like I did in this question of mine: What happens if I omit the pullup resistors on I2C lines?).

Edit 2: I still thinking you're not setting the time as you imagine. That's because the code below sets the values you defined into the C++ object called RTC (which sits in the Arduino's RAM), but it fails to set them to the RTC IC:

void setup() {
    RTC.start();
    RTC.setSeconds(00);
    RTC.setMinutes(00);
    RTC.setHours(05);
    RTC.setDays(06);
    RTC.setMonths(3);
    RTC.setYears(2014);
    RTC.writeTime();      // I think this line is failing

    // But the lines above succeed in setting the time into RAM (but not on the RTC)
}

Then, you print the time inside loop(), but using the values in RAM:

void printTime(byte type) {
    Serial.print(int(RTC.getMonths())); // these values come from RAM, not the RTC
    Serial.print(int(RTC.getDays()));
    Serial.print(RTC.getYears());
    Serial.print(int(RTC.getDays()));
    Serial.print(int(RTC.getHours()));
    Serial.print(int(RTC.getMinutes()));
    Serial.print(int(RTC.getSeconds()));  
}

To confirm that, try running the code below:

...
void setup() {
    RTC.start();
} 
void loop() {
    RTC.readTime(); 
    printTime(0);
    delay(1000);
}

If time isn't set, it will print a bunch of zeroes like 0/0/0 0:0:0. If time is set, then it will print 3/6/2014 5:0:0

Best Answer

Related Solutions

Electronic – Arduino low power consumption and multiplexer

Electronic – arduino – Problem using DS1337 – RTC

Related Topic