Simple RTC Counter Idea for an IC?

[Boscoe]

Hi all,

I wonder if anyone has come across a part like this in the wild? I think it probably could be done with a few ICs however the power consumption will be high I think. It would be trivial in an FPGA or CPLD but again power consumption would be crazy.

I have designed a lot of ultra low power products that have programmable periodic wake up intervals in which to perform some action. These intervals could be between 1s to hours with precision of seconds. Existing RTCs and MCU internal RTCs always seem to be primarily focused on calendar functionality however my products only need second based periodic wakeup - it doesn't care about the exact date. The products are critical infrastructure which require reliability in the 99.99999% range.

Now, again, MCU based and existing RTCs usually have a second wise wakeup timer however what if I now need two, or three etc...? You start running out of products very quickly so you end up having to perform calculations based on a higher frequency wakeup interval which introduces a lot of room for error in the program and the HW and increase power consumption massively.

TLDR: I would love an RTC that just has a 32bit counter for UNIX time and 1s precision. Then, it has a set of registers for modulo interrupt generation with an option offset. Perhaps 8 registers for 8 different interval times. There could be a control and status register to enable / disable some interrupts and check to see which interrupt has just been generated. It could all be controlled over I2C easily. Perhaps a little bit of SRAM or EEPROM could be handy in there, too. For my applications, sub 1uA consumption would be required.

What do people think? Silly idea or could be useful?

[Dave]

Rolling your own ASIC for this seems like a ridiculous solution to what could simply be done with a microcontroller with an integrated RTC.

You can easily keep track of the 8 events you want it to trigger on in RAM and just set the RTC trigger to the event that is supposed to be triggered first. Pulse the corresponding output (or whatever you need to do), update the list and set the next trigger.
If reliability is crucial, use whichever standard you need to adhere to and design your system with some redundancy that allows you to meet said standard.

[Peabody]

The only RTC that I've had extensive experience with is the DS3231.  I have used it for very low power circuits where its open drain alarm output is used to switch on the power to the rest of the circuit.  The DS3231 when powered through its battery input will keep time at about 2uA, so to the first approximation the entire circuit will draw no power except when the alarm triggers.  That can save a lot of battery life if not everything in the circuit can be put into sleep mode.  So you would use a coin cell for the RTC, which should last for years.

However, I believe your processor would have to program in the next alarm time before powering down.  Also, I don't think this would work well if you want to wake up every second or every few seconds.  You would be powering up and down too often.

I believe it does do UNIX time, but I've never tried that.  I don't know how that would work with the alarm settings.

[spostma]

parts to consider IMO:
- MCP7952X/MCP7951X: SPI RTC with alarms, optionally with EEPROM
- CD4060: 1Hz timer with 32768Hz crystal
- TPL5110: 1% accurate timer with selectable time intervals: 100 ms to 7200 s

[emece67]

.

[Benta]

Forget about FGPA, any of them will have a higher current consumption.

Heh, heh. Been there, done that. I looked for a CPLD for a project (not even an FPGA), and didn't find a single one on the market with a current consumption less than 100 mA.

[Terry Bites]

The MCP7941X alarm has to be reset my the micro to be ready again. You could write back a new alarm setting to ALM1 or ALM2 at this point.

[pjetson]

Would something like the TPL5110 work for you?

https://www.ti.com/lit/gpn/TPL5110

The TPL5111 is similar, but with an enable output rather than built-in power switching.

Peter