I'm designing a circuit with STM32F407 and a DS3231 chip clock. The MCU has a separate VBat pin that can be used and on the other hand, the DS3231 chip has its own RTC. I'm wondering can I use a single RTC battery for both circuits? I found some circuits that suggest using a common Schottky diode like bat54c before connecting the battery to MCU. Should I use it too or not?
Thanks.
Electronic – Use one RTC battery for two circuits
rtcstm32
Related Solutions
This PIC, and all other PIC18's that I know of, have an internal oscillator block. There is no need for an external crystal, even if you didn't have the 32kHz output. The only time you would need to use an external XTAL or clock would be:
- To synchronize multiple devices to the same clock,
- If you need a very specific frequency (for serial communications, etc), or
- If you need more accuracy that the internal oscillator would provide.
You configure this INTOSC block by setting bits in the OSCCON register. You can choose from eight values from 31kHz to 8MHz to run the system clock.
In the OSCCON register, set the SCS1 bit to use the internal oscillator block. Then, set the IRCF2:IRCF0 bits to choose your frequency. This picture is from Section 2.4.1 of the datasheet:
This is convenient because you can choose a slow frequency for lower power usage, and then increase it if you need to.
You could, of course, use the 32kHz RTC output, but it may just be unnecessary complexity. Also, keep in mind that the PIC18F4550 has a different oscillator architecture than other PIC18's because of the way it clocks it's USB module. This won't stop you from using the 32kHz signal, but it will require a different configuration that you may be used to.
Assuming you want to turn the relay on if either an RTC brings an open drain output low or the Raspberry Pi provides a certain logic state, the below circuit would do this:
simulate this circuit – Schematic created using CircuitLab
The Rpi GPIO should be brought high to keep the relay in, and it should be push-pull, not open drain.
The Rpi outputs are not guaranteed 5V tolerant so they cannot be pulled up to 5V reliably. You need a catch diode across the relay, as shown. Further, it is unlikely that either the RTC or the Raspberry Pi will be very happy directly driving a relay- in your circuit about all the relay current goes through the Raspberry Pi or RTC.
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Best Answer
This is perfect example of XY Problem.
Without studying the datasheets you've made several incorrect assumptions and decided to use external RTC, which created new power supply problem for you. Here are the assumptions:
The DS3231 accuracy is ±2 ppm. The basic accuracy of STM32F407 is 5 ppm. However it also has digital calibration feature that allows you to output clock to external pin for measurement and subsequent calibration in 0.95 ppm steps.
So, the achievable accuracy of MCU RTC is 2 times better than external RTC.
DS3231 consumes 110 µA in standby mode. STM32F407 requires only 4 µA in standby mode with SRAM backup or 3.3 µA without backup. Note, that even if you use external interrupt to wake up MCU, both of them will be consuming power in standby.
So, the power consumption with DS3231 is 33 times more than with internal RTC only.
Here is a quote from the datasheet: "Two 32-bit registers contain the seconds, minutes, hours (12- or 24-hour format), day (day of week), date (day of month), month, and year. Compensations for 28-, 29- (leap year), 30-, and 31-day months are performed automatically." Not only that, but it also can be programmed for Daylight Saving Time compensation, something that DS3231 cannot do.
So, RTC in STM32F407 can do everything DS3231 does, and more.
To summarize, there is no need whatsoever to use DS3231 with STM32F407.
Having said that, there is a way to reduce power consumption even more, by using nano-power external timer, like TPL5110 to switch MCU power supply OFF completely. In this case (called VBAT mode) the RTC will continue running, consuming only 0.96 µA from backup battery. The timer itself adds negligible 35 nA, for total consumption under 1 µA (3 µA savings, compared to standby mode).