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ATMAGE328pb and NRF24 - 2.0V.

Tags atmega328pb  nrf24  Sensors  Low Power 

As a follow-up on yesterdays blog post, I’ve tried to modify one of my sensor PCB to run at a lower voltage. First I switched from the external 16MHz crystal to the internal 8MHz oscillator. This should allow us to go as low as 2.7V, by enabling the clock divide 8 fuse bit we can step this down to 1 MHz. At 0-4MHz we can go down to 1.8V. Ideally I run it at 4 MHz but I have no crystal that fits on the existing footprint nor are there for sale.

After playing a bit with the baudrates, that no longer fits, I could flash in my firmware and this all worked out of the box.

This should be perfect from running of a Sodium-Ion battery, the LDO I use is happy enough to go below its official output voltage, so it perfectly protects against over voltage of a fully charged battery.

In idle (while waking up periodically to directly go to sleep again) the current is now ~11µA@3.3V but ~5µA@2V, this should be good enough. Switching to the TPS62743 will improve the idle usage, but I am not sure its worth the extra cost. When wakeup it runs around 3.5mA with peak up to 29mA while sending (at max power). It is awake for a significant longer time (because of low clock frequency). But at a lower power consumption. This should also make it easier to run it off coin cells. With a wakeup every 2.2 minutes we get a usage of 34µAh. This is around 2/3 of a year on a cr2032. Increasing the interval between sampling to every 5 minutes we can stretch this to around 1.6 years. Probably slightly longer as the consumption drops as the voltage drops.

5m sensor @ 3V

Now to test sensors that will go down to this voltage. First to try is the SHT31. This works fine down to 2.2V, lower the sensor would no longer respond, but it gives a good opportunity to still send an low-battery message. A quick test at 2.2V shows that it does affect the range of the nrf module. The average usage at a 5 minute interval of less then 10µAh might make a strong case for the TPS62743. Even better would be if we could drop down to 1.8V wake up, ramp up to a higher voltage, send and then drop down again.

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