Sonntag, 11. September 2016

DIY ATmega328 Clock - improved power saving

I built a clock as fun and training project two years ago. There is always room for improvement - first I had a 7 segment LED which I multiplexed at 1kHz and used the time in between to sleep the µC. I tested several RTCs, from DS1302 over 1307 to the DS3231. The DS3231 is in place with a Nokia5110 display now for half a year and led to significant better time keeping and less power usage.

ATmega328 based DIY Clock.
Several further sensors are attached - the Si7021 measures temperature and humidity. Cool thing about the IC is that it does its measurement and goes to sleep immediatly, consuming less than 1µA.
Another sensor is a BH1750 which is used for only activating the display when there is enough light to read it. As soon as it is dark, the display gets disabled, further dropping the current drawn.

Today I added two further improvements to bring the power consumption down a little more. For one, I modified the Wire-library of the Arduino GUI to not activate the I2C pullup resistors. On those sensors, there are Pullups in place already.
Find your arduino-directory, go to hardware\arduino\avr\libraries\Wire\src\utility and open twi.c - there you can search for "pullup" and comment out both lines where the SDA/SCL pins are set to 1 (which enables the pullup resistor on an input pin).

Power consumption via 1R shunt on the oscilloscope.
This led to the current drop to 0µA every now and then.
So I decided that 100kHz I2C speed may be a little slow and keep the µC awake longer than necessary. So I added the line
at the end of the setup()-routine.

This value fits for 16MHz µCs and results in 400kHz I2C speed. Looking at he graph it can be seen that the I2C reading is now taking only a short fragment of a second instead of nearly half of it as it did before. The values are close to 1mV for 1mA, but with such low values, this setup is distorted by noise a lot and doesn't show the precise load values. The multimeter shows peaks of ~3mA and sleep periods with real 0µA.
Before these changes, with a proper low-quiescient-current ultra-low-dropout regulator like the MCP1700, the LiIon battery with 2400mAh lasted for exactly three months. Let's see how long the new battery will last this time! I'll update the article when the battery is empty.

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