![]() Initially I used the millisecond clock() function available to C programs on Windows, doubting that, I added code to look at an NTP server, in fact the Windows clock() value kept in sync with NTP. How accurate is it? I set up code on the Arduino to print a character to the serial port every 8 seconds and then compared results to the clock on a PC running a program watching the serial port. Waking every 8 seconds does have a power cost see Waking from sleep mode. An alternative is to use a Real Time Clock chip (see Logger). The maximum sleep time is 8 seconds, which is the same as using the built in watchdog timer (see Sensor). All I had to do was changeĮverything worked and I had a Pro Mini running on 1 microamp with an accurate clock. I found ready to run software in the form of Asynchronous timer example by Nick Gammon. First one shows original Pro Mini, second with annotation, third modified Pro Mini with added clock crystal The link consists of three solder pads the middle one is normally connected to one of the other two (see low power), the link is part of a potential divider across the output side of the voltage regulator.Ī crystal was obtained from a non-functioning wall clock (see repairing quartz analogue clocks) and soldered on to the pins of the Atmega 328 chip.Ĭlick to enlarge photos. I guessed correctly this was because it has a link which can be changed to allow 3.3 V or 5 V operation disconnecting both sides of the link allowed the desired around one microamp consumption. After all this destruction the board still used around 3 mA when sleeping. It would also have been a better idea to cut the tracks to the crystal because I could then have reconnected it if necessary. In my Logger project I show how low power operation can be achieved by cutting a single track on the board - a much better idea - but one which does not work with this design (see low power). A fine soldering iron was not good enough, I might as well have chiselled the components off the board with a small screwdriver, or as I effectively ended up doing, pulling them off with tweezers. I wanted to remove the power regulator and power LED to reduce power consumption and thought I could remove the crystal at the same time. I set the fuses by connecting a USBASPI programmer to the Pro Mini, and then used this command line (for avrdude):Īvrdude -p m328p -c usbasp -P USB -U lfuse:w:0圎2:m Disconnect the normal crystal/resonator.Set the fuses on the processor so that it uses its internal 8 MHz RC oscillator for a clock. ![]() For 16 MHz boards burn an 8 MHz boot loader (or else see 'Bricked').
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