MyProjects: september 2009

For some time i've wanted a binary wrist watch and a couple of days ago when I felt for doing a small project that could be realised fairly quickly i finally came to it.

Here's a list of some basic hardware "features" i strived for:

Fit inside a wrist watch
Use as few parts as possible
Surface mount components where possible
Only minute and hour representation is necessary
Driven by button cells for "long time" ie months without replacing
Leds to represent time
24 hour format
As single sided pcb layout as possible since hand connecting vias is a drag

Now, why reinvent the wheel so I fired up my favourite search engine but I couldn't find anything to satisfy my needs. Most of the binary watches were based on the megax8 (which in my taste is way too big and has too many features for a project like this). Finally I chose a between a tiny24 and a tiny25 and the went with the tiny24, mainly because I would have to use all the pins of the tiny25 and therefore making programming of the device problematic. An advantage of using the tiny24 is that it's 16-bit counter's got power consumption characteristics in par with both the tiny25's and the megax8 8-bit counters. This results in that we don't have to wake the CPU as often and thus save power. The datasheets are very scarse on details regarding current consumption when a 32.768kHz crystal is used so this is just an assumption made by looking at frequency=0.1MHz in the Idle Supply Current vs. Low Frequency (0.1-1.0MHz) table. Current consumption values of the timers are found in table Additional Current Consumption (percentage) in Active and Idle mode.

The circuit

The MCU is a Tiny24 (14 pin).
The leds are charlieplexed.
Q1 is a 32,768kHz crystal
The pushbutton is connected to INT0.

The software
To conserve power the microcontroller is put to sleep as soon as it doesn't do anything useful (TIMER1 has to be active so we use "idle mode").
The timekeeping part of the software is based on using TIMER1 to trigger an interrupt every minute which wakes the CPU and updates the time.
When the button is pressed an INT0 interrupt wakes the CPU and the time is displayed.
binwatch.c
binwatch.h
display2.S
Makefile

The PCB
It's a double sided pcb with microcontroller and leds on the top layer and battery and crystal on the bottom layer.
The battery holder is a low profile battery holder for cr2032 batteries.
The button is smd and placed on the right side of the following pictures.
The PCB is 3 cm in diameter.

top

bottom

The watch
The donor watch was an old broken lcd watch. I removed the button used to set time and replaced it with a small screw with conical head and it all fit quite nicely.
The leds looks much stronger "irl".