Clock with Nokia 5110 LCD Display

Another clock? I didn't really need another clock, but this project nevertheless has its compelling features. First, I stumbled across a very interesting LCD display, the Nokia 5110. I also found a cheap Chinese pushbutton rotary encoder, very similar to the CTS encoder I had used before. I wanted to give these gadgets a try. To make the project more interesting I decided to do it with no RTC clock chip.


The Nokia 5110 is a 48 x 84 pixel graphic display. This makes it feasable to use a small font for programming menus, and a large font for the time display. Support code was available on the vendor's web site, which saved me a lot of development time. The display is rated for 2.7 to 3.3 volts, so I chose to power the project with a 3.3V regulator and 3V backup battery.

the projectcircuit board

Its hard to see in the photo, but there's a pushbutton rotary encoder below the display. It is a model EC11-1B-18T made by Changzhou Xinze Electronic Co. It is very similar mechanically and electrically to the CTS series 290 rotary encoder I used before. As a bonus it is easy to panel mount, and seems to work quite well.

I chose an ATmega88PA for this project. It is a good match for the number of IOs required, and flash memory size. Also the chip is a picopower chip, which seemed to be a good idea since this project must draw minimal power when offline. A tiny 32.768 KHz watch crystal is connected to TOSC1 and TOSC2. This provides clocking for timer/counter 2. Divided properly, the result is an interrupt every second, great for running a clock.

When the clock is unplugged or power fails, the time must be maintained in the processor. This means a backup battery, and power consumption must be kept to a minimum. In asynchronous mode timer/counter 2 keeps running even when the processor is placed in power-saving sleep. Each second an interrupt wakes up the processor so that the clock can be updated, then sleep is resumed. The result is a drop in power consumption from 3 to 4 mA when powered up to 3 uA when powered from the battery.

PD2/INT0 is tied to the power supply. It is programmed to produce an interrupt whenever there is a level change. This lets the program know when power goes up and down. When power goes down, the LCD is powered off and power saving sleep mode entered. When power comes back on, normal operation is resumed.

If you are puzzled by some of the code to do with sleep mode, have a look at AVR134 Real-Time Clock using the Asynchronous Timer (PDF, sample code). There are a few tweaks in the code that are not obvious, but shown in the application note. Believe me if you omit anything sleep mode will not work as expected.

Programming the clock is simple. Press the encoder button and programming mode is entered. A series of menus are shown to get the 12/24 hour format, the hours, minutes and seconds. A final press of the button returns to normal time display. One nice thing, you don't have to wait until the seconds become zero to set the clock, like you do with a RTC chip. Spin the rotary to any appropriate seconds value, and when the seconds match, press the button and the time is set.

The value of C1 and C2 depend on the rated load capacitance of the watch crystal you use. Unfortunately I used one from my partsbin and I didn't know what the rated capacitance was. I started with large capacitors and found the clock ran too slow. I gradually reduced the capacitors until the clock ran correctly. A tiny variable trimmer is useful for getting the clock right on time. I ended up with 2 to 4 pF on each leg of the crystal, so the crystal was probably a 6 pF load type.

Download C source code for the clock

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