I got my hands on a 4-digit clock DIY kit. Soldering it is rather easy, and it works as advertised.
One downside is its power appetite : it slurps a whopping 100mA constantly, which means that operating it from a battery is going to be an very tedious & expensive proposition.
Upon close look, there is actually a 5V DC connector and a 3V one. The 5V is obviously for a wall socket DC adapter, and the 3V one is for a battery indeed, but only to retain the time, as it cannot power the LED, nor the alarm. I specifically didn’t want the alarm to sound, so i didn’t solder the PNP, nor the buzzer.
Quick hack to reduce the power usage
That made me think that the biggest contributor to the power is maybe the 4 digit 7 segments LED. Upon close look it shows that the 4 digits are lit in a circular fashion, but for a particular digit, the OFF state of a segment is done via pulling the IO pin low, sourcing the current via the IO pin instead of via the LED. Very simple but not very efficient.
So, I changed the pull-up 470Ω resistor array to a 10kΩ one to limit the current passing via the LED or the IO pins. This worked by immediately lowering the 100mA to 7 mA. The luminosity was rather low, but I don’t mind as this works perfectly in a dark room. As it is the purpose of that clock, and before it was too bright anyway.
That enabled a pack of 3 AA NiMH cells (2000mAh) to last a little less than a week before needing to recharge. Not bad for such a small hack.
That said, I do really like that the hour-minute separator blinks, as it shows it is working, that there’s still enough battery. This requires to reprogram the MCU.
This clock is powered by the venerable and well-known AT89C2051. The annoying part is that programming it requires a HVPP1 that operates on 12V and with a parallel bus. This means that I cannot simply leverage an USBasp programmer.
As usually, the AT89 official programmer is quite expensive, and not super easy to use, so I have to come up with another alternative.
It ticks mostly all of my needs, smashing many of the existing projects into a pretty nice feature-full version :
- On board 12V DC generation.
- Using a hardware-based 12V regulation (Zener).
- Comes with a portable CLI-based programmer on the desktop.
- Arduino UNO, less expensive than the Arduino MEGA.
Fortunately the lock bits were not set, so I could dump the firmware before to do some analysis. I struggled for a while in not being able to dump the firmware via the programmer. But it turned out there’s a small timing bug. Once fixed, the dumping works flawlessly.
I convert it to ASM via dis51, but unfortunately it doesn’t really make sense to me.
So, while the UNO programmer HAT is working nicely, I still want to be able to leverage a bare ATmega328 for it. My ideal goal would be to leverage avrdude, emulating an USPasp device.
High Voltage Parallel Programmer ↩