RTC Working Not Working
eZ80 for RC2014™ and compatible backplanes
An eZ80 CPU Module that works within the RC2014™ ecosystem
16 Jul 2024
Well this was an interesting day. Spent much of it getting the Real Time Clock working. Created code for the on-chip firmware to initialise and configure the on-chip RTC. Added a RTC driver in my RomWBW branch to interface with it. All working perfectly.
My interface module only provides a small 2 pin connector for connecting an external 3V power source. I hacked up a connection to a 3V button battery. (The current PCB design does not have room for a battery holder).
At the end of the day - after getting it all working, I thought I would try to power the RTC with the main 3.3V power on the board. I jumpered the main 3.3V line to the RTC power input. But I think I might have over-volt’ed the CPU - as no matter what I did thereafter, the RTC would not work anymore. Inspected the clock/crystal with the oscilloscope, and measured all 0V - no oscillations!
The rest of the chip worked fine.
I had a closer look at how I traced the power lines for the RTC, and realised that it was a direct connection. I think at least I should have had a decoupler capacitor. Looking at the reference design for the eZ80 dev board, it looks like they also have a 100 ohm resistor between the main 3V3 line and the RTC power input. Hmm live and learn!
I will redesign the PCB - I think I can fit a small battery holder on the PCB - and just power it directly from a battery always.
I did try and replace the crystal, in the hope that maybe it was the fault - but alas - no luck. I will have to soldered up another eZ80. Time for some squinting to hand solder those tiny pins!