Battery Power and clocks
eZ80 for RC2014™ and compatible backplanes
An eZ80 CPU Module that works within the RC2014™ ecosystem
10 Aug 2024
A new PCB arrived last week. This iteration, V1.5, has a number of improvements, including a small battery holder for CR1216/1225 3V Lithium Batteries.
This battery supplies power to the on-chip Real-Time-Clock circuit. So now the eZ80 remembers the time.
But in addition to keeping time, I now have a known and consistent clock signal. I can use this clock signal for other features:
- Generating a 50Hz or 60Hz system level timer counter (similar to RomWBW HBIOS system timer).
- Detect the current CPU operating frequency.
As the main CPU oscillator is mounted in a 8 PIN socket, I can easily change the frequency that drives the CPU. I have a number of compatible oscillators on hand that I can test the board and CPU at: (7.3728 MHz, 14.745 MHz, 16 MHz, 18.432 MHz, 20 MHz, 24 MHz, 25 MHz, 32 MHz).
The eZ80 variant I have selected (eZ80F92) is rated at a maximum of 20 MHz. But my experiments demonstrate that it can works just as reliability when overlocked to 32MHz. Maybe even more.
When I change the clock frequency though, without changing the UART’s selected clock divider for its baud rate generator, I needed to adjust my PC’s serial baud rate. For example, if I double the clock frequency, I would need to double the baud rate of my connecting serial adapter.
As I now can reliably detect at boot up, the current installed CPU frequency, I have updated the software to configure the on-chip baud rate divider to always ensure I have the desired baud rate (typically configured to 115200).
This speeds up the testing of the system at different CPU clock frequency, across a range of RC2014/RCBus modules – helping me to verify I have good level of general compatibility.
RomWBW Boot
Here is an example of a current boot configuration for the system.
