The SPI ePaper screens does not include any fonts and need to generated by code. To make a graphical rich display you need lots of storage.
- On board Flash of your MCU.
- SD card.
- Flash chip.
The on board flash of MCU chips is normally up to 512Kbytes. The high end with more Flash are normally big chips and high powerful chips that use power.
SD cards have lots of merit. Easy to get and work with. But is power hungry and needs 3.3 V. Can't use a coin cell with out a boost converter. For a clock that updates every minute not good.
I'm using SPI Flash chip. They use not mush power and can run to 1.8V/2V The are uselessly in bits, so the 32Mbits I use is 4Mbytes. Can hold lots of different font styles
Format and layout
Each pixel is a bit. The internal resolution of the 2.13 display is 250 x 128 , so to fill up the screen will take 4000 bytes
My clock works with a left side
and a right side
For a given clock font 24 x 2 (24 / am pm) images left . And 60 for minutes for right .