The Pokémon mini is a hand held created by Nintendo R\&D3 around 1999. It is based around the Epson S1C88 Core (then Timex/Seiko), and it derives most peripherals from this system.
The machine has a 4 KiB on-board OTP BIOS, 4 KiB of internal RAM (shared with the LCD Controller) and a 21-bit cartridge bus. The entire system is controlled by writing to 256 hardware registers, most of which are Open-Bus.
Little-endian 8-bit model 3 S1C88V20 (aka E0C88V20) clocked 4.00 MHz. This is a S1C88 family chip, potentially customized for Nintendo (as there is no public documentation for the V20).
It has both 8-bit and 16-bit register as well as 8-bit page registers for extended addressing.
External bus with 21 bits for address & 8 bits for data.
96x64 monochrome LCD, 1 bit per pixel (bpp), running at around a 75±10 Hz refresh rate. Toggling pixels allows to add a 3rd color with half tone.
It contains an on-glass controller which communicates on registers $20FE and $20FF as well as its own timer discussed below in Timers.
The rate at which the image is redrawn and copied to the LCD (and which stages of rendering are enabled) is configurable.
Single-channel square-wave with adjustable pulse-width played through a piezo speaker.
3 levels of volume: 0%, 50%, 100%. Commercial games achieve 25% and 75% by adjusting the pulse-width to achieve a weaker sound.
Can read back timer values (since sound is assigned to Timer 3).
For commercial games, hold the C button while booting up to start muted.
There are two oscillators and several timers/counters in the system. Additionally, the LCD has its own oscillator and timer, whose count value is exposed to the system.
OSC1 is 32768 Hz and is a low-power oscillator which cannot be disabled. OSC3 is 4.00 MHz and can be disabled by software. Entering sleep mode (but not halt mode) disables the oscillators.
1.5V (converted to 3.3V internally) with 1 AAA Battery