Input/output: the Pico 2 W
Everything modern goes through a Raspberry Pi Pico 2 W: USB, the microSD card, Wi-Fi, the clock. Like the video, it presents only 32 registers to the processor.
The role of the Pico 2 W
The Raspberry Pi Pico 2 W (RP2350 chip) takes care of:
- USB: it is the host of a 7-port hub;
- the microSD card, over SPI, with the FAT file system;
- Wi-Fi;
- the real-time clock and the I²C bus;
- a debugging serial console;
- supplying the boot code, which the 9K writes into PSRAM.
Its 32 registers appear in the page $00:9E00 – $00:9EFF. According to the register map
(v0.1, to be validated), the Pico mainly serves as a mailbox: the processor writes arguments, then
a command code (open, read or write a file, read the clock…), and the Pico signals completion. The
keyboard, the mouse, the gamepads and the serial console have direct registers. See The register
map.
USB: a 7-port hub
The Pico’s USB port feeds a 7-port FE2.1 hub:
- 5 USB-A sockets at the rear (two stacked doubles and one single);
- an internal 2 × 5 connector for 2 front-panel sockets.
Each port has its own 500 mA resettable fuse. The ports are USB Full Speed (12 Mbit/s), shared. On the software side, the Pico will use the TinyUSB library, configured to handle at least 7 devices. The Pico is programmed over SWD (3 pins).
The microSD card
The microSD reader is connected to the Pico over SPI, and it is the Pico that manages the FAT file system. The processor does not read the sectors itself: it requests file operations from the Pico, through its registers.
The clock and other connections
- Real-time clock DS3231 (fallback: PCF8563), with a CR2032 battery, on the Pico’s I²C.
- A stacked double DE-9 connector, wired like Atari and Sega joysticks.
- An internal I²C connector, a debugging serial link on the Pico.
- A reset button and status LEDs: power, bus, SD card.