Nova64/CARTRIDGE_PINOUT.md

# Nova64 Cartridge Pinout Overview

This document provides a compact, high-level technical view of the Nova64 cartridge connector from the cartridge developer perspective.

## Purpose

A concise 32-pin cartridge interface for expansion modules, storage, I/O, and optional power delivery.

## Connector Goals

- single edge connector for compact form factor
- separate power, data, and control signals
- support for smart cartridges and passive media
- keep SDMMC always available for cartridge-level storage access
- leave space for future expansion without extra pins
- describe pin numbering and signal roles from the cartridge edge view

## 32-Pin Layout

| Pin | Signal        | Role                                    | Default idle state |
| --: | ------------- | --------------------------------------- | ------------------ |
|   1 | GND           | ground                                  | grounded           |
|   2 | VCC_3V3       | supply                                  | N/A                |
|   3 | CARTRIDGE_DET | cartridge presence detect               | pull-up            |
|   4 | POWER_CTRL    | power direction control                 | pull-down          |
|   5 | USB_FLAG      | USB-capable flag                        | pull-up            |
|   6 | USB_DP        | USB D+                                  | pull-down          |
|   7 | USB_DM        | USB D-                                  | pull-down          |
|   8 | USB_ID        | USB host/device mode                    | pull-up            |
|   9 | USB_VBUS      | VBUS sense / optional power             | pull-down          |
|  10 | SD_CMD        | SDMMC command / host->cartridge control | pull-up            |
|  11 | SD_CLK        | SDMMC clock / host->cartridge           | pull-down          |
|  12 | SD_D0         | SDMMC data0 (bi-directional)            | pull-up            |
|  13 | SD_D1         | SDMMC data1 (bi-directional)            | pull-up            |
|  14 | SD_D2         | SDMMC data2 (bi-directional)            | pull-up            |
|  15 | SD_D3         | SDMMC data3 (bi-directional)            | pull-up            |
|  16 | I2C_SCL       | I2C clock                               | pull-up            |
|  17 | I2C_SDA       | I2C data                                | pull-up            |
|  18 | I2S_SCLK      | I2S bit clock                           | pull-down          |
|  19 | I2S_WS        | I2S word select / frame clock           | pull-down          |
|  20 | I2S_SDOUT     | I2S data out (cartridge->host)          | pull-down          |
|  21 | I2S_SDIN      | I2S data in (host->cartridge)           | pull-down          |
|  22 | I2S_MCLK      | I2S master clock                        | pull-down          |
|  23 | SPI_MOSI      | SPI MOSI (host->cartridge)              | pull-down          |
|  24 | SPI_MISO      | SPI MISO (cartridge->host)              | pull-down          |
|  25 | SPI_SCLK      | SPI clock (host->cartridge)             | pull-down          |
|  26 | SPI_CS0       | SPI chip select (host->cartridge)       | pull-down          |
|  27 | CAN_H         | CAN high                                | recessive bias     |
|  28 | CAN_L         | CAN low                                 | recessive bias     |
|  29 | UART_RX       | UART RX (host->cartridge)               | pull-up            |
|  30 | UART_TX       | UART TX (cartridge->host)               | pull-up            |
|  31 | VCC_5V        | supply                                  | N/A                |
|  32 | GND           | ground                                  | grounded           |

## Signal Groups

- Ground: pins 1, 32
- 3.3V supply: pin 2
- Cartridge detection: pin 3
- Power direction: pin 4
- USB flag: pin 5
- USB: pins 6-9
- SDMMC: pins 10-15
- I2C: pins 16-17
- I2S audio: pins 18-22
- SPI: pins 23-26
- CAN: pins 27-28
- UART: pins 29-30
- 5V supply: pin 31

## Design Rationale

- Keep storage and interface signals separate from power.
- Reserve the SDMMC channel so cartridges can provide their own software/data layer.
- Include USB and serial buses for smart module support.
- Use a dedicated ID input for simple cartridge recognition.
- Provide optional audio and GPIO for versatile cartridge types.

## Notes

- This is a high-level connector map, not a full electrical specification.
- Exact pin assignments may be refined later during hardware design.
- Preferred placement for pull-ups / pull-downs is the main device, with cartridge-side resistors only used for optional or cartridge-specific functions.
- Cartridge presence should be detected via a dedicated `CARTRIDGE_DET` pin, not by ORing multiple ID signals.
- Cartridge features such as SD/storage, I2C, SPI, CAN, UART and audio should be discovered dynamically at runtime after insertion.
- For bidirectional power delivery, the system should use a dedicated power-path controller and a clear source priority scheme.
  - Monitor main device and cartridge voltage presence before enabling any power path.
  - Use POWER_CTRL as a control input for a power-path switch or enable pin, not as a current-carrying line.
  - If the main device is powered and the cartridge is not actively supplying power, keep cartridge supply disabled.
  - If the main device is unpowered and a powered cartridge is inserted, allow the cartridge to supply the system after voltage stability is confirmed.
  - Prevent backfeed by using ideal diodes or bidirectional power-path ICs that only conduct from the active source.

Suggested cartridge categories:

|          Category | Meaning                                                       |
| ----------------: | ------------------------------------------------------------- |
|     software only | cartridge provides software/application data only             |
|  special hardware | cartridge includes additional hardware functions              |
| dynamic detection | actual capabilities are determined at runtime after insertion |

USB_FLAG = 1 indicates the cartridge can present USB functionality in addition to its category.

POWER_CTRL = 1 indicates the cartridge provides power to the main device

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A short technical reference for the Nova64 cartridge connector, intended for initial design discussion.