UCMOD Connector Specification

Specification Status

Status	Revision	Date	Notes
Draft	`Draft v0.1`	August 2026	Initial draft definition of Main and Auxiliary connectors
Draft	`Draft v0.2`	September 2026	Main Connector Pin 4 redefined from `RESERVED` to `/RESET` (active-low module reset). Modules implementing the pin are responsible for ensuring the microcontroller resets when it is driven low

[!NOTE] Draft Specification: This document defines the draft specification for the UCMOD microcontroller module interface. The pinout, peripheral mappings, and physical form factors are currently under active development and evaluation.

The UCMOD specification defines a modular, standard interface between microcontroller modules and carrier boards. It allows microcontroller modules to be swapped or upgraded without requiring a complete redesign of the host carrier board.

You can follow the ongoing design and development on www.jktdev.co.uk.

Design Philosophy

The core goal of UCMOD is to establish predictable pin locations for common peripherals across a wide range of devices—from small 8-bit microcontrollers to feature-rich 32-bit microcontrollers.

Standardised Capability Locations: UCMOD standardises where capabilities appear on the connector; it does not standardise or constrain the internal capabilities of the microcontroller itself.
Predictable Fallbacks: Every signal pin has one defined primary function and a corresponding GPIO fallback (Primary Function / Generic GPIO Number).
Carrier Board Flexibility: Carrier boards are not required to use a pin for its primary peripheral. If the carrier board does not need that peripheral, the pin can be configured and used as a general-purpose I/O (GPIO).
Developer Responsibility: The specification does not remove the developer's responsibility to understand the target microcontroller's electrical characteristics, peripheral limitations, or pin mappings.

1-to-1 Microcontroller Pin Mapping Rule

[!IMPORTANT] Core Mapping Rule: A physical microcontroller pin should normally map to exactly one UCMOD signal position.

Multiple UCMOD positions connected to the same physical microcontroller pin are an exceptional implementation and must be explicitly documented, preferably using configurable links (such as 0 Ω resistors or solder bridges) rather than permanent electrical connections.

This rule ensures that signals appearing on different connector pins are electrically independent. If permanent aliasing is unavoidable, the affected positions must be clearly identified in the module's schematic and documentation.

UCMOD Main Connector (20-Pin)

The main UCMOD connector provides power, ground, and 12 standard GPIO positions mapped to primary communication and analogue interfaces.

Main Connector Pinout

Pin	Function	Pin	Function
1	`GND`	2	`3V3`
3	`GND`	4	`/RESET`
5	`I2C_SCL / GPIO0`	6	`I2C_SDA / GPIO1`
7	`UART_TX / GPIO2`	8	`UART_RX / GPIO3`
9	`SPI_MOSI / GPIO4`	10	`SPI_MISO / GPIO5`
11	`SPI_SCK / GPIO6`	12	`SPI_CS / GPIO7`
13	`AN0 / GPIO8`	14	`AN1 / GPIO9`
15	`AN2 / GPIO10`	16	`AN3 / GPIO11`
17	`RESERVED`	18	`GND`
19	`3V3`	20	`GND`

Peripheral Mapping Guidelines

I2C: Primary I2C bus (I2C_SCL on Pin 5, I2C_SDA on Pin 6).
UART: Primary serial UART interface (UART_TX on Pin 7, UART_RX on Pin 8).
SPI: Primary SPI bus with dedicated chip select (SPI_MOSI, SPI_MISO, SPI_SCK, SPI_CS on Pins 9–12).
Analogue Inputs: 4 analogue-capable inputs (AN0–AN3 on Pins 13–16).
Power Rails: Dual redundant 3.3 V logic power (3V3) and ground reference (GND) pins.
Reset: Active-low reset input (/RESET on Pin 4).
Reserved Pins: Pin 17 is reserved for future specification use and should be left unconnected on carrier boards unless specified otherwise.

Reset Signal (`/RESET`)

Pin 4 of the Main Connector is the module reset line, active low:

Carrier boards may assume that driving /RESET low resets the module and that releasing it high allows normal operation.
It is the responsibility of the UCMOD module to ensure that the microcontroller actually resets when /RESET is driven low. A module that implements this pin must provide whatever circuitry is required (for example, a pull-up, reset supervisor, or logic that maps the low level onto the microcontroller's native reset mechanism) so that an active-low level on Pin 4 reliably resets the module.
A module that cannot support an externally driven reset is an exception to this expectation and must explicitly document that /RESET is non-functional on that module.
When not driven by the carrier board, /RESET must be treated as a no-connect by the carrier board; the module is responsible for ensuring its microcontroller boots normally in this state.

UCMOD Auxiliary Connector (20-Pin)

For more capable microcontrollers requiring additional interfaces, the UCMOD Auxiliary Connector provides secondary communication busses, dedicated interrupt lines, CAN bus connectivity, and microcontroller-specific UNIQUE pins.

Auxiliary Connector Pinout

Pin	Function	Pin	Function
1	`GND`	2	`AUX_UART_TX / GPIO12`
3	`3V3`	4	`AUX_UART_RX / GPIO13`
5	`AUX_I2C_SCL / GPIO14`	6	`AUX_I2C_SDA / GPIO15`
7	`AUX_SPI_SCK / GPIO16`	8	`AUX_SPI_MISO / GPIO17`
9	`AUX_SPI_MOSI / GPIO18`	10	`AUX_SPI_CS / GPIO19`
11	`AUX_IRQ / GPIO20`	12	`UNIQUE / GPIO21`
13	`UNIQUE / GPIO22`	14	`UNIQUE / GPIO23`
15	`UNIQUE / GPIO24`	16	`UNIQUE / GPIO25`
17	`AUX_CAN_TX / GPIO26`	18	`3V3`
19	`AUX_CAN_RX / GPIO27`	20	`GND`

Signal Details

Secondary Busses (AUX_): The AUX_ prefix denotes secondary communication interfaces (secondary UART, I2C, and SPI) provided in addition to the main connector.
Interrupt Request (AUX_IRQ): Pin 11 provides a dedicated external interrupt line.
CAN Controller Interface: Pins 17 and 19 provide logic-level AUX_CAN_TX and AUX_CAN_RX lines. This definition is controller-agnostic, supporting Classical CAN, CAN FD, or other compatible implementations connecting to an external transceiver on the carrier board.
UNIQUE Pins (Pins 12–16):
Dedicated escape hatch for microcontroller-specific capabilities that do not fit into common peripheral mappings (such as DAC outputs, dedicated VREF+, specialized timer channels, or USB signals).
If a carrier board does not need the unique function, these pins fall back to standard GPIO21 through GPIO25.
[!WARNING] > Carrier Board Portability: > Carrier boards that depend on specific UNIQUE functions become tied to the particular microcontroller family providing that feature. For maximum module interchangeability, carrier designs should prefer using standard peripheral pins or use UNIQUE pins solely as GPIOs.

Module Form Factors

The UCMOD physical standard is planned around two standard form factors:

Form Factor	Connectors Implemented	Target Microcontrollers	Use Case
Small	Main Connector Only (20-pin)	ATtiny1614, CH32V003 / CH32V004 / CH32V005	Compact, cost-effective nodes and simple peripheral controllers
Large	Main + Auxiliary Connectors (2 × 20-pin)	STM32G0B1CBT6, ESP32-C6 / ESP32-S3, RP2040, GD32V series	Feature-rich controllers needing dual busses, CAN, USB, or high I/O counts

Standardized mechanical dimensions and connector placements (to be defined) will ensure that any Small or Large UCMOD module can be cleanly seated on compatible carrier boards.