Connector Standards
Debug connectors come in several standard pinouts. Knowing which one your board uses determines what probe and cable you need. This page covers the common standards you will encounter.
ARM Cortex Debug Connector (10-pin, 1.27mm)
Section titled “ARM Cortex Debug Connector (10-pin, 1.27mm)”The most common connector on modern Cortex-M boards. 10 pins, 1.27mm (0.05”) pitch, dual-row. Defined by ARM in the CoreSight technical reference.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | VTref | 2 | SWDIO/TMS |
| 3 | GND | 4 | SWCLK/TCK |
| 5 | GND | 6 | SWO/TDO |
| 7 | Key (no pin) | 8 | TDI |
| 9 | GNDDetect | 10 | nRESET |
Notes:
- Pin 7 is keyed (no pin present) to prevent reverse insertion
- Supports both SWD (pins 2, 4, 6, 10) and JTAG (pins 2, 4, 6, 8, 10) on the same connector
- SWO (pin 6) is optional — used for Serial Wire Output trace data
- VTref (pin 1) is the target voltage reference, not a power supply
Common part numbers:
- Header: Samtec FTSH-105-01-F-DV-K (shrouded) or equivalent 2x5 1.27mm
- Cable: Samtec FFSD-05-D-xx (flat flex), or standard 10-pin 1.27mm ribbon
Legacy ARM JTAG Connector (20-pin, 2.54mm)
Section titled “Legacy ARM JTAG Connector (20-pin, 2.54mm)”The original ARM debug connector. 20 pins, 2.54mm (0.1”) pitch. Still found on older boards, evaluation kits, and FPGA dev boards. Half the pins are GND.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | VTref | 2 | VSupply |
| 3 | nTRST | 4 | GND |
| 5 | TDI | 6 | GND |
| 7 | TMS / SWDIO | 8 | GND |
| 9 | TCK / SWCLK | 10 | GND |
| 11 | RTCK | 12 | GND |
| 13 | TDO / SWO | 14 | GND |
| 15 | nRESET | 16 | GND |
| 17 | DBGRQ | 18 | GND |
| 19 | DBGACK | 20 | GND |
Notes:
- Even-numbered pins (except 2) are all GND — this provides shielding for the signal lines
- Pin 2 (VSupply) is optional target power
- RTCK (pin 11) is return clock for adaptive clocking — rarely used on modern targets
- DBGRQ/DBGACK are legacy signals from the ARM7/9 era
- SWD signals are aliased on pins 7, 9, 13 — a 20-pin connector can carry SWD without an adapter
ARM Cortex Debug+Trace Connector (20-pin, 1.27mm)
Section titled “ARM Cortex Debug+Trace Connector (20-pin, 1.27mm)”Extended version of the 10-pin connector with ETM trace signals. 20 pins, 1.27mm pitch. Used when you need instruction trace capture.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | VTref | 2 | SWDIO/TMS |
| 3 | GND | 4 | SWCLK/TCK |
| 5 | GND | 6 | SWO/TDO |
| 7 | Key (no pin) | 8 | TDI |
| 9 | GNDDetect | 10 | nRESET |
| 11 | GND/Key | 12 | TRACECLK |
| 13 | GND/Key | 14 | TRACEDATA[0] |
| 15 | GND | 16 | TRACEDATA[1] |
| 17 | GND | 18 | TRACEDATA[2] |
| 19 | GND | 20 | TRACEDATA[3] |
Notes:
- Pins 1—10 are identical to the 10-pin connector
- Pins 12—20 carry parallel trace data (4-bit wide)
- A 10-pin cable works on a 20-pin header (connects to the first row only)
- mcjtag does not currently support trace capture, but the connector carries SWD/JTAG signals that mcjtag uses
STDC14 (STMicroelectronics 14-pin)
Section titled “STDC14 (STMicroelectronics 14-pin)”ST’s 14-pin connector, found on newer STM32 Nucleo and Discovery boards. 1.27mm pitch.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | NC | 2 | VTref |
| 3 | NC | 4 | SWDIO/TMS |
| 5 | GND | 6 | SWCLK/TCK |
| 7 | GND | 8 | SWO/TDO |
| 9 | Key (no pin) | 10 | TDI |
| 11 | NC | 12 | nRESET |
| 13 | VCP_RX | 14 | VCP_TX |
Notes:
- Pins 2—12 map to the standard ARM 10-pin signals (with offset)
- Pins 13—14 provide a Virtual COM Port (VCP) for UART through the ST-Link — useful for serial debug output
- Backwards-compatible: a 10-pin cable can mate with pins 2—12 (with a 14-to-10 adapter or careful alignment)
Probe-to-Connector Compatibility
Section titled “Probe-to-Connector Compatibility”Which probes work with which connectors without adapters:
| Probe | 10-pin (1.27mm) | 20-pin (2.54mm) | 20-pin (1.27mm) | STDC14 |
|---|---|---|---|---|
| DAP-Link | Varies by board | Varies by board | — | — |
| ST-Link V2 | — | Yes (included cable) | — | — |
| ST-Link V3 | — | — | Yes (STDC14 adapter available) | Yes |
| J-Link | Yes (included adapter) | Yes (included cable) | Yes (adapter) | — |
| FTDI-based | Depends on breakout board | Depends on breakout board | — | — |
Most probes ship with a 20-pin 2.54mm cable. For 10-pin 1.27mm targets, you typically need:
- A 20-to-10 adapter board (Segger, Olimex, and others sell these)
- Or a probe that natively supports 10-pin (J-Link with the needle adapter)
Adapter Cables
Section titled “Adapter Cables”Common adapters for converting between connector standards:
| From | To | Part / Source |
|---|---|---|
| 20-pin 2.54mm | 10-pin 1.27mm | Segger J-Link 20-to-10 adapter, Olimex ARM-JTAG-20-10 |
| 20-pin 2.54mm | 20-pin 1.27mm | Segger 20-pin adapter |
| STDC14 | 10-pin 1.27mm | ST STDC14-to-MIPI10 adapter |
When in doubt, buy a probe that matches your board’s connector natively. Adapters add length and potential signal integrity issues at higher clock speeds.
Tag-Connect (Pogo Pin)
Section titled “Tag-Connect (Pogo Pin)”Tag-Connect is a pogo-pin system that eliminates the need for a permanent header on the target board — just a pattern of pads and locating holes on the PCB.
| Model | Pins | Pitch | Use Case |
|---|---|---|---|
| TC2030 | 6 | 1.27mm | SWD (minimal) |
| TC2050 | 10 | 1.27mm | Full ARM 10-pin |
| TC2070 | 14/20 | 1.27mm | JTAG with trace |
Tag-Connect comes in two variants:
- Legged (TC20xx-NL) — retaining clips hold the connector against the board hands-free
- No-leg (TC20xx) — you hold it in place manually, useful for quick production checks
Tag-Connect is popular for production programming and space-constrained designs where a permanent header is not practical. The footprint on the PCB is much smaller than a shrouded connector.
For mcjtag use, the cable plugs into your probe’s standard connector on one end and presses onto the board’s pads on the other.
Identifying Your Board’s Connector
Section titled “Identifying Your Board’s Connector”If you are not sure which connector your board has, check these things:
- Count the pins. 10-pin and 20-pin are most common. If it is 14-pin, it is almost certainly STDC14.
- Measure the pitch. 1.27mm pins are visibly smaller and closer together than 2.54mm.
- Look for a key. Shrouded connectors with a notch or missing pin are keyed to prevent reverse insertion.
- Check the board schematic. The connector is usually labeled in the silkscreen or documentation.
For wiring details and probe configuration, see Hardware Setup.