JTAG / SWD Connector Pinout
ARM 10-pin Cortex Debug and 20-pin JTAG pinouts + the two-wire SWD wiring.
What it does: Look up each pin of the ARM debug connectors and work out the minimum wires SWD needs.
When to use it: When soldering a debug header, connecting a J-Link/ST-Link, or hand-wiring SWD.
Disclaimer: This result is a reference estimate. For actual production, refer to the device datasheet / local regulations as authoritative.
| Pin | Signal | Notes |
|---|
MEANS Pure SWD? needs only SWDIO/SWCLK/GND (+VTREF for stability).
No history yet. Each calculation is automatically saved to this device.
How to use the JTAG/SWD pinout reference
Pick a connector → identify the two SWD wires → check power and orientation.
- 01
Pick a connector
Modern ARM boards mostly use the Cortex 10-pin (1.27 mm); older boards use the 20-pin JTAG (2.54 mm).
- 02
Identify the two SWD wires
To debug an ARM chip only, SWD needs just SWDIO + SWCLK (plus GND/VTREF).
- 03
Check power and orientation
VTREF lets the probe match the target voltage level; nRESET is active-low. Find pin 1 by the connector key/notch.
Minimal SWD wiring
Just want to debug over SWD? These few wires are enough.
| Signal | Necessity | Notes |
|---|---|---|
| SWDIO | Required | Bidirectional data |
| SWCLK | Required | Clock (driven by the probe) |
| GND | Required | Common ground |
| VTREF | Strongly recommended | Lets the probe match the target IO levels |
| nRESET | Recommended | Reset / recover when the connection hangs |
| SWO | Optional | Trace / printf output |
ARM SWD interface convention.
Common questions, answered in 3 minutes
How are JTAG and SWD related?
SWD is ARM's two-wire debug interface (SWDIO+SWCLK), reusing JTAG's TMS/TCK pins; the same connector can run either JTAG or SWD, and SWD uses fewer wires.
Does VTREF have to be connected?
Strongly recommended. The probe uses it to sense the target IO voltage and match levels; without it the levels may mismatch or the target may not be detected.
What is the minimum number of wires for SWD only?
SWDIO, SWCLK and GND — three wires — will run; adding VTREF is more reliable, adding nRESET helps recover from a hung target, and SWO is used for trace/printf.
Why is pin 7 of the 10-pin empty?
That is the keying notch (KEY), used to guarantee a single plug orientation; it is not a signal pin.
Can I draw power from pin 19 of the 20-pin?
It depends on the adapter. Some J-Link/ST-Link units supply target power on pin 19, others leave it unconnected — always check the manual for your specific debugger, as miswiring can damage the target board.
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| 10-pin | ARM Cortex Debug | ARM CoreSight |
| 20-pin | ARM JTAG (legacy) | ARM / adapter convention |
The power pins (especially pin 19 of the 20-pin) vary by adapter; always check your debugger's manual and the target board schematic before wiring.