COLL-ARM20-14P-050 · 20-pin ARM to STDC14 (1.27 mm)

Short description

Adapter board from the 20-pin ARM debugger connector (2 × 10 pin, 2.54 mm) to the 14-pin 1.27 mm (0.05″) connector (1.27 mm) used by the ST-LINK V3 and by Tag-Connect's STDC14 cables (Plug-of-Nails™). The debug lines run through series resistors; every signal is brought out individually for probing.

Connections

Full pin assignment: further down this page.

Compatible: SEGGER J-Link, Keil ULINK, ST-LINK/V2 and the other debuggers using the classic 20-pin 2.54 mm ARM connector.

Easily confused: There is also a 20-pin Cortex connector on 1.27 mm (IAR I-Jet, Keil ULINK Pro, SEGGER J-Trace) — same pin count, half the pitch.

Application

With the ST-LINK V3, ST made the 14-pin 1.27 mm connector the standard, and Tag-Connect offers its TC2030, TC2050 and ECV3 cables in an STDC14 version. Anyone who wants to run these cables from a debugger with the classic 20-pin connector — J-Link, ULINK, older ST-LINKs — needs the transition from 2.54 to 1.27 mm. This board is that transition, for all STDC14 cables at once.

Series resistors — signal smoothing

nTRST, TDI, SWDIO, SWCLK, RTCK and SWO each run through 39 Ω. The resistor sits directly in the signal path and matches the driver output to the line, so overshoot and ringing come out smaller. That pays off above all with longer cables and high clock rates, where the edges would otherwise start to ring. The technical term for this is series termination; what is at stake is signal integrity on the run between debugger and target.

The 39 Ω are a default, not a fixed value. The resistors are 0603 and can be swapped with an ordinary soldering iron.

Five values are included

Ten resistors each in 22 Ω, 27 Ω, 33 Ω, 39 Ω and 43 Ω come with the board — 0603 package, the same as fitted. That lets you match the damping to your own circuit without ordering anything first: measure, swap the value, measure again.

Reset can be isolated

The reset line carries a 0 Ω resistor. It can be desoldered, which separates the debugger's reset from the target — useful wherever the target circuit drives its own reset.

Test points for the oscilloscope

Every signal is additionally brought out to its own via. There is also a bare copper area with a via as a ground pickup, allowing a short ground connection at the probe instead of the long clip — the clip's ground loop creates ringing that is not present in the circuit at all.

Powering the target from the debugger — solder bridge, open as shipped

The bridge is open as shipped; the debugger reads the target's operating voltage via VREF and feeds nothing in. Closing the bridge puts the debugger's supply on the target side. Which voltage your debugger carries there is in its documentation; check it against the target circuit before closing the bridge.

The Virtual COM Port stays reachable

On pins 13 and 14 of the 14-pin connector, the ST-LINK V3 carries a UART — the Virtual COM Port many developers use for their console output. A 20-pin debugger does not have these lines. This board therefore brings them out to a separate six-pin pad field, together with VREF and ground. A pin header can be soldered in there — it is not included. A USB-to-UART adapter with matching I/O voltage — 3.3 V for a 3.3 V target — then attaches to it, and the target's console stays available even though the debugger provides none. Via VREF, a level-shifting adapter can pick up the target's I/O voltage directly.

Both UART lines run through 100 Ω each. That limits the current should target and adapter operate at different I/O voltages. Pins 1 and 2 are reserved by ST and are only passed through.

Pin assignment

Which signal sits where, and what is in the signal path:

nTRST on a jumper — shipped fitted

On SEGGER J-Link, Keil ULINK and the 20-pin ST-LINK/V2, pin 3 of the debugger connector is the nTRST output for JTAG. On this board it goes through 39 Ω and a pin jumper to pin 11 on the 14-pin side. The jumper is fitted as shipped; JTAG with TRST works without any intervention.

In ST's assignment pin 11 is GNDDETECT, corresponding to pin 9 of the ARM Cortex 10-pin connector. ARM's reference assignment puts this pin on ground; many target boards follow it, others pull it high (Silicon Labs starter kits) or leave it open. If your target ties it to ground, pull the jumper — the debugger's nTRST output then stays unloaded. No soldering iron needed.

Whether the target sees the pin at all depends on the cable: the 6-pin Tag-Connect cables (TC2030, ECV3-06) carry no GNDDetect — there the jumper can always stay in. With the 10- and 14-pin cables (TC2050, ECV3-10, ECV3-14) the pin does reach the target.

The six-pin pad field:

Matching cables

All STDC14 versions from Tag-Connect:

Everything on one board

Damping, connector conversion and test points sit on one board. Assembled from separate parts, that means several adapters in a chain — each with its own transition in the signal path, and every transition is a place where the edge you were trying to smooth takes a hit.

Technical data

• Matching cables: all Tag-Connect cables with STDC14 connector (see table)
• Interface: ARM Cortex SWD and JTAG (TDI, RTCK connected), SWO routed
• Series resistors 39 Ω: nTRST, TDI, SWDIO, SWCLK, RTCK, SWO — 0603, replaceable
• Reset line: 0 Ω, desolder to isolate
• nTRST: 39 Ω via pin jumper, fitted as shipped, removable without soldering
• Six-pin pad field, header not fitted: VREF, STDC14 pins 1, 2, 13, 14 (100 Ω each), ground; 13/14 = Virtual COM Port UART
• Test points: one via per signal, plus a bare ground area with via
• Powering the target from the debugger: solder bridge, open as shipped

Included

The adapter board and the resistor set: ten each of 22 Ω, 27 Ω, 33 Ω, 39 Ω and 43 Ω in 0603 package. Cables, debugger and the pin header for the six-pin pad field are not included.