Type-C SWD VCP USB One-Cable ST-LINK

Abstract
This paper proposes an ST-LINK/V2-1 wiring scheme based on the Type-C interface debug accessory mode. It achieves simultaneous support for SWD debugging, VCP virtual serial port, and downstream USB debugging functions with a single connection, and supports reversible insertion, significantly improving convenience compared to traditional connection methods. To verify the feasibility of the scheme, a PCB was designed based on the STM32F103CBT6 and CH334F, validating the design. The designed PCB, in addition to the proposed one-line Type-C male connector, provides a small-size standard STDC-14 debug interface for ST-LINK/V3 Minie power supply and implements overcurrent and reverse current protection for downstream power supply, demonstrating practical value. Finally, this paper provides detailed schematics, PCB design, and BOM for readers' reference.
Introduction
When designing a microcontroller minimum system, the design of the debug interface significantly affects the PCB layout and debugging convenience. Common debug interface designs based on pin headers, test points, etc., suffer from the following problems:

large size, occupying PCB space,
requiring dedicated ribbon cables or probes to connect to the debug interface
, inconsistent pin order requiring repeated reference to pin definitions and DuPont wire connection (time-consuming and laborious), easy to connect incorrectly,
insufficient protection measures for the 5V or 3V3 power supply provided by the debugger (potentially causing backflow into the computer's USB interface) , and
the debug interface being enclosed inside the development board, requiring additional openings on the casing.

While the design of integrating ST-LINK on the development board, as used in the official STM32 Discovery series development boards, improves the convenience of connecting to the computer, an additional USB cable is needed to connect to the development board if the MCU's USB function is required simultaneously. Furthermore, the onboard ST-LINK design significantly increases the development board's area and cost.
The solution proposed in this paper reuses the Type-C interface on the development board, achieving SWD debugging, VCP virtual serial port, and downstream USB connection with a single connection without any additional area or components. The MCU being debugged only needs a common, inexpensive 12/16-pin Type-C interface to simultaneously use SWD debugging and USB functionality, supporting reversible insertion. If the development board does not require USB functionality, the USB data cable can be reused as a VCP virtual serial port, and the debugger can automatically recognize the USB reused as VCP. If the development board uses a 24-pin Type-C interface, SWD debugging, VCP virtual serial port, and USB functionality can be performed simultaneously. For the 24-pin interface, the routing on the debugger side has been optimized, reducing the complexity of routing fan-out on the development board side.
It is worth noting that, based on the Debug Accessory mode defined by the Type-C specification, the Type-C debug port on the development board can also be used as a regular USB data interface, and the development board can detect whether a debugger or a regular USB cable is connected to the debug interface, thus allowing for more application scenarios. The JTAG interface, a standard feature
in relevant work
, and the large 20-pin Simplified Adapter on the genuine ST-LINK V2 are examples of improving the wiring experience through standardized debug interface pin definitions. However, the 20-pin connector is simply too large and unsuitable for small development boards. The ST-Link V3 series' STDC-14 debug interface significantly reduces size compared to the 20-pin Simplified Adapter; however, the risk of reverse connection still exists on the development board side, and the cheapest (but still expensive) ST-LINK/V3 Minie cannot provide downstream power. The STLINK from GeekSTM32 Electronics Development Network is inexpensive and can be directly connected to their designed development boards via a 7-pin 2.54mm header; however, the double-layer header design also carries the risk of reverse connection, and a considerable amount of extra space on the development board is required for the debug interface. The
minoseigenheer debug pin design,
based on a Type-C interface debug accessory mode, provides an SWD over USB-C solution that enables SWD debugging and downstream USB via the Type-C interface, serving as one of the inspirations for this paper. However, this solution lacks VCP virtual serial port functionality and requires a 24-pin Type-C interface on the development board, increasing cost and wiring complexity.
This solution addresses this by adjusting the Rd pull-down resistor values ​​on the two CC lines within the specified tolerance range on the development board side. This allows detection of the female insertion direction from the male side (instead of the usual female side), enabling the reversal of the pinout on the debugger side. This allows for two-wire debugging of SWDIO and SWCLK using the two SBU pins. Furthermore, based on the detection of the Rd pull-down resistors, the analog switch can be controlled to switch D+ and D- for USB or VCP serial port use, providing as many functions as needed on the 16-pin Type-C interface.
The
Type-C specification requires that the receiving end (Sink) have 5.1kR pull-down resistors, referred to as Rd, connected to both CC1 and CC2; the supply end has pull-up resistors or current sources of a certain value, Rp, connected to both CC1 and CC2 to pull the voltage on the CC line to a certain value. Different voltage levels on the CC line indicate the maximum current that the receiving end is allowed to draw during the initial connection establishment. Since a correct CC connection only allows connection to one side of the CC pin, the other CC pin must either be open or have a 1kR pull-down resistor Ra to enable VCONN power. Therefore, the connection status of Rp, Rd, and Ra on the two CC lines can distinguish the type of connected device and the insertion direction.



CC1
CC2
Status




Rp Rd Not Connected
Rp Rd Not Connected
Rp


Rd Not Connected
Rp-Rd 0.25V-2.04V
Connect to device using ordinary cable


Rp-Rd 0.25V-2.04V
Rp Rd Not Connected
Connect to device using ordinary cable


Rp-Ra -0.25V-0.15V
Rp-Rd 0.25V-2.04V
Connect to device using E-Marker cable, CC1 is VCONN


Rp-Rd 0.25V-2.04V
Rp-Ra -0.25V-0.15V
Connect to device using E-Marker cable, CC2 is VCONN


Rp-Rd 0.25V-2.04V
Rp-Rd 0.25V-2.04V
Debug Accessory Mode


Rp-Ra -0.25V-0.15V
Rp-Ra -0.25V-0.15V
audio accessory mode



Different voltages on the Rp-Rd connection can also distinguish power supply capabilities. Therefore, the Type-C interface can differentiate the types and insertion orientations of various connected devices without transmitting PD messages, enabling debug accessory mode. As shown in the table above, since a standard Type-C connector has only one CC pin on the male side, debug accessory mode requires a specially designed male connector directly connected to the female port. The development board's female port controls the Mux to change the wiring by detecting whether both CC pins simultaneously have vRd-Connect voltage to indicate a debug accessory connection. (In the case of using only USB 2.0, the solution in this paper does not require detection and can be directly connected).
A standard Type-C debug accessory mode allows pins 2, 3, 6, 7, 8, 10, and 11 (7 pins in total) to be used to transmit custom debug signals without insertion orientation detection. However, this requires the connector to have a 24-pin Type-C interface, and the wiring on the development board side is more complex. A simple 16-pin Type-C interface only uses pins 6, 7, and 8. If USB functionality is retained and no additional components are added to the development board, only pin 8 (SBU pin) is actually usable. Therefore, insertion orientation detection must be implemented to transmit the SWDIO and SWCLK signals on pins A8 and B8 respectively.
Type-C insertion orientation detection and pin sequence reversal are typically performed on the female side, but to avoid adding components to the female side, this paper places them on the male side. The Type-C specification provides a large tolerance range for the Rd resistor. By using slightly different Rd resistors on both sides of the female side and the same Rp resistor on the male side, a comparator can compare the voltages on the two CC lines to determine the insertion orientation of the female side. Then, an analog switch can be used to swap the pin sequence of the SWDIO and SWCLK signals without requiring firmware modifications to the debugger. The two SBU pins on the development board can be directly connected to the MCU without requiring additional components.
The above design allows simultaneous use of SWD debugging and USB functionality on a 16-pin Type-C port. For development boards that do not support or require USB functionality, can the USB data line be reused as a USART serial port? This is possible. Due to the complex voltage levels of USB D+ and D- and USART, a comparator is used to detect the voltage divider resistor of CC1 to switch the analog switch, achieving USB and VCP multiplexing. For the complete 24-pin female port, VCP and the remaining debugging signals are brought out from the high-speed signals on both sides. The additional signals are double-sided on the debugger male connector side, so the development board female port side only needs a single-sided connection, reducing the wiring difficulty on the development board side.
The USB section incorporates a small-package CH334 Hub, enabling simultaneous connection of the debugger and development board to the host computer in a single cable. The STDC-14 debug interface utilizes an undefined pin to add 5V power supply and is protected by a TPS2553 chip to prevent overcurrent and reverse flow. A red LED next to the USB male connector indicates a power supply failure. The overcurrent protection current is designed to be 1.5A, which basically meets most needs and can be adjusted. (Note: Genuine ST-LINK/V2-1 firmware has the function of controlling the load switch using GPIO pins, which can limit the current to 100mA during the initial USB handshake to meet the USB specification. In fact, almost all USB interfaces do not strictly implement the 100mA limit, so this function is not implemented in this design.)
Experiments and discussions
have verified that this debugger can be flashed with ST-LINK/V2-1 firmware and works normally, supporting reversible insertion. DAPLink firmware can be flashed and recognized by the host computer PyOCD, but the SWD debugging function has not yet been tested. Short circuit protection and reverse current protection functions have been verified, and the LED lights up when there is a power failure. The following are precautions:

CH334 crystal-free function: In order to save PCB space, this design needs to use the internal crystal oscillator of CH334. Although, according to replies 1 and 2 on the Qinheng forum, the latest CH334 chipsets, except for the CH334S package, all support crystal-free functionality, as of September 2024, the LCSC surface-mount CH334F does not support crystal-free operation and cannot function properly. However, the CH334F sold by this Taobao store in August 2024 does support crystal-free operation. This design has two versions, CH334F and CH334P; it is recommended to confirm with the seller whether crystal-free functionality is supported.
STM32 Capacity: Please use a 128kB STM32F103CBT6; otherwise, ST-LINK/V2-1 firmware cannot be flashed. Other domestic alternative chips are theoretically feasible, but have not been tested.
Firmware Flashing Method: The debugger's own SWD debugging interface also uses a one-line solution to connect to the host computer's Type-C female connector. When building your first debugger, you can purchase a Type-C male/female connector test board. Connect the test board to your computer using the female connector on the debugger, and then use a regular ST-LINK firmware to flash it.
Before wiring, connect the ground wire, then use a multimeter in voltage mode to measure the voltage of pins A5, A8, B5, and B8 on the test board. The pin with 3.3V is SWDIO, and the pin diagonally opposite SWDIO is SWCLK. The other end of the test board can be directly connected to the computer's USB port. The wiring diagram is shown below.
Download, install, and open STM32CubeProgrammer. Click "Connect" in the upper right corner to connect the debugger. After a successful connection, the target chip information will be displayed in the lower left corner.
Flash the widely circulated STLinkV2.J28.M18.bin firmware.
Note that this firmware version may not work properly and requires a firmware upgrade. Click "Disconnect" to disconnect the test board and the other debugger, leaving only this debugger connected.
After plugging the debugger into the computer, without clicking any other buttons, directly click the "Firmware Upgrade" button
to upgrade the firmware and it will be usable normally.
After successfully flashing one debugger, you can use the male connector of this debugger to flash another debugger without needing to use the test board again.


The DAPLink board has a USB re-enumeration circuit, requiring DAPLink firmware compatible with ST-LINK/V2-1, such as DAP103
board thickness and 0.8 stacked 4-layer board, JLC04161H-3313 stacked board (1-4 layers of gold plating coupon can be used).
The Type-C DRP debugger can only act as a source, not a sink. If the target development board does not use a PD protocol chip, or if the PD protocol chip requires an external pull-down resistor, you can directly place the Rd resistor as designed in this article. If the PD chip has a built-in Rd resistor, it may be possible to achieve direction detection by connecting a pull-down resistor in parallel on one side of CC, but this has not been verified.

Conclusion:
No more padding.
Recommended
reading:

Ruan Xingzhi "DIY: Homemade DAPLink"
Arya Voronova "All About USB-C: Resistors And Emarkers"

Appendix
ST-LINK Indicator Light Meaning
The meanings of the two LEDs next to the STM32F103CBT6 are as follows:

Red flashing: Powered on, not connected to computer (most likely CH334 does not support crystal oscillator-free operation)
Solid red: Connected to computer, not connected to target chip
Red and green flashing alternately: Data transmission in progress
Green solid: Last data transmission successful
Red and green solid: Last data transmission failed

The LED next to the Type-C male port has the following meanings:

Not lit: Power supply is normal
Light lit: Power supply abnormal, possibly overcurrent or reverse flow

BOM
main component Taobao reference prices:

STM32F103CBT6: 2.2 yuan (free shipping)
CH334F / CH334P: 1.5 yuan (1 yuan shipping fee) (Note batch)
XC6206P332MR: 2 yuan (20 pieces, free shipping)
S9013: 2 yuan (100 pieces, free shipping
) TPS2553: 0.6 yuan (free shipping)
RS2227XUTQK10: 0.52 yuan (1 yuan shipping fee)
CH443K: 0.12 yuan (1 yuan shipping fee)
RS8901XF: Shipping fee: 0.45 yuan; 1 yuan shipping
fee; 16-pin Type-C surface mount female connector
: 2.8 yuan (10 pieces, free shipping); 24-pin Type-C male connector: 0.48 yuan; shipping fee: 1.5

yuan. The total cost of the main components for a single debugger is less than 10 yuan. According to the BOM exported from LCSC EDA, the price of five LCSC surface mount boards with all components is approximately 140 yuan.
More pictures.