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Defined: dongle: A small device that plugs into a computer and serves as an adapter .... [snip]
In the OpenOCD case, this generally refers to a small adapter that attaches to your computer via USB or the parallel port.
There are several things you should keep in mind when choosing a dongle.
There are many USB JTAG dongles on the market, many of them based on a chip from “Future Technology Devices International” (FTDI) known as the FTDI FT2232; this is a USB full speed (12 Mbps) chip. See: http://www.ftdichip.com for more information. In summer 2009, USB high speed (480 Mbps) versions of these FTDI chips started to become available in JTAG adapters. Around 2012, a new variant appeared - FT232H - this is a single-channel version of FT2232H. (Adapters using those high speed FT2232H or FT232H chips may support adaptive clocking.)
The FT2232 chips are flexible enough to support some other transport options, such as SWD or the SPI variants used to program some chips. They have two communications channels, and one can be used for a UART adapter at the same time the other one is used to provide a debug adapter.
Also, some development boards integrate an FT2232 chip to serve as a built-in low-cost debug adapter and USB-to-serial solution.
These devices also show up as FTDI devices, but are not protocol-compatible with the FT2232 devices. They are, however, protocol-compatible among themselves. USB-JTAG devices typically consist of a FT245 followed by a CPLD that understands a particular protocol, or emulates this protocol using some other hardware.
They may appear under different USB VID/PID depending on the particular product. The driver can be configured to search for any VID/PID pair (see the section on driver commands).
There are several OEM versions of the SEGGER J-Link adapter. It is an example of a microcontroller based JTAG adapter, it uses an AT91SAM764 internally.
Raisonance has an adapter called RLink. It exists in a stripped-down form on the STM32 Primer, permanently attached to the JTAG lines. It also exists on the STM32 Primer2, but that is wired for SWD and not JTAG, thus not supported.
STMicroelectronics has an adapter called ST-LINK. They only work with STMicroelectronics chips, notably STM32 and STM8.
For info the original ST-LINK enumerates using the mass storage usb class; however, its implementation is completely broken. The result is this causes issues under Linux. The simplest solution is to get Linux to ignore the ST-LINK using one of the following methods:
Texas Instruments has an adapter called ICDI. It is not to be confused with the FTDI based adapters that were originally fitted to their evaluation boards. This is the adapter fitted to the Stellaris LaunchPad.
Nuvoton has an adapter called Nu-Link. It is available either as stand-alone dongle and embedded on development boards. It supports SWD, serial port bridge and mass storage for firmware update. Both Nu-Link v1 and v2 are supported.
ARM has released a interface standard called CMSIS-DAP that simplifies connecting debuggers to ARM Cortex based targets http://www.keil.com/support/man/docs/dapdebug/dapdebug_introduction.htm.
The two well-known “JTAG Parallel Ports” cables are the Xilinx DLC5 and the Macraigor Wiggler. There are many clones and variations of these on the market.
Note that parallel ports are becoming much less common, so if you have the choice you should probably avoid these adapters in favor of USB-based ones.
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