## 8 Debug Adapter Configuration

Correctly installing OpenOCD includes making your operating system give OpenOCD access to debug adapters. Once that has been done, Tcl commands are used to select which one is used, and to configure how it is used.

> Note: Because OpenOCD started out with a focus purely on JTAG, you may find places where it wrongly presumes JTAG is the only transport protocol in use. Be aware that recent versions of OpenOCD are removing that limitation. JTAG remains more functional than most other transports. Other transports do not support boundary scan operations, or may be specific to a given chip vendor. Some might be usable only for programming flash memory, instead of also for debugging.

Debug Adapters/Interfaces/Dongles are normally configured through commands in an interface configuration file which is sourced by your `openocd.cfg` file, or through a command line `-f interface/....cfg` option.

    source [find interface/olimex-jtag-tiny.cfg]

These commands tell OpenOCD what type of JTAG adapter you have, and how to talk to it. A few cases are so simple that you only need to say what driver to use:

    # jlink interface
    interface jlink

Most adapters need a bit more configuration than that.

### 8.1 Interface Configuration

The interface command tells OpenOCD what type of debug adapter you are using. Depending on the type of adapter, you may need to use one or more additional commands to further identify or configure the adapter.

Config Command: interface name

  Use the interface driver name to connect to the target.

Command: interface_list

  List the debug adapter drivers that have been built into the running copy of OpenOCD.

Command: interface transports transport_name+

  Specifies the transports supported by this debug adapter. The adapter driver builds-in similar knowledge; use this only when external configuration (such as jumpering) changes what the hardware can support.

Command: adapter_name

  Returns the name of the debug adapter driver being used.

### 8.2 Interface Drivers

Each of the interface drivers listed here must be explicitly enabled when OpenOCD is configured, in order to be made available at run time.

Interface Driver: amt_jtagaccel

  Amontec Chameleon in its JTAG Accelerator configuration, connected to a PC’s EPP mode parallel port. This defines some driver-specific commands:

  Config Command: parport_port number

    Specifies either the address of the I/O port (default: 0x378 for LPT1) or the number of the `/dev/parport` device.

  Config Command: rtck [`enable`|`disable`]

    Displays status of RTCK option. Optionally sets that option first.

Interface Driver: arm-jtag-ew

  Olimex ARM-JTAG-EW USB adapter This has one driver-specific command:

  Command: armjtagew_info

    Logs some status

Interface Driver: at91rm9200

  Supports bitbanged JTAG from the local system, presuming that system is an Atmel AT91rm9200 and a specific set of GPIOs is used.

Interface Driver: cmsis-dap

  ARM CMSIS-DAP compliant based adapter.

  Config Command: cmsis_dap_vid_pid [vid pid]+

    The vendor ID and product ID of the CMSIS-DAP device. If not specified the driver will attempt to auto detect the CMSIS-DAP device. Currently, up to eight [vid, pid] pairs may be given, e.g.

        cmsis_dap_vid_pid 0xc251 0xf001 0x0d28 0x0204

  Command: cmsis-dap info

    Display various device information, like hardware version, firmware version, current bus status.

Interface Driver: dummy

  A dummy software-only driver for debugging.

Interface Driver: ep93xx

  Cirrus Logic EP93xx based single-board computer bit-banging (in development)

Interface Driver: ft2232

  FTDI FT2232 (USB) based devices over one of the userspace libraries.

  Note that this driver has several flaws and the `ftdi` driver is recommended as its replacement.

  These interfaces have several commands, used to configure the driver before initializing the JTAG scan chain:

  Config Command: ft2232_device_desc description

    Provides the USB device description (the iProduct string) of the FTDI FT2232 device. If not specified, the FTDI default value is used. This setting is only valid if compiled with FTD2XX support.

  Config Command: ft2232_serial serial-number

    Specifies the serial-number of the FTDI FT2232 device to use, in case the vendor provides unique IDs and more than one FT2232 device is connected to the host. If not specified, serial numbers are not considered. (Note that USB serial numbers can be arbitrary Unicode strings, and are not restricted to containing only decimal digits.)

  Config Command: ft2232_layout name

    Each vendor’s FT2232 device can use different GPIO signals to control output-enables, reset signals, and LEDs. Currently valid layout name values include:

    - - axm0432_jtag Axiom AXM-0432
    - - comstick Hitex STR9 comstick
    - - cortino Hitex Cortino JTAG interface
    - - evb_lm3s811 TI/Luminary Micro EVB_LM3S811 as a JTAG interface, either for the local Cortex-M3 (SRST only) or in a passthrough mode (neither SRST nor TRST) This layout can not support the SWO trace mechanism, and should be used only for older boards (before rev C).
    - - luminary_icdi This layout should be used with most TI/Luminary eval boards, including Rev C LM3S811 eval boards and the eponymous ICDI boards, to debug either the local Cortex-M3 or in passthrough mode to debug some other target. It can support the SWO trace mechanism.
    - - flyswatter Tin Can Tools Flyswatter
    - - icebear ICEbear JTAG adapter from Section 5
    - - jtagkey Amontec JTAGkey and JTAGkey-Tiny (and compatibles)
    - - jtagkey2 Amontec JTAGkey2 (and compatibles)
    - - m5960 American Microsystems M5960
    - - olimex-jtag Olimex ARM-USB-OCD and ARM-USB-Tiny
    - - oocdlink OOCDLink
    - - redbee-econotag Integrated with a Redbee development board.
    - - redbee-usb Integrated with a Redbee USB-stick development board.
    - - sheevaplug Marvell Sheevaplug development kit
    - - signalyzer Xverve Signalyzer
    - - stm32stick Hitex STM32 Performance Stick
    - - turtelizer2 egnite Software turtelizer2
    - - usbjtag "USBJTAG-1" layout described in the OpenOCD diploma thesis
  Config Command: ft2232_vid_pid [vid pid]+

    The vendor ID and product ID of the FTDI FT2232 device. If not specified, the FTDI default values are used. Currently, up to eight [vid, pid] pairs may be given, e.g.

        ft2232_vid_pid 0x0403 0xcff8 0x15ba 0x0003

  Config Command: ft2232_latency ms

    On some systems using FT2232 based JTAG interfaces the FT_Read function call in ft2232_read() fails to return the expected number of bytes. This can be caused by USB communication delays and has proved hard to reproduce and debug. Setting the FT2232 latency timer to a larger value increases delays for short USB packets but it also reduces the risk of timeouts before receiving the expected number of bytes. The OpenOCD default value is 2 and for some systems a value of 10 has proved useful.

  Config Command: ft2232_channel channel

    Used to select the channel of the ft2232 chip to use (between 1 and 4). The default value is 1.

  For example, the interface config file for a Turtelizer JTAG Adapter looks something like this:

      interface ft2232
      ft2232_device_desc "Turtelizer JTAG/RS232 Adapter"
      ft2232_layout turtelizer2
      ft2232_vid_pid 0x0403 0xbdc8

Interface Driver: ftdi

  This driver is for adapters using the MPSSE (Multi-Protocol Synchronous Serial Engine) mode built into many FTDI chips, such as the FT2232, FT4232 and FT232H. It is a complete rewrite to address a large number of problems with the ft2232 interface driver.

  The driver is using libusb-1.0 in asynchronous mode to talk to the FTDI device, bypassing intermediate libraries like libftdi of D2XX. Performance-wise it is consistently faster than the ft2232 driver, sometimes several times faster.

  A major improvement of this driver is that support for new FTDI based adapters can be added competely through configuration files, without the need to patch and rebuild OpenOCD.

  The driver uses a signal abstraction to enable Tcl configuration files to define outputs for one or several FTDI GPIO. These outputs can then be controlled using the `ftdi_set_signal` command. Special signal names are reserved for nTRST, nSRST and LED (for blink) so that they, if defined, will be used for their customary purpose.

  Depending on the type of buffer attached to the FTDI GPIO, the outputs have to be controlled differently. In order to support tristateable signals such as nSRST, both a data GPIO and an output-enable GPIO can be specified for each signal. The following output buffer configurations are supported:

  - - Push-pull with one FTDI output as (non-)inverted data line
  - - Open drain with one FTDI output as (non-)inverted output-enable
  - - Tristate with one FTDI output as (non-)inverted data line and another FTDI output as (non-)inverted output-enable
  - - Unbuffered, using the FTDI GPIO as a tristate output directly by switching data and direction as necessary

  These interfaces have several commands, used to configure the driver before initializing the JTAG scan chain:

  Config Command: ftdi_vid_pid [vid pid]+

    The vendor ID and product ID of the adapter. If not specified, the FTDI default values are used. Currently, up to eight [vid, pid] pairs may be given, e.g.

        ftdi_vid_pid 0x0403 0xcff8 0x15ba 0x0003

  Config Command: ftdi_device_desc description

    Provides the USB device description (the iProduct string) of the adapter. If not specified, the device description is ignored during device selection.

  Config Command: ftdi_serial serial-number

    Specifies the serial-number of the adapter to use, in case the vendor provides unique IDs and more than one adapter is connected to the host. If not specified, serial numbers are not considered. (Note that USB serial numbers can be arbitrary Unicode strings, and are not restricted to containing only decimal digits.)

  Config Command: ftdi_channel channel

    Selects the channel of the FTDI device to use for MPSSE operations. Most adapters use the default, channel 0, but there are exceptions.

  Config Command: ftdi_layout_init data direction

    Specifies the initial values of the FTDI GPIO data and direction registers. Each value is a 16-bit number corresponding to the concatenation of the high and low FTDI GPIO registers. The values should be selected based on the schematics of the adapter, such that all signals are set to safe levels with minimal impact on the target system. Avoid floating inputs, conflicting outputs and initially asserted reset signals.

  Config Command: ftdi_layout_signal name [`-data`|`-ndata` data_mask] [`-oe`|`-noe` oe_mask]

    Creates a signal with the specified name, controlled by one or more FTDI GPIO pins via a range of possible buffer connections. The masks are FTDI GPIO register bitmasks to tell the driver the connection and type of the output buffer driving the respective signal. data_mask is the bitmask for the pin(s) connected to the data input of the output buffer. `-ndata` is used with inverting data inputs and `-data` with non-inverting inputs. The `-oe` (or `-noe`) option tells where the output-enable (or not-output-enable) input to the output buffer is connected.

    Both data_mask and oe_mask need not be specified. For example, a simple open-collector transistor driver would be specified with `-oe` only. In that case the signal can only be set to drive low or to Hi-Z and the driver will complain if the signal is set to drive high. Which means that if it’s a reset signal, `reset_config` must be specified as `srst_open_drain`, not `srst_push_pull`.

    A special case is provided when `-data` and `-oe` is set to the same bitmask. Then the FTDI pin is considered being connected straight to the target without any buffer. The FTDI pin is then switched between output and input as necessary to provide the full set of low, high and Hi-Z characteristics. In all other cases, the pins specified in a signal definition are always driven by the FTDI.

  Command: ftdi_set_signal name `0`|`1`|`z`

    Set a previously defined signal to the specified level.

    - - `0`, drive low
    - - `1`, drive high
    - - `z`, set to high-impedance

  For example adapter definitions, see the configuration files shipped in the `interface/ftdi` directory.

Interface Driver: remote_bitbang

  Drive JTAG from a remote process. This sets up a UNIX or TCP socket connection with a remote process and sends ASCII encoded bitbang requests to that process instead of directly driving JTAG.

  The remote_bitbang driver is useful for debugging software running on processors which are being simulated.

  Config Command: remote_bitbang_port number

    Specifies the TCP port of the remote process to connect to or 0 to use UNIX sockets instead of TCP.

  Config Command: remote_bitbang_host hostname

    Specifies the hostname of the remote process to connect to using TCP, or the name of the UNIX socket to use if remote_bitbang_port is 0.

  For example, to connect remotely via TCP to the host foobar you might have something like:

      interface remote_bitbang
      remote_bitbang_port 3335
      remote_bitbang_host foobar

  To connect to another process running locally via UNIX sockets with socket named mysocket:

      interface remote_bitbang
      remote_bitbang_port 0
      remote_bitbang_host mysocket

Interface Driver: usb_blaster

  USB JTAG/USB-Blaster compatibles over one of the userspace libraries for FTDI chips. These interfaces have several commands, used to configure the driver before initializing the JTAG scan chain:

  Config Command: usb_blaster_device_desc description

    Provides the USB device description (the iProduct string) of the FTDI FT245 device. If not specified, the FTDI default value is used. This setting is only valid if compiled with FTD2XX support.

  Config Command: usb_blaster_vid_pid vid pid

    The vendor ID and product ID of the FTDI FT245 device. If not specified, default values are used. Currently, only one vid, pid pair may be given, e.g. for Altera USB-Blaster (default):

        usb_blaster_vid_pid 0x09FB 0x6001

    The following VID/PID is for Kolja Waschk’s USB JTAG:

        usb_blaster_vid_pid 0x16C0 0x06AD

  Command: usb_blaster (`pin6`|`pin8`) (`0`|`1`)

    Sets the state of the unused GPIO pins on USB-Blasters (pins 6 and 8 on the female JTAG header). These pins can be used as SRST and/or TRST provided the appropriate connections are made on the target board.

    For example, to use pin 6 as SRST (as with an AVR board):

        $_TARGETNAME configure -event reset-assert \
              "usb_blaster pin6 1; wait 1; usb_blaster pin6 0"

Interface Driver: gw16012

  Gateworks GW16012 JTAG programmer. This has one driver-specific command:

  Config Command: parport_port [port_number]

    Display either the address of the I/O port (default: 0x378 for LPT1) or the number of the `/dev/parport` device. If a parameter is provided, first switch to use that port. This is a write-once setting.

Interface Driver: jlink

  Segger J-Link family of USB adapters. It currently supports only the JTAG transport.

  > Compatibility Note: Segger released many firmware versions for the many harware versions they produced. OpenOCD was extensively tested and intended to run on all of them, but some combinations were reported as incompatible. As a general recommendation, it is advisable to use the latest firmware version available for each hardware version. However the current V8 is a moving target, and Segger firmware versions released after the OpenOCD was released may not be compatible. In such cases it is recommended to revert to the last known functional version. For 0.5.0, this is from "Feb 8 2012 14:30:39", packed with 4.42c. For 0.6.0, the last known version is from "May 3 2012 18:36:22", packed with 4.46f.

  Command: jlink caps

    Display the device firmware capabilities.

  Command: jlink info

    Display various device information, like hardware version, firmware version, current bus status.

  Command: jlink hw_jtag [`2`|`3`]

    Set the JTAG protocol version to be used. Without argument, show the actual JTAG protocol version.

  Command: jlink config

    Display the J-Link configuration.

  Command: jlink config kickstart [val]

    Set the Kickstart power on JTAG-pin 19. Without argument, show the Kickstart configuration.

  Command: jlink config mac_address [`ff:ff:ff:ff:ff:ff`]

    Set the MAC address of the J-Link Pro. Without argument, show the MAC address.

  Command: jlink config ip [`A.B.C.D`(`/E`|`F.G.H.I`)]

    Set the IP configuration of the J-Link Pro, where A.B.C.D is the IP address, E the bit of the subnet mask and F.G.H.I the subnet mask. Without arguments, show the IP configuration.

  Command: jlink config usb_address [`0x00` to `0x03` or `0xff`]

    Set the USB address; this will also change the product id. Without argument, show the USB address.

  Command: jlink config reset

    Reset the current configuration.

  Command: jlink config save

    Save the current configuration to the internal persistent storage.

  Config: jlink pid val

    Set the USB PID of the interface. As a configuration command, it can be used only before ’init’.

Interface Driver: parport

  Supports PC parallel port bit-banging cables: Wigglers, PLD download cable, and more. These interfaces have several commands, used to configure the driver before initializing the JTAG scan chain:

  Config Command: parport_cable name

    Set the layout of the parallel port cable used to connect to the target. This is a write-once setting. Currently valid cable name values include:

    - - altium Altium Universal JTAG cable.
    - - arm-jtag Same as original wiggler except SRST and TRST connections reversed and TRST is also inverted.
    - - chameleon The Amontec Chameleon’s CPLD when operated in configuration mode. This is only used to program the Chameleon itself, not a connected target.
    - - dlc5 The Xilinx Parallel cable III.
    - - flashlink The ST Parallel cable.
    - - lattice Lattice ispDOWNLOAD Cable
    - - old_amt_wiggler The Wiggler configuration that comes with some versions of Amontec’s Chameleon Programmer. The new version available from the website uses the original Wiggler layout (’wiggler’)
    - - triton The parallel port adapter found on the “Karo Triton 1 Development Board”. This is also the layout used by the HollyGates design (see http://www.lartmaker.nl/projects/jtag/).
    - - wiggler The original Wiggler layout, also supported by several clones, such as the Olimex ARM-JTAG
    - - wiggler2 Same as original wiggler except an led is fitted on D5.
    - - wiggler_ntrst_inverted Same as original wiggler except TRST is inverted.
  Config Command: parport_port [port_number]

    Display either the address of the I/O port (default: 0x378 for LPT1) or the number of the `/dev/parport` device. If a parameter is provided, first switch to use that port. This is a write-once setting.

    When using PPDEV to access the parallel port, use the number of the parallel port: `parport_port 0` (the default). If `parport_port 0x378` is specified you may encounter a problem.

  Command: parport_toggling_time [nanoseconds]

    Displays how many nanoseconds the hardware needs to toggle TCK; the parport driver uses this value to obey the `adapter_khz` configuration. When the optional nanoseconds parameter is given, that setting is changed before displaying the current value.

    The default setting should work reasonably well on commodity PC hardware. However, you may want to calibrate for your specific hardware.

    > Tip: To measure the toggling time with a logic analyzer or a digital storage oscilloscope, follow the procedure below:

    >     > parport_toggling_time 1000
    >     > adapter_khz 500

    > This sets the maximum JTAG clock speed of the hardware, but the actual speed probably deviates from the requested 500 kHz. Now, measure the time between the two closest spaced TCK transitions. You can use `runtest 1000` or something similar to generate a large set of samples. Update the setting to match your measurement:

    >     > parport_toggling_time <measured nanoseconds>

    > Now the clock speed will be a better match for `adapter_khz rate` commands given in OpenOCD scripts and event handlers.

    > You can do something similar with many digital multimeters, but note that you’ll probably need to run the clock continuously for several seconds before it decides what clock rate to show. Adjust the toggling time up or down until the measured clock rate is a good match for the adapter_khz rate you specified; be conservative.

  Config Command: parport_write_on_exit (`on`|`off`)

    This will configure the parallel driver to write a known cable-specific value to the parallel interface on exiting OpenOCD.

  For example, the interface configuration file for a classic “Wiggler” cable on LPT2 might look something like this:

      interface parport
      parport_port 0x278
      parport_cable wiggler

Interface Driver: presto

  ASIX PRESTO USB JTAG programmer.

  Config Command: presto_serial serial_string

    Configures the USB serial number of the Presto device to use.

Interface Driver: rlink

  Raisonance RLink USB adapter

Interface Driver: usbprog

  usbprog is a freely programmable USB adapter.

Interface Driver: vsllink

  vsllink is part of Versaloon which is a versatile USB programmer.

  > Note: This defines quite a few driver-specific commands, which are not currently documented here.

Interface Driver: hla

  This is a driver that supports multiple High Level Adapters. This type of adapter does not expose some of the lower level api’s that OpenOCD would normally use to access the target.

  Currently supported adapters include the ST STLINK and TI ICDI.

  Config Command: hla_device_desc description

    Currently Not Supported.

  Config Command: hla_serial serial

    Currently Not Supported.

  Config Command: hla_layout (`stlink`|`icdi`)

    Specifies the adapter layout to use.

  Config Command: hla_vid_pid vid pid

    The vendor ID and product ID of the device.

  Config Command: trace source_clock_hz [output_file_path]

    Enable SWO tracing (if supported). The source clock rate for the trace port must be specified, this is typically the CPU clock rate. If the optional output file is specified then raw trace data is appended to the file, and the file is created if it does not exist.

Interface Driver: opendous

  opendous-jtag is a freely programmable USB adapter.

Interface Driver: ulink

  This is the Keil ULINK v1 JTAG debugger.

Interface Driver: ZY1000

  This is the Zylin ZY1000 JTAG debugger.

> Note: This defines some driver-specific commands, which are not currently documented here.

Command: power [`on`|`off`]

  Turn power switch to target on/off. No arguments: print status.

Interface Driver: bcm2835gpio

  This SoC is present in Raspberry Pi which is a cheap single-board computer exposing some GPIOs on its expansion header.

  The driver accesses memory-mapped GPIO peripheral registers directly for maximum performance, but the only possible race condition is for the pins’ modes/muxing (which is highly unlikely), so it should be able to coexist nicely with both sysfs bitbanging and various peripherals’ kernel drivers. The driver restores the previous configuration on exit.

  See `interface/raspberrypi-native.cfg` for a sample config and pinout.

### 8.3 Transport Configuration

As noted earlier, depending on the version of OpenOCD you use, and the debug adapter you are using, several transports may be available to communicate with debug targets (or perhaps to program flash memory).

Command: transport list

  displays the names of the transports supported by this version of OpenOCD.

Command: transport select transport_name

  Select which of the supported transports to use in this OpenOCD session. The transport must be supported by the debug adapter hardware and by the version of OpenOCD you are using (including the adapter’s driver). No arguments: returns name of session’s selected transport.

#### 8.3.1 JTAG Transport

JTAG is the original transport supported by OpenOCD, and most of the OpenOCD commands support it. JTAG transports expose a chain of one or more Test Access Points (TAPs), each of which must be explicitly declared. JTAG supports both debugging and boundary scan testing. Flash programming support is built on top of debug support.

#### 8.3.2 SWD Transport

SWD (Serial Wire Debug) is an ARM-specific transport which exposes one Debug Access Point (DAP, which must be explicitly declared. (SWD uses fewer signal wires than JTAG.) SWD is debug-oriented, and does not support boundary scan testing. Flash programming support is built on top of debug support. (Some processors support both JTAG and SWD.)

Command: swd newdap ...

  Declares a single DAP which uses SWD transport. Parameters are currently the same as "jtag newtap" but this is expected to change.

Command: swd wcr trn prescale

  Updates TRN (turnaraound delay) and prescaling.fields of the Wire Control Register (WCR). No parameters: displays current settings.

#### 8.3.3 CMSIS-DAP Transport

CMSIS-DAP is an ARM-specific transport that is used to connect to compilant debuggers.

#### 8.3.4 SPI Transport

The Serial Peripheral Interface (SPI) is a general purpose transport which uses four wire signaling. Some processors use it as part of a solution for flash programming.

### 8.4 JTAG Speed

JTAG clock setup is part of system setup. It does not belong with interface setup since any interface only knows a few of the constraints for the JTAG clock speed. Sometimes the JTAG speed is changed during the target initialization process: (1) slow at reset, (2) program the CPU clocks, (3) run fast. Both the "slow" and "fast" clock rates are functions of the oscillators used, the chip, the board design, and sometimes power management software that may be active.

The speed used during reset, and the scan chain verification which follows reset, can be adjusted using a `reset-start` target event handler. It can then be reconfigured to a faster speed by a `reset-init` target event handler after it reprograms those CPU clocks, or manually (if something else, such as a boot loader, sets up those clocks). See Target Events. When the initial low JTAG speed is a chip characteristic, perhaps because of a required oscillator speed, provide such a handler in the target config file. When that speed is a function of a board-specific characteristic such as which speed oscillator is used, it belongs in the board config file instead. In both cases it’s safest to also set the initial JTAG clock rate to that same slow speed, so that OpenOCD never starts up using a clock speed that’s faster than the scan chain can support.

    jtag_rclk 3000
    $_TARGET.cpu configure -event reset-start { jtag_rclk 3000 }

If your system supports adaptive clocking (RTCK), configuring JTAG to use that is probably the most robust approach. However, it introduces delays to synchronize clocks; so it may not be the fastest solution.

NOTE: Script writers should consider using `jtag_rclk` instead of `adapter_khz`, but only for (ARM) cores and boards which support adaptive clocking.

Command: adapter_khz max_speed_kHz

  A non-zero speed is in KHZ. Hence: 3000 is 3mhz. JTAG interfaces usually support a limited number of speeds. The speed actually used won’t be faster than the speed specified.

  Chip data sheets generally include a top JTAG clock rate. The actual rate is often a function of a CPU core clock, and is normally less than that peak rate. For example, most ARM cores accept at most one sixth of the CPU clock.

  Speed 0 (khz) selects RTCK method. See FAQ RTCK. If your system uses RTCK, you won’t need to change the JTAG clocking after setup. Not all interfaces, boards, or targets support “rtck”. If the interface device can not support it, an error is returned when you try to use RTCK.

Function: jtag_rclk fallback_speed_kHz

  This Tcl proc (defined in `startup.tcl`) attempts to enable RTCK/RCLK. If that fails (maybe the interface, board, or target doesn’t support it), falls back to the specified frequency.

      # Fall back to 3mhz if RTCK is not supported
      jtag_rclk 3000