Introduction to the API

Using pyOCD’s Python API, you have extreme flexibility and precise control, and can do anything SWD allows, at the expense of more complexity compared to pyocd commander. Using pyOCD like this is particularly useful for situations where other debuggers become ineffective, such as device and board bringup, or automated testing.

This document assumes familiarity with the Arm CoreSight debug architecture.

See the architecture documentation for an overview of the classes and how they are connected.


pyOCD provides a handful of helper routines that make it very easy to enumerate and connect to available debug probes. These routines are all available as static methods on the ConnectHelper class in pyocd.core.helpers.

ConnectHelper.session_with_chosen_probe() is the primary connection helper. This method returns a single Session object, or None. If only a single probe is available, a new session for that probe is returned immediately. But if there are multiple probes available, then by default, it will present a simple console UI that lets the user select which probe they want to use. Other options allow automatically picking the first available probe.

One of the most useful parameters for session_with_chosen_probe() is unique_id. Pass whole or part of a probe’s unique ID (aka serial number) to programmatically select a specific probe.

Session options may be passed to session_with_chosen_probe() in two ways. The options parameter accepts a dictionary of session options. Or, you may pass options as keyword parameters. The two methods may be combined.

DP access

The DP is controlled through an instance of the DebugPort class (in pyocd.coresight.dap). You get the DebugPort object via the ‘dp’ attribute of the target instance, i.e.,

The DebugPort class has read_reg(addr) and write_reg(addr, data) methods. ‘addr’ must be an integer in the set (0x0, 0x4, 0x8, 0xC).


x =, 0x1)

For completeness, the DebugPort class also has readDP(addr), writeDP(addr, data), readAP(addr), and writeAP(addr, data) methods. They work as described below, except that the readAP() and writeAP() methods require the APSEL in the address (i.e., 0x010000fc to read ID of APSEL=1). The AP will automatically be selected.

AP access

CoreSight APs are represented with AccessPort classes defined in pyocd.coresight.ap. These include the MEM_AP subclass and AHB_AP subclass of that.

To get the AP objects you can use the ‘aps’ attribute of the DebugPort. This attribute is a dict with the keys being the APSEL number and values being AccessPort instances. For instance, use[1] to get the AP with APSEL=1, assuming it exists (if not, you’ll get an IndexError exception).

AccessPort also has read_reg(addr) and write_reg(addr, data) methods. For these methods, addr is an integer of the register offset. Note that you do not need to include the APSEL in the address, and you do not need to modify the DP’s SELECT register prior to accessing AP registers. The AP will automatically be selected in the DP as required.

The MEM_AP/AHB_AP class has the memory access methods that are available on the target, but the access is, of course, performed through that specific AP. This is particularly useful for multicore devices or Cortex-A class devices.

Example showing access of the proprietary MDM-AP of NXP Kinetis MCUs:

mdm_ap =[1]
idr = mdm_ap.read_reg(0xfc) # Read IDR.
mdm_ap.write_reg(0x4, 0x1)

Reset control

To control reset, there are several options.

DebugPort has methods for driving the hardware reset signal:

  • DebugPort.reset(), asks the debug probe to perform a hardware reset of the target.
  • DebugPort.assert_reset(asserted) to directly control the nRESET signal. Pass True to drive nRESET low, False to drive high.

A wider range of reset options is provided by these Target methods:

  • Target.reset(reset_type=None). Normally performs a software reset unless the optional parameter is set to False.
  • Target.reset_and_halt(reset_type=None) to perform a halting reset. Again, the reset defaults to software but may be set to hardware.

The reset_type parameter on the Target reset methods can be set to one of the Target.ResetType enums:

  • ResetType.HW: Hardware reset using the nRESET signal.
  • ResetType.SW: Uses the core’s default software reset method.
  • ResetType.SW_SYSRESETREQ: Software reset using SYSRESETREQ, which usually resets the entire system on most MCUs.
  • ResetType.SW_VECTRESET: Software reset using VECTRESET, only available on v7-M targets. This resets only the core itself. If requested on non-v7-M targets, it will fall back to SW_EMULATED.
  • ResetType.SW_EMULATED: Restores the core to reset conditions by writing registers. However, this will not trigger a reset vector catch.

The CortexM objects have default_reset_type and default_software_reset_type properties that let you control the overall default reset type (any one of the ResetType enums), as well as the default if ResetType.SW is selected, respectively.

Another option for performing a halting reset is by setting vector catch with the target’s set_vector_catch() method, then using a normal reset. This has the benefit of always halting at reset, if you leave the vector catch enabled.

Example timed reset using the DP:

import time

dp =

# Timed reset.


You are encouraged to look through the code to see what additional functionality is available. The most interesting places to look at are:

  • defines Target class, which is the main API.
  • pyocd.core.coresight_target: Represents the chip as a whole, provides access to DP and APs, as well as each of the cores.
  • pyocd.coresight.cortex_m: CortexM class to control a core, implements Target API and adds some stuff.
  • pyocd.flash.loader: high level flash programming of raw binary data.
  • pyocd.flash.eraser: high level flash erasing.
  • pyocd.flash.file_programmer: high level file programming.
  • pyocd.flash.flash: low level flash programming API in the @ref pyocd.flash.flash.Flash “Flash” class. Each flash memory region has a @ref pyocd.flash.flash.Flash “Flash” class instance associated with it, accessible from the flash property on the region. To get the boot flash memory region, call target.memory_map.get_boot_memory(). The flash attribute of this region then returns the boot flash memory’s @ref pyocd.flash.flash.Flash “Flash” instance.