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4.2 Serial and Debug Adapter (OpenSDA)
OpenSDA is an open-standard serial and debug adapter. It bridges serial and debug communications
between a USB host and an embedded target processor as shown in Figure 4. The hardware circuit is
based on a Freescale Kinetis K20 family microcontroller (MCU) with 128 KB of embedded flash and an
integrated USB controller. OpenSDA features a mass storage device (MSD) bootloader, which provides
a quick and easy mechanism for loading different OpenSDA Applications such as flash programmers,
run-control debug interfaces, serial-to-USB converters, and more. Refer to the OpenSDA User's Guide
for more details.
OpenSDA is managed by a Kinetis K20 MCU built on the ARM® Cortex™-M4 core. The OpenSDA circuit
includes a status LED (D4) and a pushbutton (SW1). The pushbutton asserts the Reset signal to the
KL25Z target MCU. It can also be used to place the OpenSDA circuit into Bootloader mode. SPI and
GPIO signals provide an interface to either the SWD debug port of the KL25Z. Additionally, signal
connections are available to implement a UART serial channel. The OpenSDA circuit receives power
when the USB connector J7 is plugged into a USB host.
4.2.1 Debug Interface
Signals with SPI and GPIO capability are used to connect directly to the SWD of the KL25Z. These
signals are also brought out to a standard 10-pin (0.05") Cortex Debug connector (J6). It is possible to
isolate the KL25Z MCU from the OpenSDA circuit and use J6 to connect to an off-board MCU. To
accomplish this, cut the trace on the bottom side of the PCB that connects J11 pin 1 to J11 pin 2. This
will disconnect the SWD_CLK pin to the KL25Z so that it will not interfere with the communications to
an off-board MCU connected to J6.
Figure 4. OpenSDA High-Level Block Diagram
Figure 5. SWD Debug Connector
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