JTAG and SWD are two common interfaces used for debugging and programming ARM Cortex microcontrollers. Both provide similar capabilities but have some key differences.
Overview of JTAG
JTAG stands for Joint Test Action Group. It is an industry standard interface originally designed for testing printed circuit boards using boundary scan. The JTAG interface has been widely adopted for debugging and programming embedded systems and microcontrollers.
Some key characteristics of JTAG are:
- Uses 4 or 5 pins – TCK, TMS, TDI, TDO, and optionally TRST.
- Supports daisy chaining of multiple chips on the same JTAG interface.
- Robust and reliable due to its lineage from boundary scan testing.
- Built-in sanity and integrity checks during data transfers.
- Compatible debug interface across various ARM Cortex-M cores.
The downsides of JTAG include:
- Typically slower interface speed than SWD.
- Requires more pins than SWD.
- Complex state machine protocol.
Overview of SWD
SWD stands for Serial Wire Debug. It is a debug interface developed by ARM as a slimmer alternative to JTAG. SWD uses only 2 pins – SWCLK and SWDIO, compared to the 4 or 5 pins used by JTAG.
Key characteristics of SWD include:
- Uses just 2 pins – SWCLK and SWDIO.
- Faster interface speed than JTAG, especially at higher frequencies.
- Simpler protocol than JTAG.
- Not limited to scan chain testing like JTAG.
- Supports multiprocessor debugging via switch matrix.
- Provides equivalent debug capabilities as JTAG.
Downsides of SWD are:
- No built-in integrity checks like JTAG.
- Not as widespread use and support as JTAG.
- Lacks support for boundary scan testing.
Key Differences Between JTAG and SWD
Let’s summarize some of the main differences between the JTAG and SWD interfaces:
Number of Pins
JTAG requires 4 pins (TCK, TMS, TDI, TDO) or 5 pins (with TRST). SWD uses only 2 pins (SWCLK, SWDIO). SWD provides equivalent debug capability with fewer pins.
SWD supports faster interface clock speeds than JTAG, especially at higher frequencies like 20-50 MHz. SWD can operate at up to 50 MHz whereas JTAG maxes out around 10-20 MHz.
JTAG uses a complex state machine based protocol with 16 different states to coordinate communication. SWD uses a much simpler handshaking protocol between the debugger and target device.
Boundary Scan Support
A key capability of JTAG is its support for boundary scan testing of circuit boards with multiple chips. SWD does not support boundary scan capabilities.
JTAG has built-in parity and integrity checks to ensure reliable data transfers. SWD has no internal integrity checks, but does allow checksums to be added at higher levels.
SWD includes a switch matrix architecture to support coordinated debugging of multiprocessor devices. JTAG does not directly have this capability.
Adoption and Use
JTAG is an older standard that has been very widely adopted across the industry. SWD is newer but is gaining popularity in ARM Cortex devices. JTAG is still more universally supported.
JTAG vs SWD on ARM Cortex MCUs
Modern ARM Cortex-M series microcontrollers support both JTAG and SWD interfaces. This provides flexibility to choose which interface to use.
For ARM Cortex-M debugging, SWD is generally preferred because it provides equivalent functionality to JTAG with fewer pins and faster speed. The simplicity of SWD also makes it easier to use and integrate into your application.
However, JTAG can still be useful for ARM Cortex-M devices when you need:
- Boundary scan or chain testing capability.
- Compatibility with legacy tools that only support JTAG.
- Very long signal traces where maximum reliability is needed.
SWD is less susceptible than JTAG to noise issues on long trace lengths. But very long traces may still require the robustness of JTAG instead of SWD.
JTAG vs SWD on Other ARM Devices
On other ARM architecture devices like Cortex-A application processors, the choice between JTAG and SWD is less straightforward:
- Some ARM application processors only support JTAG debug.
- JTAG provides TRACE capabilities not available on SWD.
- Boundary scan chain testing is still an advantage of JTAG.
But SWD does offer these benefits on ARM application processors:
- Faster interface speed than JTAG.
- Simpler 2-pin connection.
- Supports ARM CoreSight debugging architecture.
For new ARM application processors, SWD is increasingly becoming the preferred debug interface. But legacy JTAG support remains important for wider tool compatibility.
JTAG and SWD Used Together
The JTAG and SWD interfaces can be used together to combine some of their benefits. There are a few ways to use JTAG and SWD together:
- Transition from JTAG to SWD for faster data transfer after initial setup over JTAG.
- Use JTAG for boundary scan testing and SWD for debug during development.
- Connect both interfaces to provide redundant debugging access if one interface fails.
Using both JTAG and SWD requires up to 6 pins total to support both interfaces. The added flexibility can be beneficial for some applications.
Summary of JTAG vs SWD
In summary, JTAG and SWD offer two different approaches to debugging ARM devices, each with their own pros and cons:
- JTAG is an older, proven standard that offers boundary scan and robust operation but requires more pins and operates slower.
- SWD is newer, simpler, and faster but lacks built-in integrity checks and boundary scan capabilities.
For most ARM Cortex-M development, SWD provides a good balance of simplicity and performance for debugging. JTAG still has advantages for boundary scan testing or legacy tool compatibility.
Both interfaces continue to be useful and are often provided together on new ARM processors to give developers flexibility. Understanding the tradeoffs helps select which interface makes sense for a particular application.