ARM SWO ITM Console Bidirectional Standard I/O Retargeting

The ARM Cortex M architecture has many features which are underused, probably simply because engineers are not aware of it. SWO (Single Wire Output) is a single trace pin of the ARM Cortex-M CoreSight debug block. trace pin uses the ITM (Instruction Trace Macrocell) on ARM Cortex. It provides a serial output channel, at a high speed higher than the usual UART, because it is clocked at half or a quarter of the core clock frequency, depending on the core and implementation.

As such, it is an ideal high speed output channel to send text or data to the host. This is how it is usually used, but what is unknown to many: it can be used in a bidirectional way with the help of the debugger.

The topic of this article: how to redirect standard I/O like printf() or scanf() using the SWO ITM console: means both sending *and* receiving data over the SWO debug channel: that way I can use it as a kind of UART with a single pin only.

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Standalone SWO

SWO (Single Wire Output) in ARM cores is probably one of the most under-used features. Which is surprising, because SWO can be very useful. In a nut shell: SWO is a single wire output pin/signal channel which can provide lots of different data, like PC sampling for coverage information, interrupt tracing data or ‘uart-like’ text packets.

SWO output from application
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SWO with ARM Cortex-M33

One major concern of embedded system development is performance. Microcontroller have more MHz than in the past, but embedded applications are all about timing and real-time aspects.

The challenge is: how to get data off the target in realtime? SWO can help with this….

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SWO with NXP i.MX RT1064-EVK Board

With the cost of an single pin, many ARM Cortex-M boards including the NXP i.MX RT1064 can produde SWO data: think about a pin able to stream data out of the chip in realtime. For example interrupt activity which otherwise might be hard to capture:

SWO Interrupt Trace

SWO Interrupt Trace

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ARM SWO Performance Counters

In “Cycle Counting on ARM Cortex-M with DWT” I have used the ARM DWT register to count the executed cycles. With the MCUXpresso IDE comes with a very useful feature: it can capture the ARM SWO (Single Wire Output) trace data. One special kind of trace data is the ‘cycle counter’ information which is sent through SWO.

SWO Counters

SWO Counters

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Tutorial: Using Single Wire Output SWO with ARM Cortex-M and Eclipse

As a standard procedure, I add some console functionality to my embedded applications. That way I have a command line interface and can inspect and influence the target system. One interesting hardware feature of ARM Cortex-M is Single Wire Output (SWO): it allows to send out data (e.g. strings) over up to 32 different stimulus ports, over a single wire.

swo-pin-on-arm-debug-header

swo-pin-on-arm-debug-header

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