The AVR MCU OCD messaging system is a side-channel in the on-chip debug module. It is used extensively in some OCD variants to communicate with the core when it is stopped, but is not used by the system during Run mode. It can be used by end-user code to send messages to the debugger at run time. In Run mode, the debugger constantly polls the OCD for run/stop status, and at the same time picks up any messages. AVR MCU OCD messaging is a channel for code instrumentation without using any dedicated pins (other than the debug pins). Messages are single 8-bit values and are by default sent to Atmel Studio and displayed in the Output window as hex values, unless routed to Data Visualizer.
There is no standard way to use OCD messaging. The techniques shown in these examples each have advantages and disadvantages, and make use of different resources on the target device. Not all AVR devices support OCD messaging, and not all applications are suited to the use of OCD messaging. It is essentially a side-channel of the on-chip-debug system.
Enable AVR MCU OCD messaging in the Code Profiling Configuration dialog, then connect the AVR MCU OCD messaging source to a graph plot sink and start a debug session. Messages will not appear unless code is instrumented accordingly.
The most typical use-case for AVR MCU OCD messaging is ASCII printf-style debugging displayed on a terminal, as demonstrated in the examples. However, it could be used to transport any 8-bit data values, or even a composite structure. Messages can, for example, be sent from an ADC sample-complete interrupt, writing the 8-bit value of an ADC sample directly to the OCD message register. This can then be plotted directly onto a graph in Data Visualizer.
The ̔defaultʼ OCD message channel to Atmel Studio operates at a fixed sample rate with 50 ms period. When enabled from Data Visualizer, the polling loop makes use of ̔spareʼ cycles in the debugger to read and transport OCD messages. This leads to a higher throughput, but is also less deterministic in timing.