Microcontroller board design tips

20.06.2012 17:52

I've been working with VESNA for a better half of a year now and over time I have come to know some oversights that were made in the original hardware design that make development of software for VESNA unnecessary complicated. Of course, mistakes like these are only obvious in hindsight, so I'm sharing here a few tips that should prevent them in future designs.

Have a JTAG port always accessible. JTAG is what allows you to do any kind of debugging beyond simple printfs. Together with an on-chip debugger and the GNU debugger it makes it possible to debug firmware running on the microcontroller in much the same way as an executable on your PC. You can inspect parts of the CPU's address space, program new firmware right from the debugger and set break and watchpoints. However a JTAG port takes between 4 and 6 pins and many microcontrollers allow you to turn it off and remap other peripherals to those pins.

Relying on the functionality hidden behind JTAG pins is a bad idea and should only be used as a last resort. Doubly so on a board that is meant for multiple purposes and is going to see a lot of software development. VESNA has several peripherals on the core board that can only be used after JTAG is turned off. Needless to say, these are a pain to develop drivers for. This means that people avoid using those parts of the hardware as far as possible and in the cases when the use is unavoidable, the software is badly tested and bugs tends to linger around for longer than for other parts of the system.

Straightforward physical accessibility is also important. On VESNA, JTAG pins are routed through the general-purpose expansion connector and we had several cases where accessing the connector was a simple geometric impossibility. It also means that all expansion boards have to be designed with the debugability of the underlying core board in mind. Having a separate connector would be a much better choice. When designing a board, think how it will be mounted and what kind of expansions will there be in the future. These should not make the debug port inaccessible.

Use a separate serial port for diagnostics and debug messages. The Cortex M3 microcontroller used on VESNA has several hardware UART peripherals, but only one is accessible on a convenient connector, which means that one serial line is used for data transfer as well as random debug messages used during development. This violates the rule of separation, which means that debug messages jumbled up together with data are not a rare occurrence.

Advice regarding accessibility of the JTAG port applies to the diagnostic port as well. VESNA's serial line is available on the connector that is also used for external power. This means that in some configurations the serial line simply can't be connected, making debugging hard. A much better design choice would be to bundle JTAG and a diagnostic serial console on the same connector and leave the power supply separate.

Actually, I have come to learn that special care should be given to accessibility of the circuit in general. It should be possible to run the basic core board in a way that makes it possible to probe most nets and microcontroller pins with an oscilloscope or a logic analyzer. The positioning of the JTAG on the expansion connector on VESNA means that in order to use a debugger you have to use a special break-out debug board which, until a later redesign, used to cover half of the core board, making a part of the circuit inaccessible to a oscilloscope probe without a soldering iron and some wires. When a bug hits that requires you to use a debugger and an oscilloscope you really don't need any additional complications.

In short, design for debugability, both software and hardware. It will make people happier.

Posted by Tomaž | Categories: Digital


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