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Using the 'J' port

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Hi All,


New the MSP430 so please excuse potential ignorance :)


I am working on a project using the MSP430FR5739 which will be used to fire some pyro igniters (high powered rocketry).

Port J would be easiest to use with regards to PCB layout, but as this is also the JTAG port i'm just checking what state they are initialised in, and if I should be careful of anything when using them to prevent accidental misfire?


I will be using SBW to debug/program.


Thanks in advance!




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Take a look in the family guide (http://www.ti.com/lit/ug/slau272c/slau272c.pdf) in section 1.8 (page 40).  


The JTAG pins default to GPIO unless the SYSJTAGPIN bit of the SYSCTL register has been set (in software).  GPIO pins default to hi-z inputs (pull up/down disabled).  


So for general operation you should be fine.  That said, I would not try and flash/debug the chip with live rounds attached.

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If the part starts up in normal application mode, then Port J defaults to GPIO as @@mbeals mentioned. However, if you start up the part in debug mode (don't think it matters if it's JTAG or SBW), then the pins will come up for their different JTAG/SBW functions rather than as normal GPIO - some will be outputs. I think it is the TEST pin that selects whether they should be JTAG/SBW or normal GPIO - normally you have this pulled down internally (and sometimes people put an external pull-down here too) and your debugger will drive the TEST pin high to enable the pins for JTAG use.


So definitely be careful!

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