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For this PCB there will be a voltage regulator that provides 3.3V from 12 VDC. There will also be a 6-pin connector for doing In system programming via SPI. The SPI programmer is going to be an Arduino Uno, which will provide its target uC (and the whole board) with Vcc. My question is how should I route the Vcc on the board ? Should there be any kind of decoupling (jumper ?) between the voltage regulator to be used when in ISP mode ?

Went to How should power be supplied for in-system programming? but - not quite what I am looking to know.

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  • \$\begingroup\$ What voltage does the Uno provide to the target? \$\endgroup\$ – Finbarr Mar 15 '17 at 10:20
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Assuming your programmer board can supply sufficient current at 3.3V then you can simply connect the programming header to the main 3.3V rail and it should all work fine. Most voltage regulators will cope with that without any issues. You could always play safe and add a diode to block the programming voltage from reaching the regulator but that then lowers your normal 3.3V rail but the diode forward voltage drop.

If your programmer can only supply sufficient current at some other voltage (e.g. 5V) then it may be possible to supply that voltage to the 12V rail and power the system up that way, it depends entirely on the voltage regulator used on the board. In that situation put some small series resistors (10-100R) on all the data lines in order to limit currents on them while power supplies come up. And for hopefully obvious reasons doing that with the 12V also connected has the potential to be very bad.

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  • \$\begingroup\$ The second part sounds interesting. Probably I won't be able to put it into practice with this L78L33ABUTR - st.com/resource/en/datasheet/l78l.pdf that I am using, but if this is the case why not place small series resistor on each data line regardless of the voltage I feed to the regulator ? \$\endgroup\$ – kellogs Mar 15 '17 at 11:10
  • \$\begingroup\$ And would you say this small current (100 mA) regulator could handle some 3V in reverse ? \$\endgroup\$ – kellogs Mar 15 '17 at 11:11
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    \$\begingroup\$ The series resistors shouldn't hurt no matter what although depending on the trace lengths, capacitance etc... they may slightly decrease the maximum clock speed you can use reliably. The part should be OK, I think there will be some leakage back on to the 12V rail but it shouldn't cause any issues. The regulator has a 2V dropout so you'd need at least 5.3V on the 12V rail to drive things that way. \$\endgroup\$ – Andrew Mar 15 '17 at 11:25

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