[PIC] Power of board through the UART board?

[urnuj]

Hi

I have realized that my PCB board containing the pic33ep512MU814 remained power on as the LED can still be seen lighted after I have removed the power supply.

Another board which is a slave to the main processor is connected to the main board through UART and it is still powered on.

I would like to ask what is the reason for this that the main board is able to remain powered on through the UART?

Thanks a lot for help.

Regards

 

[shaiko]

I would like to ask what is the reason for this that the main board is able to remain powered on through the UART?

Theoretically, yes. But it would be a very strange design.
You'll have to post both of the board's schematics to get a more educated answer.

Does the LED gets powered off when you disconnect the UART ?

 

[betwixt]

It's difficult to say without seeing your schematics but the usual reason is there is enough voltage and current available through the serial data lines to make the static protection diodes at the UART input conduct. Normally, when the circuit is powered up, they conduct any excess voltage to the power and ground lines to protect the IC and do nothing if the serial signals are less than VDD. When VDD is removed, the serial voltages are higher than VDD so the protection diodes conduct and to some extent pull the VDD line to a higher voltage, giving the appearance the power is still turned on.

Brian.

 

[urnuj]

Yes its strange and actually I do not want it to be this way. As it would obstruct the normal working of the main board causing problems when debugging the project.

So now I have to disconnect the UART and debug the project and connecting it back after the debugging started and connect it before running through the initialization code.

Regards
And yes the LED get powered off when the UART cable is disconnected.

 

[KlausST]

Hi,

you say UART board.... but what interface?
Is it RS232? Then the signal levels are +/-3V at least but it can be as high as +/-12V. So it is quite normal that the levels are higher than VCC. But with RS232 there should be nor protection diodes to VCC.
On RS232 idle the signals should be high. High with RS232 is negative level. I doubt it could produce positive voltage on VCC.

Only schematic can tell.

Klaus