Please re-read posts #5 and #6. The real question is do you need it? Has someone told you that you need additional protection?
The datasheet (of every CMOS Chip, it's inherent to the CMOS technology) told me that
Max voltage on I/O pins: -0.3V to VCC + 0.3V
When Vcc = 0 there can be max 0.3V on the I/O pins.
Actually it will power the supposedly unpowered ICs, because almost all chips have these protection diodes built-in.
Ok I will try it. Serial resistors + clamping diodes.
I only have to take care that the sum of the current that flows though the I/Os doesn't exceed the 10-20mA so that the internal diode doesn't burn out.
For the USB or SPI could I use very high resistence value, it doesn't matter, 3.3k-10kΩ for instance?
Does the USB/SPI spécifications allows it?
You need to be far more specific - we can't comment on your exact question unless we know what it is?.
Well imagine a PCB with:
- a PIC controller that uses USB and/or SPI to connect to a computer or another PIC on another board
- a voltage regulator for the PIC and other Chip (I2C chips for instance), that can be switch off or not.
That's it.
Questions:
* Can I keep the USB cable connected to the PIC even if the power supply of the PIC is switched off (voltage regulator output disbaled)?
* Can I keep the USB cable connected to the PIC even if the power supply is NOT pulleg in -> no ground -> open circuit. Latchup still possible?
* One solution would be to use the USB power supply to power up the PIC (and only the PIC), so no latch-up possible with the USB data lines connected to the PIC. But it would require to have a common ground between the PCB and the USB ground. In that case how can I avoid that too much current flows back to the PC through the USB ground (when we assume that there are other device on the PCB powered by an external power supply and that the consume more then the 500mA allowed by the USB standart).
Is it a problem? Will the current only flows thought the external power supply ground or also the USB ground???
Is it clearer?
(Sorry for my bad english :/ )
Ok I will try then the serial resistor on all IO + camping diodes.
I would be glad to get an answer about the last question (common USB ground).
Thank you!
pic are not that easy to break and the D+ and D- are data pins not power pins. The in current is in Microamps so is SPI
There is no clamping diodes on the D- and D+ pins just pullup's look at the data sheet.
Ok thank you for the confirmation.
So as long the current is limited there are no concern about a possible latch-up.