Usb to ttl modules - connection to pics at 3.3v

[rsocor01]

yes 4k7 in series with the usb ttl module is needed , i am also thinking there must be usb -ttl modules that have a 3.3v signal in/ out level option

FTDI makes their USB/serial for 5V and they also have a 3.3v version. Have you tried the 3.3v FTDI USB version?

[tumbleweed]

You'll have the same problem with a 3V3 converter if you apply an input signal to an unpowered PIC unless the pin doesn't have protection diodes.

[Charlie]

Agreed - anything you use can't be allowed to supply power. The PIC input should be connected to the PIC supply through a resistor, and then a diode (or equivalent) connected in series in the signal line, with the anode connected to the input pin. The serial converter can then only pull low. When it's high, the diode makes it high impedance, and the resistor pulls high to the PIC VDD. When power is removed everything will then truly shut down.
This problem has nothing to do with 5 V vs 3 V. It's all about a low impedance signal source, idle state high, able to supply enough current to power the PIC through the protection diode.

[Dick Ivers]

Charlies's approach sounds good. Would it not be wise to use a Schottky diode to keep the low input to the PIC as low as possible?

[Charlie]

Can if you like, or if signal speed is an issue, but otherwise it doesn't matter. Any old diode in the box will do.