How to Prevent Ghost Powering When Using USB/TTL Adapter:
As many of you may be aware, microcontrollers (as well as many other CMOS devices) can incorporate internal ESD protection diodes. These ESD diodes can cause the device to be "ghost powered" by an external device connected to an I/0 Pin that is confifured as an input. This is because the internal ESD diode provides a path from the I/0 pin to the internal Vdd rail of the chip. This can even cause ghost powering of an entire board if preventative steps are not taken
This ghost powereing can cause problems, such as the chip not resetting when powered off/on or MCLR not working as expected or even chip programing to to fail. This ghost powering can/will happen when a USB/TTL adapter with serial data idling high is connected to a microcontroller. This ghost powering can be prevented by adding a diode and a pullup resistor on the PIC RX Pin.
The attached diagram shows how to connect a TTL-USB adapter while eliminating the ghost powering of a PIC microcontroller. Note the use of D1 and R2.
How to Prevent Ghost Powering When Using USB/TTL Adapter:
As many of you may be aware, microcontrollers (as well as many other CMOS devices) can incorporate internal ESD protection diodes. These ESD diodes can cause the device to be "ghost powered" by an external device connected to an I/0 Pin that is confifured as an input. This is because the internal ESD diode provides a path from the I/0 pin to the internal Vdd rail of the chip. This can even cause ghost powering of an entire board if preventative steps are not taken
This ghost powereing can cause problems, such as the chip not resetting when powered off/on or MCLR not working as expected or even chip programing to to fail. This ghost powering can/will happen when a USB/TTL adapter with serial data idling high is connected to a microcontroller. This ghost powering can be prevented by adding a diode and a pullup resistor on the PIC RX Pin.
The attached diagram shows how to connect a TTL-USB adapter while eliminating the ghost powering of a PIC microcontroller. Note the use of D1 and R2.
Last edit: William Roth 2017-05-01
Good Tip,
Thanks