Using a Proto USB Board

zaphoid

I have a Propeller Proto USB board and am·using some pin headers and jumping over to a bread board.·

Have been following one of the Propeller Education Labs, I/O and Timing Basics. Fun stuff, thanks for leading me along.

Had a volt meter hooked across Vss and Vdd and it read the 3.0 volts, normal right... Then I pulled the battery plug out.· The volt meter showed 1.5 volts.

What gives, the battery is disconnected....

Pulled the USB cable from the PC.· Voltage goes to zero.


So, here is the question, Does the board protect my PC, or is it possible that I could make a bone head move and push lots of volts into the USB cable?


I kind of also think that USB power line is 5V but what do I know.

BradC

zaphoid said...
I have a Propeller Proto USB board and am using some pin headers and jumping over to a bread board.

Have been following one of the Propeller Education Labs, I/O and Timing Basics. Fun stuff, thanks for leading me along.

Had a volt meter hooked across Vss and Vdd and it read the 3.0 volts, normal right... Then I pulled the battery plug out. The volt meter showed 1.5 volts.

What gives, the battery is disconnected....

Its quite likely that the voltage is coming from the TX line on the FTDI USB chip. That device sources its power from the USB and not the regulators on board.
What you are seeing is not at all unusual and nothing to worry about.

zaphoid said...


Pulled the USB cable from the PC. Voltage goes to zero.

Perfectly normal.

zaphoid said...
So, here is the question, Does the board protect my PC, or is it possible that I could make a bone head move and push lots of volts into the USB cable?

Quite unlikely unless you blow up the FTDI chip. They are pretty well protected.

zaphoid said...
I kind of also think that USB power line is 5V but what do I know.

You know enough to be correct. The USB power line is about 5V. I have a board here I use for testing that I've jumpered the +5V from the USB socket to the 5V bus on-board so it's entirely USB powered. This is not recommended practice, but it sure works.

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Cardinal Fang! Fetch the comfy chair.

Mike Green

The USB power lines supply 5V approximately. If the power is turned off on the Protoboard, the USB interface chip will still output transmit data from the PC to the Propeller at approximately 3V since that's the logic high voltage. That 3V will "sneak" through the protective diode to the Propeller's internal power bus and provide something on the order of 2.3V there and that'll appear on the Vdd pins of the Propeller. It can be lower because the USB voltage isn't guaranteed to be 5V. It may be lower.

A PC's USB connection is supposed to protect the PC from static and some applied voltage from out on the USB bus. There at least is protective circuitry in the USB interface in the PC. Whether it can "stand up" to the external voltage or not depends on the specific parts used and their specification.

Philldapill

The 1.5V could also be the residual voltage left in the capacitors... The regulators are LDO, but could be dropping 1.5V. This may cause the capacitors to not bleed off very quickly, keeping the 1.5V charge in them.... Or it's the USB. [noparse]:)[/noparse]