Finding mystery voltage.

[Thric]

A little background:
I recently created a PCB that has an onboard USB to UART converter to program my propeller chip. I got the board a week ago and populated the minimum components (the propeller, the EEPROM and its hardware, a CP2101, usb port, voltage regulator, etc.) and then hooked it up to my computer. After a few issues with figuring out were to get the drivers, my computer was finally happy viewing a COM port.

The problem:
Even though i'm able to connect the CP2101 to my computer i'm not able to download any programs from the Propeller tool. The tool scans the port but says 'no propeller chip was found'. I've doubled checked the TX and RX pins (TX from the CP2101 goes to pin 31 while RX from CP2101 goes to pin 30) and also checked whether 3.3 volts were present on the reset line (it is), but i'm still not able to download.

The Mystery:
Whenever I plug the usb in without applying power to the rest of the board (i don't give power to the voltage reg, which powers the prop) i find 2.7 volts present in the supply net (the net that connects the 3.3v from the reg to the prop, eeprom, etc.) vs. ground. Is this suppose to happen? The USB circuit is separate except for ground, tx, rx, and reset.

Thanks

[Bits]

Are you toggling the REsn pin on the propeller correctly?

As far as the mystery voltage could it be parasitic spawning from the USB IC?

[Mike Green]

1) The USB to serial adapter has to reset the Propeller when DTR changes state. That's how the Propeller Tool forces the Propeller to start the ROM bootloader. Make sure the CP2101 is configured to do that. Look at how the PropPlug is designed for an example using the FTDI chip. There may also be a problem with the CP2101 I/O driver preventing proper operation of DTR.

2) You forget that the TX line from the CP2101 to Propeller pin 31 is a source of power when it's high (the idle state). There are protective diodes connected from each I/O pin to the Vdd and Vss busses on the Propeller chip and these will conduct when the Propeller doesn't have any other source of power. 3.3V minus one diode drop (0.6V) is roughly 2.7V.

[Cluso99]

I have used a CP2102 with the transistor circuit on DTR and it works with the prop fine.