Persistent SerTxd transmission

[beb101]

I asked about persistent SerTxd transmission when the power supply to a Picaxe circuit was disconnected. The details are here,

http://www.picaxeforum.co.uk/showthread.php?20601-CP2102-Puzzle

The answer was that it had something to do with flow control relative to the USB adapter, but it has nothing to do with flow control. I am using this exact Goeytex circuit,

http://www.picaxeforum.co.uk/showthread.php?18940-USB-communication-and-power&highlight=Goeytex+CP2102

I decided to measure voltages with the following results,

USB to Board: Connected, Power Supply to Board: Connected
======================================
board gnd to USB TXD: 3.3V
board gnd to USB RXD: 3.3V
board gnd to USB gnd: 0.0V
board ground to board Vin: 3.3V
board gnd to Picaxe gnd: 0.0V
board gnd to Picaxe VCC: 3.3V
board gnd to 74HC14 pin 1 A1 (from USB TXD): 3.3V
board gnd to 74HC14 pin 2 Y1 (to Picaxe serrxd): 0.0V
board gnd to 74HC14 pin 9 A4 (from Picaxe sertxd): 0.0V
board gnd to 74HC14 pin 8 Y4 (to USB RXD): 3.3V
board gnd to 74HC14 pin 7: 0.0V
board gnd to 74HC14 pin 14: 3.3V
other 74HC14 pins: NC

USB to Board: Connected, Power Supply to Board: DISCONNECTED
=========================================
board gnd to USB TXD: 3.2V
board gnd to USB RXD: 2.5V <--- ????
board gnd to USB gnd: 0.0V
board ground to board Vin: 0.0V
board gnd to Picaxe gnd: 0.0V
board gnd to Picaxe VCC: 2.5V <--- ????
board gnd to 74HC14 pin 1 A1 (from USB TXD): 3.2V
board gnd to 74HC14 pin 2 Y1 (to Picaxe serrxd): 0.0V
board gnd to 74HC14 pin 9 A4 (from Picaxe sertxd): 0.5V <-- ????
board gnd to 74HC14 pin 8 Y4 (to USB RXD): 0.0V
board gnd to 74HC14 pin 7: 0.0V
board gnd to 74HC14 pin 14: 2.5V <-- ???
other 74HC14 pins: NC

I posted a question on Yahoo Groups Electronics_101 about this behavior and received the following answer,

Quote:
"Many devices will conduct from their I/O pins to their supplies. The 2.5V is a
common voltage when this is happening - it is a diode drop (about 0.7V) plus
some IR drop down from the 3.3V supply that is driving the other pins."

What I would like to do is have the ability to turn off power to the shield and shut it down completely without removing the USB cable. Will a switch on the power and a couple of transistor switches for the USB TXD, RXD lines work here?

Baxter

 

[SAborn]

If you are powering the circuit from the USB 5 volt then there is no reason you can not just add a switch to disconnect the 5v+ supply, why do you need to disconnect the data lines.

 

[beb101]

If you are powering the circuit from the USB 5 volt then there is no reason you can not just add a switch to disconnect the 5v+ supply, why do you need to disconnect the data lines.

Well, because SerTxd is still transmitting. The Picaxe and 74HC14 are are somehow still active as evidenced by the voltage on Picaxe pin 1 and 74HC14 pin 14 even though the board Vin is 0V. It seems to me the only way to shut the board down for a clean power off reset is to disable all three lines.

Baxter

 

[premelec]

Take a look for a CMOS switch or buffer with an enable pin - use that pin to disconnect the signal lines - I don't have my CMOS logic books handy but believe such exist from long time ago [4066?] if you don't need bi-directional it's easier...

 

[SAborn]

I tried the setup here with a 4069 hex inverter and only get 0.8 volt to picaxe pin 1 with 5V+ disconnected, but if i leave the power connected to the hex inverter and disconnect the 5v+ to the picaxe then i read 0V on pin 1, so your answer might be just put a switch on the 5v+ supply to the picaxe only, problem fixed.

This would then make the circuit the same as it would be with a standard serial cable or a common usb to seral adaptor like the Rev-Ed one.

There is no need to power down the inverter chip, just treat is as if it was a part of the CP2101 adaptor.

 

[beb101]

Hi premelec and SAborn,
Great suggestions ... I completely overlooked just turning off the supply to the picaxe. I wired it into the V+ power rail which I guess was a mistake. Rethinking this, I may just use a digital IO from the Netduino with a transistor to switch the Picaxe on and off.

Thanks,

Baxter

 

[hippy]

I am using this exact Goeytex circuit

Presumably not "exact" because you then detail your measurements with ...

USB to Board: Connected, Power Supply to Board: Connected
USB to Board: Connected, Power Supply to Board: DISCONNECTED

And there is no power supply in Goeytex's circuit nor any means to disconnect power without either disconnecting the USB interface from the circuit or unplugging the USB cable.

Your pin descriptions of the 74HC14 also differ entirely from what Goeytex's circuit show.

It always helps to show what you actually have rather than something that happens to be somewhat like it.