Prop Plug Grounding Issues

CassLan

Hi, I have a Dip Prop + Eeprom + Prop Plug connections hooked into a breadboard as described in page 17 of the Prop manual.

I can program the Prop just fine, however I feel that my circuit is somehow grounding via the PropPlug, I have some shift registers for outputs that drive LEDs and if I remove the power source, the LEDs stay very dimly lit until I unplug the PropPlug. I also had a similar oddity with·composite monitor that would not display clearly unless the PropPlug was connected. I have since switched to a DC powered composite monitor, and that seems to be working.

I would like to leave the PropPlug connected to the breadboard for programming, but I'm nervous about these occurances for fear of damage to the PropPlug or my USB Port, and also I want to know that my circuit will work with or without a PropPlug connected to it.

Has anyone had similar things happen? Should I be concerned?

Thanks,

Rick

Mike Green

This happens. The PropPlug does have a ground connection (and has to have it to work). It also has some signal lines which provide a signal that changes from 0V to 3.3V and can supply a couple of milliAmps of current. That's enough to provide a little bit of power to the Propeller chip. When the power supply is turned off, there's leakage from an I/O pin through a protective diode to the chip's internal power bus. This can be enough to power the Prop and provide a little current to an LED or two, particularly when there's little else connected to the Prop.

The solution is to unplug the USB cable from the PropPlug when the Propeller setup is turned off.

The main problem is that the protective diode is not designed to handle more than about 1/2 mA of current, so it's not a good idea to leave things connected this way.

Phil Pilgrim (PhiPi)

This, along with the stray reset issue, could be solved by including a 74LVC2G07 open-drain buffer (a ten-cent part) on the Prop Plug to isolate the FTDI chip from the target board. This would entail a pullup on the Prop's RxD pin, but that's not a show-stopper. (Maybe if I repeat it often enough, it will come to pass. )

-Phil

mctrivia

any chance you have a schematic? i want to try building my own propplug for the fun of it.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

CassLan

Hey Phil, is this something I could throw on the breadboard? Or would this have to reside on the PropPlug itself before the breadboard connections? If its something I could add myself I would appreciate a bit more info if you can.

Thanks,

Rick

mctrivia

probably could not be added to bread board. I have tried adding jumper wires to rx and tx. more then 10cm total and it would not work.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Phil Pilgrim (PhiPi)

Here's a schematic for a buffered Prop Plug. This should solve the phantom reset and any other vestigial power issues. You will need a pullup on pin A31 of the Prop to its Vdd (IOW, you can't put it on the Prop Plug itself; it has to be on the target PCB), since the buffer is open-drain. The reason this works is that the 74LVC logic series includes "I_off" circuitry, which prevents pin voltages on an unpowered device from propagating to the Vdd pin. This also allows input pins to accept higher-than Vdd logic levels, which makes this series a good choice for use in logic level converters.

A non-open-drain buffer (e.g. 74LVC2G34) would solve the reset problem, but it would not eliminate the vestigial power to an unpowered Prop coming from a powered Prop Plug.

-Phil

mctrivia

what is the part labeled ferrite?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Phil Pilgrim (PhiPi)

It's a surface-mountable ferrite bead. I use DigiKey #240-2389-1-ND. Parallax omits this part, along with the 0.01uF cap. I've included it because it's specified by FTDI.

-Phil

mctrivia

ok so other then caps resitors and leds which i have tones of i need to make this:

240-2389-1-ND - Ferrite
296-23277-1-ND - 74lvc2g07
Digi-Key Part Number - ft232rl(unfortunately not available in BGA)


will add to my next order. Still a little short of the $200 min for free shipping.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Phil Pilgrim (PhiPi)

mctrivia said...
unfortunately not available in BGA

Unfortunately?!!! I consider it a favor not to have the temptation. BTW, since you seem to be a glutton for punishment, the FT232RQ (QFN32) will also work, but the pinout will be different.

-Phil

mctrivia

I like to make things small and now that I got a small reflow oven I want more practice with bga and other small components

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

mikediv

mctriva if you dont mind me asking I wanted to get into the smt stuff but I have no idea what type of small reflow oven I should but can you recommend one and where to buy and what I should pay and if I will need any extras thank you
·

mctrivia

sure.

for hand soldering I recomend the JBC AD2700(discontinued) or similar. JBC soldering irons are more expensive but worth every penny. I have not yet replaced a tip and have replaced hundreads of weler tips. You can change tips while hot which is important because smt work requires different sized tips to work with different sized components and copper traces.


for reflow work. I recomend this Black and Decker Toaster Oven works perfectly for reflowing ICs and I managed to get on sale for half price.

Important things to look for:

Quartz heating element. low speed convections is preferable.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Post Edited (mctrivia) : 3/28/2009 2:47:35 AM GMT