Prop Clip/Plug Transistor

Lord Steve · 2010-02-04 20:33

Could someone please explain what the transistor does on the Prop Clip/Plug?· And is there a waveform or clock diagram available which shows what the reset output from the transistor to the Propeller should be for that circuit for given values of DTR and over the RC time constant of the cap and res?· I'm trying to debug a board.

Thanks.
·

Peter Jakacki · 2010-02-04 23:03

The transistor acts as a simple buffer/shaper for the RC circuit whose sole purpose is to generate a short pulse on the changing edge of the DTR signal. When the DTR signal goes from low to high the capacitor passes current while it charges which primarily flows through the base of the npn transistor. The transistor acts as a switch in that the collector is pulled low when even a little base current is flowing. But as the capacitor charges less current flows in an inverse exponential fashion. At the reset pin this looks like a clean low signal for the duration of the charge time. The exact time constant is not that important, it's generating a clean short reset pulse on the changing edge of the DTR signal that's important. The R part of the RC is to handle discharge.

Unless you have a scope though you won't be able to see anything. With a voltmeter the static condition of the reset line should be high as there is only a weak 5K pullup internal to the prop when the BOE line is grounded.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
*Peter*

Lord Steve · 2010-02-04 23:19

On my scope I am not seeing any change at all on the collector.· It is always low.· I can see the DTR signal change on the scope.· I can see the opposite side of the capacitor (the side which is wired to the resistor) change with DTR.

The transistor I chose is here: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MMBT5089LT1GOSCT-ND

Is that choice a problem?
·

Peter Jakacki · 2010-02-04 23:45

There is no problem with the choice of transistor, you don't need anything special, almost any small signal npn will do. You have connected up the transistor the right way according to the datasheet haven't you? Now the base of the transistor should be below 0V normally, anything around 0.6V means that you have current flowing into the base from somewhere, check your connections.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
*Peter*

Ariba · 2010-02-05 00:41

Lord Steve said...
On my scope I am not seeing any change at all on the collector. It is always low....

The collector should be High the most time and not Low.
Have you connected the BOEn pin to ground?
This enables the internal 5k PullUp resistor at the Reset input.

Andy

Miner_with_a_PIC · 2010-02-05 01:25

The transistor looks like its there to invert the signal...ie change a DTR low to high transition into a high to low transition...the capacitor makes the pulse finite. You won't see anything on the collector as the base current brings the collector to the emitter potential which sits at ground. Without a pull-up resistor on the collector you won't see any change on your scope and neither will the prop, Ariba makes a good point...either make sure your BOEn pin is grounded as to enable the internal 5K pullup on the reset pin or use an external pull-up resistor to pull the reset pin up.

Lord Steve · 2010-02-05 02:51

Ah, thank you.· It turns out that I connected /BOE to VCC and thus was not getting the internal resistor.· Thank you each for your comments I really appreciate it.· I solder-jumped /BOE to VSS (ground) and cut the trace to VCC.· Now, the prop programs.· [noparse]:D[/noparse]

--Steve
·

Peter Jakacki · 2010-02-05 03:43

I said...
With a voltmeter the static condition of the reset line should be high as there is only a weak 5K pullup internal to the prop when the BOE line is grounded.

As I had mentioned in my first post, but of course you need to actually read the replies properly to glean these vital bits of information.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
*Peter*

Prop Clip/Plug Transistor

Comments