PDA

View Full Version : P8X32A Pin driving fet resistance graph



Tubular
11-14-2011, 09:17 AM
I thought I may as well stick this here in case others ever need it:-

This is a single sample estimate of the internal resistance of the Prop's pin driving fet transistors (output mode). Two measurement sets are recorded -
1) Resistance of the N fet, measured by connecting a 180 ohm resistor from pin to Vdd, measuring voltage drops, calculating current and hence internal resistance.
2) Resistance of the P fet, measured by connecting a 180 ohm resistor from pin to Vss, measuring voltage drops, calculating current and hence internal resistance.

When might this matter?
* When choosing (low value) resistors for DACs, such as the VGA or TV DACs, or other R2R DAC networks
* For driving very carefully selected Blue and White leds directly
* For usb protection series resistors (maybe?)
* For resonant circuit / transformer driving

One upshot of this testing - if you short an "Output High" pin with an "Output Low" pin, the current that flows is only barely over the 40mA "absolute maxiumum" from the datasheet. In my test it was only when Vsupply>3.2V that I was able to break the 40mA barrier. Tested by connecting a 1 ohm resistor in between the two pins, and increasing the supply voltage until move than 40mV appeared across the resistor.

Rayman
11-14-2011, 03:59 PM
25..30 Ohms huh... I wouldn't have thought it would be that high.

I see now how people get away with connecting an LED straight from a Prop pin to GND.

Phil Pilgrim (PhiPi)
11-14-2011, 04:40 PM
Excellent work, Tubular! It ought to become part of the Propeller datasheet. I thought the Prop was supposed to have symmetrical P vs. N drive capabilities. You've demonstrated that it does not.

I'm currently facing a much tougher situation, outputting video from a device that doesn't have nearly the Propeller's output drive. It looks like I will have to drive a high-resistance R2R network, then amplify it with a video op-amp. The devil in that detail, though, is stray capacitance and, combined with the high ladder resistance, how much it will affect the frequency response.

-Phil

David Carrier
11-14-2011, 06:05 PM
Phil,
The P and N drive have symmetric current capabilities, but the output impedance varies too much to specify that closely. The output impedance is also affected by the output current, so if you run the tests again with different load resistors, you will measure different resistances. Each die's characteristics will differ, and each pin on a die may even differ slightly. This is why most datasheets only specify a minimum and a maximum output impedance, or possibly a typical output impedance under a very specific condition. (e.g. Iout = 10 mA, Vdd = 3.3 V)

As for driving video off of a device with lower drive capabilities, try running the outputs through a buffer between the outputs and the R2R ladder. We have had a few designs where we wanted to be able to switch either a TV or a VGA output on and off, so the I/O pins could be used for other functions, and adding a buffer between the output and the R2R ladder doesn't negatively impact the circuit, but it does add an enable pin.

— David Carrier
Parallax Inc.

Tubular
11-14-2011, 06:30 PM
Excellent work, Tubular! It ought to become part of the Propeller datasheet. I thought the Prop was supposed to have symmetrical P vs. N drive capabilities. You've demonstrated that it does not.

I think they're reasonably symmetrical around the normal operating range (2.7v~3.6v). Also the 180 ohm load is a bit more demanding than the 240~270 ohm minimum normally encountered for VGA / TV out. While I'm set up I might repeat the test for 240 ohm and see how it compares. I also thought about using a current source but really most of the time we're using resistors anyway.



I'm currently facing a much tougher situation, outputting video from a device that doesn't have nearly the Propeller's output drive. It looks like I will have to drive a high-resistance R2R network,
then amplify it with a video op-amp. The devil in that detail, though, is stray capacitance and, combined with the high ladder resistance, how much it will affect the frequency response.
-Phil

Yeah can't help thinking like David a buffer near the source might be a good idea, but I guess if its digital you're faced with buffering 8 or perhaps more channels?

I've recently found stray capacitance can be useful with the prop, using a ~220k, SIL resistor array as a kind of "scroll wheel" next to a 4D display. These are the yellow (or sometimes blue, black) epoxy coated thingos often used as terminating resistors). Should have boards this week and then it gets really interesting (working out the optimum orientation for sensing through a cover, how far the stray field radiates etc)

Cluso99
11-14-2011, 10:32 PM
Thanks tubular. Interesting results. Keep up the good work and info :)

Phil Pilgrim (PhiPi)
11-14-2011, 11:43 PM
As for driving video off of a device with lower drive capabilities, try running the outputs through a buffer between the outputs and the R2R ladder.


Yeah can't help thinking like David a buffer near the source might be a good idea, but I guess if its digital you're faced with buffering 8 or perhaps more channels?

I might end up doing it that way if I can figure out how to fit buffers for 21-24 bits on the board.

-Phil

Tubular
11-15-2011, 12:11 AM
Hmm 21..24 bits, sounds decidedly *color*. Look forward to hearing more when you're ready.

I guess with that many bits you start thinking about a slave micro (eg an SX! ) and keep it in the digital domain, unless you want to do overlay or something tricky.

Tubular
11-15-2011, 12:15 AM
Just for completeness here's the same test with 270 ohm load, ie as used for TV and close to that used for the MSB of the VGA output.
Not that different to the 180 ohm test, really.

Also I checked the difference from pin to pin across P16..23, there's hardly any difference in driving impedance from one pin to another (as might be expected from being on the same die)

BigFoot
11-15-2011, 01:03 AM
Good information, I have always wondered what the drivers on resistance was.