PSCU output voltage

max711

So I have a Propeller Servo Controller USB (PSCU) http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/28830/List/0/SortField/4/ProductID/595/Default.aspx

I'm using it to drive a Gecko G340 servo controller that needs a 5v step signal. I haven't actually connected it yet.

I have a very simple SPIN program that toggles Pin_0 97 times per second. Approx 10ms high followed by 10ms low.

According to the schematic for the PSCU http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PSCUsb_A%20Schematic.pdf , I should be getting 5v from the TXB0108PW "step-up" chip.

Problem is, I'm only getting 2.8v on pin_high, dropping to 1.8v on pin_low. I'm using a Fluke 123 scopemeter and I'm pretty sure it's giving me correct readings. Interestingly, on the "low" side of the TXB0108PW chip I get 0v and 3.3v every pulse on Pin_0 as expected.

I've tried running off USB power and also putting 5v on the Vext of J2 but it makes no difference.

Is my board bad?

Thanks

max711

OK, just talked to tech support. The schematic on their website is for a Rev A board which uses the Texas instruments chip TXB0108PW and should give a 5v output, but the design has changed to the Rev B board which I got and uses a different chip that does not output 5v. Great! Anyone have a Rev A board they want to swap?

hover1

What chip did they go to on Rev B? They only have Rev a schematics up.

Try putting a 5K resistor inline with your device.

We did a lot of testing a while ago related to the problem:

http://forums.parallax.com/showthread.php?118121-Digital-servo-s-on-USB-servocontroller&highlight=PSCU+translator

Jim

max711

Thanks for the link, interesting reading. I tried the 5k resistor with no luck. My problem is a little different becasue I'm not trying to drive a servo directly, I just need a 5v signal. I think I might try pulling the output up to 5v with a resistor and see what happens. The problem is I don't fully understand how the TXB0108 chip works.

max711

I'm thinking I might be better off with a #40000 P8X32A Quick Start board and then figure out some way to convert the 3.3v outputs to 5v through some sort of converter? Is there a simple way to convert 3.3v to 5v?

JonnyMac

This would probably do the trick: http://www.onsemi.com/pub_link/Collateral/MC74VHCT541A-D.PDF.

VIH is 2v which is plenty low enough for the 3.3v output of the Propeller.

[Edit] After posting found this is the device (74HC541) that the QuickStart uses to drive the LEDs.

max711

Thanks! That looks like it would work. The schematic for the Quick Start shows 3.3v on the (74HC541) but I'm assuming it could be changed to 5v?

JonnyMac

Yes; you'll see that the device is designed for 5v (TTL). The important think is to check the VIH level of the inputs. This is 2.0 which is well below the Propellers 3.3v outputs.