Board bringup issues

Hardware questions and issues with the FV-1

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Digital Larry
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Board bringup issues

Post by Digital Larry » Thu Oct 05, 2017 11:17 pm

Finally designed a PCB. Getting 32 kHz on both crystal pins and activity on SDA and SCL when I flip the program select pins around. I believe that I programmed the chip successfully with my PICKit2. I did use socketed DIP EEPROM so I could swap it into Spin dev board to be sure.

I'm seeing a signal at the ADCL left input pin 1. Don't see anything coming out. Clip light is on solid. ADCR is pulled up to VDD through a 4.7k (I am going to use it for tap tempo input but nothing else is connected there.

DACL and DACR are just flat at about 1.6 volts.

I'm sure it's something stupid, but got any suggestions?

Thx,

DL

frank
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Post by frank » Fri Oct 06, 2017 8:30 am

Try using the internal program set to make sure the PCB and chip are functioning. If they are then try the EEPROM in the dev board and see if it works there.
Frank Thomson
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ice-nine
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Post by ice-nine » Fri Oct 06, 2017 3:54 pm

Symptom Clip LED always on

I have found that this is usually down to two things.

1. The oscillator is not working, this will always produce a constant clip LED on. Try some flux solvent to make sure everything is clean.
2. if the FV-1 is set to ext. EEPROM an the EEPROM is empty or not fully making pin contact the clip LED will be on.

As Frank says, set to internal programs and test, if that is ok then check EEPROM and dry joints.
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Digital Larry
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Post by Digital Larry » Sat Oct 07, 2017 7:17 am

Thanks guys. I have not gone fully though this, however:

a) Internal programs pass signal. I'm just looking at a scope trace using a sine wave input.

b) Same EEPROM does not make clipping light come on with FV-1 dev board.

c) what you DIDN'T know is that I have serial 1k/3.3V zeners attached to limit input voltage when connected to my cheap eBay PICKit 2 clone (it seems to want to always output at 5 volts). The zeners are right on the SDA and SCL lines and the 1k resistors go out to the programming header. The signals on SCL/SDA are around 2 volts when inactive (measured either on the FV-1 OR THE 24LC32A). I don't understand why that is happening but rather than split hairs I think I will just remove the zeners completely.

One thing I noticed when programming the EEPROM onboard is that it wouldn't work when the PICKit was plugged directly into the USB port of my laptop, but if I put it on the end of an extension USB cable it worked OK.

As Rosanne Rosannadanna used to say, "it's always something".

Digital Larry
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Post by Digital Larry » Sat Oct 07, 2017 1:47 pm

I pulled the zeners off and... it's ALIVVVEEEEEE moo0000oooowahahahahahahah!!!

:D

The specs say they could have 10 uA reverse current. As this i2c bus is lacking pullups, there's no specific control over the idle voltage if something else is leaking current.

Now to see if I can still program the EEPROM but we are moving on! 8)

Thanks for the help!

DL

frank
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Post by frank » Sat Oct 07, 2017 2:54 pm

There are pull-ups in the FV-1 on SDA and SCL so the I2C bus is not lacking them they are just internal.
Frank Thomson
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Digital Larry
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Post by Digital Larry » Sat Oct 07, 2017 4:37 pm

So is 10 uA reverse leakage through the zeners enough to pull those down by 1.3 volts and prevent reading of the EEPROM from working? It's probably fine to leave the zeners off, I'm just trying to understand what happened here.

frank
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Post by frank » Sun Oct 08, 2017 10:10 am

We would need the schematic of the programmer and the PCB to figure out what happened. And those clones often do not handle voltages properly.
Frank Thomson
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Admiral_dk
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Post by Admiral_dk » Mon Oct 09, 2017 5:31 am

Zeners are NOT alike and almost all the old models are sh!.e ..!
Datasheets are unfortunately not very thrust worthy around and below the zener point - not the same as the reverse leakage current ....

If we are talking about those with a max. of 500mW. - I exclusively use BZX79Cxx as the leaded and BZX55Cxx as SMD for the last 20 years or so to avoid the problems you get.

Digital Larry
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Post by Digital Larry » Mon Oct 09, 2017 7:46 am

I appreciate the insight, Admiral_dk. These are BZX84s. I do need to protect against 5 volts, even though on the pedal I got from Mr. Ice-9, my 5 volt only PICKit-2 clone does not blow up the FV-1, I don't feel comfortable continuing without a voltage clamp.

I'll give the ones you recommended a shot, although in my quick search here, both BZX79C and BZX55C are DO-35 (through hole). Need something in SOT-23 to work with my layout.

For reference, since I didn't supply a schematic, at the point where I am having the problem, the programmer is not connected and the only thing different compared to a stock FV-1 circuit (i2c bus wise) is that there are reverse biased zeners to ground on SDA and SCL.

I also thought that external pullups might help, but FV-1 spec sheet shows the internals are 3.75k ohms, meaning that the zeners are leaking something like 0.33 mA!

ice-nine
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Post by ice-nine » Mon Oct 09, 2017 12:38 pm

If this is a real problem Larry you could always add 5v to 3.3v level shifters on both the SDA and SCL. Used this method for interfacing the 5V Arduino to FV-1 and EEPROM. This works just fine.
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Digital Larry
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Post by Digital Larry » Mon Oct 09, 2017 2:18 pm

Thanks, Ice-9, I'm looking for an approach that will work whether the input is 5V or 3.3V. Would the level shifters do that?

ice-nine
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Post by ice-nine » Tue Oct 10, 2017 4:53 am

Digital Larry wrote:Thanks, Ice-9, I'm looking for an approach that will work whether the input is 5V or 3.3V. Would the level shifters do that?
Hard to say really if that would work reliably, but it would be quick enough to test out on breadboard.
you would need to use two level shifters, one for SDA and one for SCL, each shifter consists of 1 mosfet and 2 resistors.

also available on ebay cheaply if you just wanted to get one of these to test.

http://www.ebay.co.uk/itm/5PCS-5V-to-3- ... SwXeJYHFxc
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amz-fx
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Post by amz-fx » Sun Oct 29, 2017 6:25 am

Level Shifter:

Image

You may need to lower the value of the pullup resistors if this does not work consistently.

regards, Jack

ice-nine
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Post by ice-nine » Sun Oct 29, 2017 11:03 am

amz-fx wrote:Level Shifter:

Image

You may need to lower the value of the pullup resistors if this does not work consistently.

regards, Jack
Exactly that, Jack :)
www.stanleyfx.co.uk
"It's fairly straight forward, if you want to start it press start, you can work out the rest of the controls yourself."

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