Zener diodes at D+ D-

[Guest]

Thanks for the quick response. I also thought that the usb comms should work for it to be recognised. The waveforms don't look ideal, but it does go from limit to limit. I suppose the only other culprit could be the firmware then?

I would appreciate some clarification on the role of the zeners though. I understand their purpose, but don't understand the way they are used. With a 2k2 pullup, or even a 1k5 pullup there is not enough current to properly drive the zener into its break down region. I get only 3V on the zener, which is way too close to the lower limit is it not?

[Dieter]

Only noticed now, last post was mine but I was not logged in

[christian]

3 V is enough to be recognized as high level. When the AVR outputs +5V on D+ or D-, the diodes limit the voltage to 3.6 V, which is also accepted as high level inside the common mode range of the input amplifier, although it's slightly outside the spec.

3.6 V is a compromise between a too low level with the pull-up resistor and a too high level with the 68 Ohm series resistor.

If you want to meet the spec, then you should add a 3.3 V (or even better: 3.5 V) regulator for the AVR's power supply. But then you probably operate the AVR outside of its allowed clock range...

[Dieter]

OK, thanks. That makes sense. So in your opinion the most likely cause of my problem would be the firmware, seeing that Windows recognises the device?

[christian]

Yes. I would recommend that you run avrdude with the highest debugging level enabled and look at the communication. May be a wiring problem on the ISP end.

[sawan sharma]

From main page:

"D1 and D2 are a low cost relpacement for a low drop 3.3 V regulator chip, such as the LE33. Operating the AVR at higher voltages exceeds the common mode range of many USB chips. If you need to run the AVR at 5 V, add 3.6 V zener diodes at D+ and D- to limit the voltage."

Why should be the 3.6 V zener diodes ? (not 3.3 V) ? USB data lines work at 3.0 - 3.6 V.

Is it necessary to use zener diode when i am using 9828 voltage regulator IC to make voltage 3.3v

[christian]

Is it necessary to use zener diode when i am using 9828 voltage regulator IC to make voltage 3.3v

No. You need either the zener diodes or the voltage regulator.

[C. Brosseau]

OK. It tooks me 10 hours to make USB commmunication works. Here, I was just able to get 3.6 V, 1 Watt zener diodes. I’ve never been able to make it work.

Then I found 3.2 V, 1/4 Watt zener diodes. Didn’t work. After many many attemps, I’ve begun to use a scope to have an idea of what was going on… and it worked! It was due to the scope impedance. So, I’ve added a 500 k resistance between ground and D-. And it works!!

Also, I power the uC from USB. I need to put a 0.1 uF in parallel to Vcc and Gnd.

[declan smith]

Also, I power the uC from USB. I need to put a 0.1 uF in parallel to Vcc and Gnd.

Put a 4u7 electrolit capacitor at USB Vcc and Gnd if you supply your uc with USB bus not 0.1uf.

So, I’ve added a 500 k resistance between ground and D-. And it works!!

That's very strange to do, you didn't have to do that. You'll get another issue if you use different cable length, like more than 5 meters.
You must be had hardware wiring issue, the circuit was not that critical as long as enough ground plane or ground wiring and also a stable supply.
I tested my circuit with breadboard (which don't have ground plane) and nothing happen like that even I use long enough cable for testing. and I tested at many AVR type for my projects.

Here, I was just able to get 3.6 V, 1 Watt zener diodes. I’ve never been able to make it work.

It works fine with 1 watt zener, but 250mw is enough.
Don't forget to use 68 ohm series resistor like the circuit did.

[winelight]

Just a comment to say that my supplier sent 500mW Zeners and as you might expect, they work fine.

[Karrahahu1]

Also, the data line on an oscilloscope looks like a triangle when it goes low. I suspect I have too much capacitance on the line, but can't figure out from where.???



Ali

[KerimF]

This is just another idea.
Lately I built the circuit of EE_prog project. Since I couldn't get a zener (around 3V3), I replaced it with a conventional "white" LED (small one). It happens that its forward voltage at the nominal driving current (of D+ and D-) is about 3V3. But I also added in parallel to it a switching diode (as 1N4148) to emulate the zener in its reverse direction (acting as a forwarded diode). The circuit worked and the two LEDs also help in monitoring the level status of the D+ and D- lines.
Kerim