pandaemonium
Member
I'm taking the leap to USB interfaces for my projects, but I'm having problems with voltage limiting on the USB data lines. I prefer to use a +5v supply because many of the great IC's out there need it to run stability, but that means using a couple of 3.6v zeners on the USB data lines with a 1K5 pull-up on D- (and maybe a 15K pull-down on D+???).
I know I need to use low capacitance zeners (most people say low wattage, but it usually amounts to the same thing in that respect), but I still have no luck! I've tried about a dozen different zeners, and I always end up with a "unknown device" error from the O.S. and this is with a circuit and code known to function (a very very simple SNES controller to USB adapter that I chose to learn USB from).
From my measurements of the most recent zener I used, the data lines don't exceed 3.32v, I made a rig to check capacitance in-circuit, and it doesn't exceed 290pF and that is consistent with the data sheet (which is better than the parts specified in working circuits). And yes, I know how to build circuits and program MPU's. I've made hundreds over the last 20 years, it's just a problem with USB interfacing, and I suspect the clamping zeners, because that is always a unreliable and unpredictable way of doing things where precision is needed.
Can the USB cable be an issue? I didn't use a cheap one, resistance and capacitance should be low. Any ideas?
Perhaps I need to request the help of a electro-exorcist?
Just a thought; Is there a nice little IC out there that I can put between the MCU and the USB port that runs from a +5v supply that can do the trick? If not, maybe I should talk to the awsome engineers at Maxim!
I know I need to use low capacitance zeners (most people say low wattage, but it usually amounts to the same thing in that respect), but I still have no luck! I've tried about a dozen different zeners, and I always end up with a "unknown device" error from the O.S. and this is with a circuit and code known to function (a very very simple SNES controller to USB adapter that I chose to learn USB from).
From my measurements of the most recent zener I used, the data lines don't exceed 3.32v, I made a rig to check capacitance in-circuit, and it doesn't exceed 290pF and that is consistent with the data sheet (which is better than the parts specified in working circuits). And yes, I know how to build circuits and program MPU's. I've made hundreds over the last 20 years, it's just a problem with USB interfacing, and I suspect the clamping zeners, because that is always a unreliable and unpredictable way of doing things where precision is needed.
Can the USB cable be an issue? I didn't use a cheap one, resistance and capacitance should be low. Any ideas?
Perhaps I need to request the help of a electro-exorcist?
Just a thought; Is there a nice little IC out there that I can put between the MCU and the USB port that runs from a +5v supply that can do the trick? If not, maybe I should talk to the awsome engineers at Maxim!
Last edited: