USB based product - what protective devices to put in?

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Hi Everyone,

I'm thinking of building my first USB type product using an atmega8|16|32u2 - it will run on USB provided 5v - max power use around 75mA.

What type of protective devices would you put in?

1. PTC fuse?

2. ESD protection on the D+/D- lines?

3. TVS on the 5V provided by the USB?

4. Diode on the uC reset line?

Thanks,

Alan

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1: Fuse, definitely. You never know what you customers will try to do, and you can't depend that everything that looks like a USB connector actually is a standard USB connector.

For example, I have come across some Point of Sale equipment with a separate cash drawer, connected by what looked like a standard USB connector. However, this one had 12 volts where there should be 5... resulting in several smoked mice and keyboards when clerks who didn't know any better happily plugged things in 'the wrong way'.

A fuse only hurts your pocket book a tiny bit if it never blows. Compared to being sued, it is not any cost at all.

People are 'trained' these days to powering down and back up when there is any problem with their 'technology', so a PTC fuse will do just fine.

2: ESD protection, definitely. How often do you think you customers will walk across a carpeted room, building up a significant charge, and then plugging in your device?

3: TVS, same as 2. Put it behind the fuse.

4: Reset diode only hurts your pocketbook. If the reset line is 'exposed', then definitely.

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The USB6B1 is a protection device for the datalines and Vbus.

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Hi jayjay1974,

How does the USB6B1 compare to the USBLC6?

http://www.digikey.com/product-s...

Thanks,

Alan

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Why do you think you need any extra protection?
1. No
2. No
3. No
4. what for?

These chips meet USB standard.
Are these (1-4) additional USB requirements that have to be met?

No RSTDISBL, no fun!

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Hi,

It sounds like you are a minimalist Brutte!

I guess it is always a balance. I don't want to have products come back for repair and if I can design towards that goal with a minimal amount of added expense, then I'm for it. At the same time I don't want to add complexity, cost, or work, so there is the balance. I sure don't want someone complaining that my product killed their USB port...

Thanks,

Alan

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1. No real need
2. Atmel added protection on the USBKey board.
3. Won't hurt
4. Diode is recommended by Atmel as there is no protection diode on the reset line to Vcc because it must be able to handle 12V for high voltage programming. Therefore it's more susceptible to damage. Not an USB specific issue though.

Being USB compliant does not necessarily means it complies without additional components. Like the 27R resistors in the datalines and ferrite bead in Vbus.

So, just add the components. The USBLC6 is only $0.36 (seems to offer the same functionality as the one I mentioned, just in a small package) and just one warranty return costs more than the extra expense. I don't think you're going to make thousands of the units ;)

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OP wrote:
if I can design towards that goal with a minimal amount of added expense, then I'm for it.

Define "minimal".
The USB standard takes care of that protection. If you include everything USB requires and this approach fails, then IMHO it is a bad standard.
OP wrote:
At the same time I don't want to add complexity, cost, or work, so there is the balance.

Perhaps you should also consider this if you want to design your product "always being on a safer side". AFAIK this BOR remark also applies to U2 series.
Also take a look at PIC16F1455 if you want your design to be simple. It does not require any quartz or D+/D- resistors. It is 12MIPS at U >=2,7V and also significantly cheaper than ATMega16U2. Comes with ADC, DAC, thermometer etc.

jayjay wrote:
2. Atmel added protection on the USBKey board.
Ok, but does it mean their product does not meet USB specification without it? USBkey is not a consumer electronics device but an evaluation board where you can/will touch it with your 10 ESD guns so I do not think USB specification targets such market.
jayjay wrote:
4. Diode is recommended by Atmel(..)

Even when you simply tied nRESET to Vcc (via ext. or internal pull-up) and someone mistakenly applied 12V to V_USB, do you really think it matters if the chip enters programming mode or not? What if it actually did? If you are concerned, pull-up SCK or MOSI high and problem is gone - you will not erase it with noise on IOs without a connected programmer then. Or perhaps there are already pull-ups there..

Quote:
Being USB compliant does not necessarily means it complies without additional components.

I hope we are not discussing the need for external components (like decoupling caps or D+/D- resistors). It is about requirements/protection/components that are not specified in a datasheet of the chip.
Can you imagine whole USB "industry" to function in such way, by individually deciding which component is and which is not required for interoperability/compliance?

No RSTDISBL, no fun!

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4. It's not about inadvertently entering programming mode, but damaging the chip as there is no internal ESD diode like on all other pins.

Quote:
Can you imagine whole USB "industry" to function in such way, by individually deciding which component is and which is not required for interoperability/compliance?

It does specify, for example, that no more than 10uF of capacitance is allowed on Vbus. They even have a PC tool that takes input from your scope and decides whether it complies or not. If not, no USB logo on your product.

So yes, compliance depends on added components. Of course, you debate again whether this has anything to do with the MCU :roll:

I don't know the USB standard that well to know if there are any ESD protection requirements in them. Probably they just refer to regular CE standards. Even if there were requirements, I don't think it says the protection has to be implemented in the MCU itself. Why would they care where the actual protection device is?

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Quote:
4. It's not about inadvertently entering programming mode, but damaging the chip as there is no internal ESD diode like on all other pins.

Now I get the diode idea. As I understand this is one of Atmel's suggestions to follow requirements specified in absolute maximum ratings table.
Ok, I agree, diode solves the problem up to some kV applied to nRESET pin. You can also satisfy this constraint with adequate pull-up resistor and cap. Alternatively, if you are paranoid, you can RSTDSBL and solder this IO to GND. And many other ways of how to not violate absolute maximum table.
Quote:
Even if there were requirements, I don't think it says the protection has to be implemented in the MCU itself.

Now, the interesting question in here is if USB standard defines the ESD test conditions on D+ and D- that the chip must survive. As you suggest it may happen that internal D+ and D- clamping diodes/protection are not characterised by USB standard, chip vendor does not care as his product is USB-ready and it is a designer who must take care of that problem and add some external circuitry or the device will not pass USB certification (or die in customer's hands)..
Quote:
Why would they care where the actual protection device is?

I thought USB transceiver requirements characterize that. If the transceiver does not meet specification then it is not an USB transceiver.
Through the discussion I can see that may not be so obvious.
Atmel only stated:
    "do not apply voltages from out of [GND-1V* to Vcc+0,5V] range to D+ and D-", "mount D+ and D- resistors of 22ohms+-5%" and
    "Our chip is USB 2.0 compliant"
but can we infer from these statements that to meet USB compliance a device only requires a chip, D lines resistors and an usb connector? Or perhaps additional voltage suppressors, fancy active filters and amplifiers on D+ and D- and a gold wire from the chip to USB hub are a must or a device will not work?

On the other hand why are there USB ESD protection devices on the market when the chips are USB compliant? Anybody seen a uC with USB where the datasheet explicitly stated: "Our chip does not have USB protection circuitry and will not pass USB certification unless an external one is added."?

No RSTDISBL, no fun!

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Part of the following describes some of the weaknesses of USB
(ESD and the like, ground loops, EMI from motors, arcing from loose connections):
How to avoid a Hindenburg-level USB meltdown in harsh industrial environments by Brian Foster (B&B Electronics Manufacturing Co.) (EE Times, 6/13/2012).

"Dare to be naïve." - Buckminster Fuller