RE: gEDA-user: Announcement : gTAG schematics released

["Luis Miguel Brugarolas" <lmbruga@sire.es>]

> > 1.- I have used EZ-USB parts before the company was bought by Cypress.
> 
> Aha an experienced user user! Good! What is your over all experince of
> these chips? Are they good or have you gone further to better stuff? A
> couple of my friends are trying to use a Philips chip and a PIC.
> Apparently the Philips chip are a bit buggy?
Achor Chips provided really good devices and design environment. We didn't have any problem except for the one mentioned. It allowed us to do a nice product.

> > In my first prototype I used the RESET arangement recommended by the
> > manufacturer, which is the one used in gTAG. While this worked well
> > for some PCs it did not for others 
> [snip]
> > We measured in those PCs (well known manufacturer) the USB supply rise
> > time was very slow. You can fix for this PC but tomorrow will come
> > another slower, and you have to change once again. We solved th
> > problem using a 3-pin MAX-810SEUR-T connected to RESET, GND and +3.3V
> 
> Hmm, you're right. They usually suck, these cap-res stuff. I have a power
> supply circuit from Maxim that converts 5V to 3.3V (MAX882). I looked at
> the data sheet and it said it has a rise time of the power out of 200
> us. Hope it's enough. So it seems like I'm saved by the bell :-)
> 
> Serious, I didn't consider this at first. When you pointed it out I
> thougth "o, no", but after looking in the data sheet for the Maxim MAX882 
> I think it will work as is.

No: this has to be fixed. The problem is not in the LDO, but in the USB power from the PC, which supplies the LDO. If this supply rises slowly (it would take me time to review my companion notes on HOW MUCH), the LDO has nothing to do, and the USB bridge will not have a proper RESET if taken from R-C network.
And it is difficult to take notice of the effect as the manufacturer recommend something different. I think USB consortium specifies a 'fast' supply rise time, but it seems some manufacturers (I insist, one of the biggest) are non compliant to this, maybe due to supply rise time of the PC power supply.

Best regards

Luis Miguel Brugarolas