USB Circuit example - help needed

[PeterDove]

Hi All,

I built this small USB circuit for my PIC18F2455 - I have now been able to compile and upload the examples to the PIC. However nothing happens at all in windows, no drivers get installed, no com-ports appear NADA. Would anyone be able to let me know if my little circuit is suitable for running these demos? I realise that the LEDs wont work/ligh in the demos that use them but surely it should still work?



Thanks in advance

Peter

[Markdem]

Hi, the first thing i would do is change C3 to somthing more like a 470uf cap, see how you go then.

Have fun , Mark

[Ttelmah]

Start with needing some decoupling on the Vdd-Vss connection close to the PIC. Something like 100uF, paralleled with 0.1uF.
The, you will need to comment out the lines in the source, dealing with USB connection sensing, or make the connection for this.

Best Wishes

[PeterDove]

[PeterDove]

[Guest]

You need to put 470nF non-polarized capacitor between pin RC3 (Vusb) and VSS

[Ttelmah]

There are a series of points here. The first, is that some people have found that particular batches of the 2550, need more capacitance on Vusb, than the specified 470nF. Also, the capacitor shown here in the diagram is a polarised 0.22uF, when non polarised types will generally perform better a high frequencies. I'd start by putting the specified non polarised 470nF, but be prepared to try a higher value if problems still exist.
The second, is that as well as the Vusb decoupling, there should be decoupling close to the PIC, between Vss and Vdd. Again because of the difference in frequency performance of capacitors (something like a 10uF electrolytic capacitor will perform awfully, when dealing with 'edges' of a few nSec), the common practice is to use a fairly large polarised capacitor, with a second small non-polarised capacitor drectly in parallel, to give better high frequency performance.

Best Wishes