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PICList Thread
'[PIC] picdem fs usb schematic ?'
2006\02\02@091649 by Comfix

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Hi,
I have a question about the schematic of picdem fs usb:

On figure A3 (Introduction to the picdem fs usb board)
(document 51526a.pdf) the usb plug J1 has R3 and R4
of value 0ohm. What does that mean?
tia.

2006\02\02@094944 by Robert Young

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> -----Original Message-----
> From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu
> [.....piclist-bouncesKILLspamspam@spam@mit.edu] On Behalf Of comfixspamKILLspambtinternet.com
> Sent: Thursday, February 02, 2006 8:17 AM
> To: .....piclistKILLspamspam.....mit.edu
> Subject: [PIC] picdem fs usb schematic ?
>
>
> Hi,
> I have a question about the schematic of picdem fs usb:
>
> On figure A3 (Introduction to the picdem fs usb board)
> (document 51526a.pdf) the usb plug J1 has R3 and R4 of value
> 0ohm. What does that mean? tia.

0 Ohm = short.  Without looking at the schematic or board, three things
come to mind.  First there are alternate circuit configurations
available and the use of the 0 Ohm jumpers is selecting the "default"
form.  Second the designer wasn't sure if he needed a resistor there and
so just put one in, later found out it wasn't necessary and specified a
0 Ohm jumper instead of modifying the layout and having more blank PCBs
made.  The third possiblity is that these are the series resistors for
the D+ and D- signals from the USB connector to the PIC.  Somebody
decided that most of the time, 0 Ohms was OK but if you have problems
you can replace them with 27 Ohm, 68 Ohm or whatever it takes to clean
up the lines.  I would be supprised if they didn't include a couple of
pads for small caps on D+ and D- too.

Rob

2006\02\02@104556 by Comfix

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Hi ,
I woudn't know, I do not have the board myself. I was trying to
figure out what I need to order to connect a 18f2455 to a usb
cable. So far I managed to deduct that I need the 20mhz xtal
with the two caps and resistor (the resonator circuit.) Apart
from that, it doesn't appear as though I need anything else
(apart from the 0ohm resistors ;))
thanks.

----- Original Message -----
From: "Robert Young" <EraseMErwybeakerspam_OUTspamTakeThisOuThotmail.com>
To: "'Microcontroller discussion list - Public.'" <piclistspamspam_OUTmit.edu>
Sent: Thursday, February 02, 2006 2:49 PM
Subject: RE: [PIC] picdem fs usb schematic ?


>
>
> > {Original Message removed}

2006\02\02@115431 by Hector Martin

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comfix@btinternet.com wrote:
> Hi ,
> I woudn't know, I do not have the board myself. I was trying to
> figure out what I need to order to connect a 18f2455 to a usb
> cable. So far I managed to deduct that I need the 20mhz xtal
> with the two caps and resistor (the resonator circuit.) Apart
> from that, it doesn't appear as though I need anything else
> (apart from the 0ohm resistors ;))
> thanks.
>

Add in a cap to Vusb. Also, you can do with many oscillators (I use
8Mhz), since you have yo use the internal PLL to do USB anyway (unless
you use a 48Mhz oscillator). I use a 8Mhz xtal, PLLed up to 48Mhz for
USB, and divided to 24Mhz for the CPU.

--
Hector Martin (@spam@hectorKILLspamspammarcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc

2006\02\02@121713 by Comfix

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Hi,

> Add in a cap to Vusb. Also, you can do with many oscillators (I use
> 8Mhz), since you have yo use the internal PLL to do USB anyway (unless
> you use a 48Mhz oscillator). I use a 8Mhz xtal, PLLed up to 48Mhz for
> USB, and divided to 24Mhz for the CPU.

My plan is to program the device with my own programmer, and then to
connect it straight to the pc (without picdem fs usb). I think you are right
about the extra cap to Vusb (or VBUS as they call it). I think I also need
to ground pins 20,21 of the USB_ATTACH feature to keep the app
happy.
Do you happen to be familiar with the board, it seems there should be
a sample mouse hid device somewhere, I can't find it?

2006\02\02@130814 by Comfix

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I found that sample now.

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