How does USB work with PIC18LF2550, when Vcc=3.3V?

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kender

Joined: 19 Jun 2005
Posts: 1230
Helped: 88
Location: Stanford, SF Bay Peninsula, California, Earth, Solar System, Milky Way

08 Dec 2006 7:54 pic18lf2550

Colleagues,

If I understand correctly, the USB is a differential bus with 0 to +5V levels. Microchip makes PIC18LF2550, which can run off 3.3V. Can PIC18LF2550 communicate on the USB, when it’s powered with 3.3V?

Thanks,
Nick

KST

Joined: 12 Dec 2005
Posts: 130
Helped: 8

08 Dec 2006 8:11 how does usb work

kender wrote:

yeh but When U use 3.3v, the speed must down (CLK), and you must use buffer !
Good luck for U !

namqn

Joined: 19 Feb 2006
Posts: 172
Helped: 19

08 Dec 2006 12:49 vcc usb

kender wrote:

The power bus is 5V, but the signal levels at Low-speed/Full-speed are for 3.3V rail (min = 2.8V and max = 3.6V for high level). See Table 19-2, page 518 in USB Complete, 3rd Edition.

Cheers,

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08 Dec 2006 12:49 Ads

kender

Joined: 19 Jun 2005
Posts: 1230
Helped: 88
Location: Stanford, SF Bay Peninsula, California, Earth, Solar System, Milky Way

09 Dec 2006 3:58 usb vcc

KST wrote:

yeh but When U use 3.3v, the speed must down (CLK), and you must use buffer !

Thanks!

Your post brings up another question. D+ and D- lines on the USB are bi-directional how do I make a bi-directional buffer

Would anyone post a link to a reference design Question

Thanks again, everyone!

Kender

KST

Joined: 12 Dec 2005
Posts: 130
Helped: 8

09 Dec 2006 11:55 vcc usb post

kender wrote:

KST wrote:

yeh but When U use 3.3v, the speed must down (CLK), and you must use buffer !

Thanks!

Your post brings up another question. D+ and D- lines on the USB are bi-directional how do I make a bi-directional buffer

Would anyone post a link to a reference design Question

Thanks again, everyone!

Kender

You should search in some address as (bidirectional buffer):
www.maxim-ic.com
www.ti.com
www.analog.com
. . . . . . . so on . . . . (easy to search)
many and many for U. My mind is not good. Good luck for U !

TechToys

Joined: 16 May 2006
Posts: 151
Helped: 17

09 Dec 2006 16:48 18lf4550

Buffer in the microcontroller side known as end-points. End-points built-in the 18F2550/4550 series. There are IN and OUT endpoints as buffer for the direction in and out for the host. For IN, it is in respect to the host (PC) meaning data flowing in the PC from the mcu. For OUT, data flows from PC to mcu.

John

kender

Joined: 19 Jun 2005
Posts: 1230
Helped: 88
Location: Stanford, SF Bay Peninsula, California, Earth, Solar System, Milky Way

10 Dec 2006 23:49 pic vusb

TechToys wrote:

Buffer in the microcontroller side known as end-points. End-points built-in the 18F2550/4550 series.

Thanks, John!

From the fig. 17-1 in the datasheet (probably I should have looked in the datasheet before posting the initial question), I understand that the signal levels that PIC would put on the USB will be 0 to 3.3V even when Vcc=+5V, because the PIC has an internal 3.3V regulator. Perhaps, I should connect both the PIC's Vcc and Vusb to my external 3.3V regulator and disable the PIC's internal regulator.

What do you think?

Cheers,
Nick

TechToys

Joined: 16 May 2006
Posts: 151
Helped: 17

11 Dec 2006 4:37 vusb pic

Code:

connect both the PIC's Vcc and Vusb to my external 3.3V regulator and disable the PIC's internal regulator

Yes, it is possible to do that. The other way is to enable the internal regulator and bypass the Vusb pin with 470nF capacitor. Because you are using 3.3V, there won't be 3.3V output from the internal regulator. Instead, it would be around 3.1x to 3.2x volts depends on individual 18LF4550 chips. I have measured samples of over 20 18LF4550 chips from my development boards and found the Vusb is always higher than 3.1x volt; sometimes it reaches 3.2x volt for individual chip!

Even for the lowest Vusb, all PIC USB demo and even the Mass Storage Device routine work without any problem.

However, running at 3.3V Vdd with internal Vusb regulator on is something off the specification. I've asked Microchip Tech Support for this but they don't recommend doing so; even though my samples working.

John