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PIC18F87J50 USB osc configuration options

 
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ktallevi



Joined: 17 Dec 2005
Posts: 58

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PIC18F87J50 USB osc configuration options
PostPosted: Mon May 12, 2008 2:44 pm     Reply with quote
Im implementing USB in a mobile device. Using the PLL would draw too much current so I was hoping to just use a 24MHz crystal without the PLL enabled.

If I understand the datasheet correctly, I can use a 24Mhz crystal for USB low-speed operation. Would the USB work correctly using a 24Mhz crystal with PLL disabled?

Ideally I would like to run the device using the INTRC at 32khz to perform certain operations, then when the USB is plugged in (ill use usb-sense with an interrupt) switch over to the 24MHz crystal.

Being a mobile device I need to save as much power as possible when the USB is not connected.

thanks
PCM programmer



Joined: 06 Sep 2003
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PostPosted: Mon May 12, 2008 3:12 pm     Reply with quote
Look at this table in the PIC data sheet:
Quote:
TABLE 2-5: OSCILLATOR CONFIGURATION OPTIONS FOR USB OPERATION

The 5th entry shows that you can use a 24 MHz crystal or oscillator
(in HS or EC mode, respectively) to provide the PIC's clock frequency.
There is a note in the PLL Division column that says this configuration
is only valid for low-speed USB operation.
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