Alternate clock speeds

General discussions about V-USB, our firmware-only implementation of a low speed USB device on Atmel's AVR microcontrollers
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Alternate clock speeds

Post by marco » Wed Oct 03, 2007 3:23 am

I have an ATmega designs that operationally needs to support RS232/RS485 for remote control functions. But for diagnostics, configuration and data logging readout we are considering to support a USB port so a PC can be plugged in directly to do these functions and the unit is not expected to operational at that stage.

However, I am concerned about the clock speeds that are suggested for the
USB interface of 12 or 16MHz as for proper RS232 baud rates, my clock speeds needs to be 14.745MHz.

Can the USB operate correctly with the 14.745Mhz clock or is it so tightly coupled to instruction timings that it must be 12 or 16MHz?



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Post by christian » Sun Oct 07, 2007 9:19 pm

The clock rate must be correct within a couple of 100 ppm (parts per million). 14.75 MHz is definitely out of this range.

Please note that asynchronous protocols usually tolerate clock rate deviations of more than 1%. You can usually trim the baud rate precisely enough from any high frequency clock, unless you need really fast transfers.

BTW: We now also have a 15 MHz module, if that helps.

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