hello,my friends!!
I want to know, is it possible to use internal clock for communication between at91sam7s256 and computer via usb.
I know that when I want to program chip with sam_ba and via usb, I have to use 18.432MHz external crystal, but I want to know that ,if I choose another way to program it ,so can I delete external crystal and use internal clock source.

thanks a lot for your attention & answers!!!!!!!!!!

Seems pretty clear from the documentation you cannot use the internal RC (SCLK) to drive the USB. Most of these types of RC are well outside the accuracy specs for USB anyway, and external crystals are pretty much commodity items ($0.10-0.20).

hello,my friend!
thanks you,for your suggestion, and excuse me for my bad English writing.

I have another question,too.
Is it possible to use internal RC for usual project (projects that don't need any usb connection);and how can I do that?

for example assume that I want to design only a LED blinker; I choose a way for programming without any use of usb , I don't want to use external crystal , is it possible or not.
if it is possible, so,how??

thanks a lot,my best friend.
have a good time!!!

The manual you want for the part is here
http://www.atmel.com/dyn/resources/prod ... oc6175.pdf

It will run slowly (22-42 KHz, pg 30) from the internal SCLK (Slow Clock)
You can program the CSS (master clock selection) to b'00 to select the Slow Clock, for the PMC_MCKR (pg 209).

Blinking the LED's should not be a problem, serial ports will likely be unusable. There are better part choices for your hypothetical application.

Try the LED's on a dev/eval board, program the PIO, and loop toggling the GPIO of your choice, try without any delays as it will be relatively slowly, check period on a scope to calibrate software delay loops.

There are other parts with 40 KHz, 8 MHz, internal RC clocks, that require no external oscillators/crystals. Serial will work with such parts. USB is going to require a crystal based source, absent some serious characterization/calibration of individual parts.