Re: [avr-chat] ATMega 645 USART problem.

[Joerg Wunsch]

As Marc Moreau wrote:

> After much trial and frustration I found the problem.  The clk/8
> fuse is programmed by default in 325/645.  After reading the
> documentation in that section multiple times I discovered the odd
> default.

It's not all that `odd': together with the 8 MHz RC oscillator, this
is the only way to guarantee an AVR as shipped can be programmed
throughout the entire target voltage range.

The difference to older AVRs is that the RC oscillator of modern AVRs
now exclusively runs at 8 MHz, and can no longer be switched between
like 1, 2, 4 or 8 MHz.  Instead, you are now supposed to use the CLKPR
to scale it down when needed.  The CKDIV8 fuse is doing nothing else
but pre-loading CLKPR.  On the pro side, the new RC oscillator is much
less dependant on Vcc and Tamb than the older ones used to be.

-- 
cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)