AVR-Chat

Indeed,

   I generally sample the bit stream at the fastest rate I can, between 
16 and 60X the clock rate, then arrange subsequent checks to be centered 
in the rest of the bits.  I've gotten a 4MHz 12C508A to run reliably on 
its internal RC oscillator at 9600 bps and a Tiny12 at 1MHz at 9600 bps. 
  The 12C508 was totally on a timed FSM, the Tiny12 had the advantage of 
having an IRQ that I could use on a timer.  All in assembly of course, 
you probably couldn't get a receiver to work well using a high level 
language.

fun stuff,
DLC

Don Kinzer wrote:
> --- In AVR-Chat@yahoogroups.com, James Washer <washer@t...> wrote:
> 
>>Is this correct?
> 
> 
> Yes.
> 
> As you may know, the trick to bit banging a serial input is to arrange 
> to sample the input line consistently somewhere near the middle of the 
> bit window.  Generally, this means that you need to sample the input 
> line at some multiple of the bit frequency (probably 4x is the lowest 
> practical multiple, 8x or higher is even better if you can afford the 
> overhead) and then arrange to sample the input line at multiples of 
> the bit time after a start bit has been detected.  This process is 
> complicated by the fact that the transmitter's clock can have 
> virtually any phase relationship to the receiver's clock so you have 
> to design your software to accommodate the worst-case clock skew.  
> Moreover, the transmitter's clock and the receiver's clock can vary 
> from the ideal frequency in opposite directions and the resultant 
> error is cumulative over the 9 to 11 bits of the character being 
> received.
> 
> Compared to reception, bit bang transmission is a cinch.
> 
> I implemented a 4-channel bit bang full duplex UART in assembly 
> language for the ZX microcontrollers; they are based on a mega32 
> running at 14.7456MHz.
> 
> Don
> ZBasic Microcontrollers
> http://www.zbasic.net
> 
> 
> 
> 
> 
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 

-- 
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Dennis Clark          TTT Enterprises
www.techtoystoday.com
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