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'High speed UARTs and PICs'
1996\06\13@110926 by Newfound Electronics

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G,day again,

Thanks to all who gave tips on tapping the crystal oscillator. Tomorrow
I shall eat the pudding!


Now can anyone help set me straight (if that's possible) on the
situation with the built-in UARTS on the PICs. Yes I know there
has been a lot of discussion about them but I didn't take any
notice until I need to know.

I want a 15.2K baud link. Failing this, 57600 or even 38.4K baud.
For this application a software UART is out of the question,
(Though it can be achieved, it is just not suitable.)

Questions:

1) Can the UART in the PIC16C64/65/74 WORK CORRECTLY at these high baud
  rates? This would require BRGH =1 and isn't this supposed to be bugged?
  Do any of the devices work as stated? Are there work-arounds?

2) I might even use the 17C4x family. Does anyone know what OBTAINABLE
  crystal Frequency I should use for 38.4K, 57.6K or 115.2K Baud. All the
  high baud rates with the usual crystals are really odd-ball. I want to
  interface it with a PC at standard baud rates.

Can any superior intellect enlighten me?

Thank-you and Regards,

Jim

1996\06\13@114947 by Ray Gardiner

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>I want a 15.2K baud link. Failing this, 57600 or even 38.4K baud.
>For this application a software UART is out of the question,
>(Though it can be achieved, it is just not suitable.)
>
>Questions:
>
>1) Can the UART in the PIC16C64/65/74 WORK CORRECTLY at these high baud
>   rates? This would require BRGH =1 and isn't this supposed to be bugged?
>   Do any of the devices work as stated? Are there work-arounds?
>

I have been using the 16C74 at 156250 bps for an RS485 multidrop network for
some time now and not experienced any problems with BRGH=1.

I can't speak for the other parts. But I can recommend the 16C74.

1996\06\13@142027 by Richard Klosinski

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At 01:52 AM 6/14/96 +1100, you wrote:
>>I want a 15.2K baud link. Failing this, 57600 or even 38.4K baud.
>>For this application a software UART is out of the question,
>>(Though it can be achieved, it is just not suitable.)
>>
>>Questions:
>>
>>1) Can the UART in the PIC16C64/65/74 WORK CORRECTLY at these high baud
>>   rates? This would require BRGH =1 and isn't this supposed to be bugged?
>>   Do any of the devices work as stated? Are there work-arounds?
>>
>
>I have been using the 16C74 at 156250 bps for an RS485 multidrop network for
>some time now and not experienced any problems with BRGH=1.
>
> I can't speak for the other parts. But I can recommend the 16C74.
>

I dont know about your application, but I have had many problems with the
BRGH set. My product must be used on any PC, and works fine on 95% on the PCs
out there. A customer loaned me a laptop that would not work with my product.
After doing all the engineering stuff, like analyzing data stream, code, and
pulling my hair out, I decided to review the April 96 errata on the 74. One
now-documented-feature was that the user "may see a high number of
communication
errors" if the BRGH is set.

So, to make a long story short, I replaced the xtal with a 7.3728mhz,
changed the
firmware, and it fixed the problem. Thank you Mchip.

Note that the problem only occured after sending about 7k of continuous data.

Richard

Parallax Inc, t - 1.5 days

1996\06\13@152000 by Ray Gardiner

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> I dont know about your application, but I have had many problems with the
> BRGH set. My product must be used on any PC, and works fine on 95% on the PCs
> out there. A customer loaned me a laptop that would not work with my product.
> After doing all the engineering stuff, like analyzing data stream, code, and
> pulling my hair out, I decided to review the April 96 errata on the 74. One
>now-documented-feature was that the user "may see a high number of
>communication
> errors" if the BRGH is set.
>
> So, to make a long story short, I replaced the xtal with a 7.3728mhz,
> changed the
> firmware, and it fixed the problem. Thank you Mchip.
>
> Note that the problem only occured after sending about 7k of continuous data.
>


My application involved sending short packets, not 7K continuously,
maybe a small mismatch in baudrate over a long transmission makes a
difference. Also because the BRGH changes from x64 to x16 might make for
more sensitivity to slightly mismatched baud rates. Would changing sampling
frequency per bit increase susceptability to noise?

Yes I saw the Errata. But not until much later! (such is life)

We ran extensive error-rate tests to prove the system. The nett
result is that the 16C74 SCI works fine in my book. Only problem that
I can recall was getting the initialization sequence right.  And remembering
to toggle CREN!.....

I used 4.9152 Mhz xtal and spbrgh=01,brgh=1.




Ray Gardiner, 104 Macintosh Street, Shepparton, Victoria 3630,  Australia
spam_OUTrayTakeThisOuTspamnetspace.net.au

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