Sean

Assuming that the data line is being held for a nominal 16 cycles, then for=
 at least 14 of those cycles, the data is stable at a FF on a clock edge. T=
he only time that metastability might be an issue is at the first and last =
as the data changes.

Veronica

On 2011-07-02, at 10:19 PM, Sean Breheny wrote:

> Hi Veronica,
>=20
> Yes, I understand how UARTs work. Just as all logic circuits, they
> depend on internal consistency. If two signals within the UART are
> both supposed to be at the same value as the sampled input, but they
> instead differ because they come from two different logic gates which
> were "asked" to interpret an invalid logic level and they did so
> differently, then all bets are off. The 16x sampling is intended to
> help reduce noise sensitivity - it has nothing to do with the problem
> I am talking about.
>=20
> Sean
>=20
>=20
> On Sun, Jul 3, 2011 at 12:30 AM, Veronica Merryfield
> <veronica.merryfield@gmail.com> wrote:
>>=20
>> On 2011-07-02, at 8:57 PM, Sean Breheny wrote:
>>=20
>>> Let's make things more concrete: let's say I have two PICs
>>> communicating via their internal USARTs in asynchronous mode.
>>=20
>> In which case it doesn't matter a jot if the clocks are the same or not.=
 From a PIC data sheet on the UART...
>>=20
>> "The data recovery block operates at 16 times the baud rate, whereas the=
 main receive serial shifter operates at the baud rate. After sampling the =
UxRX pin for the Stop bit, the received data in UxRSR is transferred to the=
 receive FIFO (if it is empty).
>>=20
>> The data on the UxRX pin is sampled three times by a majority detect cir=
cuit to determine if a high or a low level is present at the UxRX pin."
>>=20
>> Two things to note -
>> 1. the sampling of the data pin is at 16x the baud rate. This is common =
practise for most UARTs and variants.
>> 2. the UART logic is looking for a level not an edge.
>>=20
>> However, there are other reasons why a common clock might not be a good =
idea but it can work out.
>>=20
>>=20
>>=20
>> --
>> http://www.piclist.com PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>=20
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist


--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.