[time-nuts] Pictic II mods

[Peter Putnam]

List Members,

How casually the time-nuts treat data timing... on one hand, errors of 
parts in ten to the minus 12 are cause for great concern, and on the 
other hand, throw in an extra stop bit and hope for the best.

Consider how a UART receives data from an asynchronous data transmitter.

The beginning of a received START bit starts a clock running at 16 times 
the data rate. At 8 clock ticks, the middle of the start bit is 
established. The middle of the first data bit is sampled 16 clock ticks 
later, as are the succeeding bits. The center of an "on-speed" STOP bit 
occurs on the 144th tick. If the incoming stream is fast, such that the 
stop bit is ending at 144 ticks, or slow, such that the stop bit is just 
beginning at 144 ticks, timing errors can be expected. The maximum 
combined receiver and remote transmitter clock error can then be 
established as 144 +/- 8  or  +/- 5%. Errors are not cumulative; each 
incoming byte restarts the process.

The penalty for this tolerance of clock errors is the overhead involved; 
ten bits are transmitted for each 8-bit byte.

Peter Putnam




On 7/2/2010 10:01 PM, Chuck Harris wrote:
> Hi,
>
> I only worry about the UART working with other UARTS.  And at the 1% 
> spec,
> the PIC UARTs , in my experience, always do....
> If you are concerned, use 2 stop bits.
>
> -Chuck Harris
>
> Bob Camp wrote:
>> Hi
>>
>> A lot depends on just what you worry about. Most clock specs for 
>> serial com are in the 0.01 to 0.1% range.
>>
>> Bob
>
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