On Thu, 26 Feb 2009 13:18:34 -0800 (PST), ssylee
<s...@gmail.com> wrote:

>On Feb 26, 12:23 pm, Raveninghorde <raveninghorde@invalid> wrote:
>> On Thu, 26 Feb 2009 11:24:05 -0800 (PST), ssylee
>>
>>
>>
>> <staniga...@gmail.com> wrote:
>> >On Feb 26, 2:15 am, IanM <look.in.my....@totally.invalid> wrote:
>> >> ssylee wrote:
>> >> > On Feb 25, 8:08 pm, John Larkin
>> >> > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> >> >> On Wed, 25 Feb 2009 20:10:45 -0500, "Stephen D. Barnes"
>>
>> >> >> <stephendbar...@cavtel.net> wrote:
>> >> >>> ssylee wrote:
>> >> >>>> On a clearer note from my last message, I am hoping to have the PIC
>> >> >>>> receive the data between the rates of 15300 and 15800, even though
>> >> >>>> they may not be standard baud rates. Think you can make any
>> >> >>>> suggestions based on these rates? Thanks.
>> >> >>>> On Feb 25, 4:11 pm, "Stephen D. Barnes" <stephendbar...@cavtel.net>
>> >> >>>> wrote:
>> >> >>>>> ssylee wrote:
>> >> >>>>>> I'm trying to implement the USART on a PIC18F4680 running on an
>> >> >>>>>> external crystal to read a SCI signal that ranges from 15.3 to 15.8
>> >> >>>>>> kHz. I know that using an external crystal of over 2 MHz or more would
>> >> >>>>>> induce more significant errors in the USART module divider. How bad or
>> >> >>>>>> much more suitable is using a 32 kHz crystal instead? Thanks.
>> >> >>>>> What is the baud rate of the serial data you wish to receive
>> >> >>>>> on the PIC? There are several registers that have to be
>> >> >>>>> correctly configured to communicate at a specific baud rate
>> >> >>>>> wit a given Fosc frequency. Provide more details and your
>> >> >>>>> configuration code and possibly myself or others could lead
>> >> >>>>> you in the right direction.
>> >> >>>>> Stephen D. Barnes
>> >> >>> Again, what is the correct baud rate of the serial device
>> >> >>> sending the data (Published in it's documentation possibly)?
>> >> >>> This info is critical to your success. The baud rate of the
>> >> >>> PIC will have to be set at one specific value. It will not
>> >> >>> be able to receive a serial data stream that varies by 500 baud.
>> >> >> UARTs can usually tolerate several per cent of baud rate to clock
>> >> >> mismatch. If he aims for around 15.55Kbaud, the extremes will be under
>> >> >> 2% off.
>>
>> >> >> Most UARTs need an 8x or 16x clock, and usually have additional
>> >> >> scalers.
>>
>> >> >> John
>>
>> >> > Thanks for your advices on this. I guess this chip isn't as easy to
>> >> > work with as I thought. I will reorganize the information about the
>> >> > RFID tag that I'm trying to read and then ask for suggestions about
>> >> > modules that are easier to use in order to read the tag data. Thanks.
>>
>> >> RTFM! Page 4 paragraph 2:
>>
>> >> > The SCI encoder uses an 8-bit shift register to send the received data bytewise (least
significant bit first) to
>> >> > the microcontroller with a transmission rate of 15.625 kbaud (± 1.5 %), one start bit (high) and
one stop bit
>> >> > (low), but no parity bit (asynchronous mode indicated by the SYNC bit of the mode control
register
>> >> > permanently low).
>>
>> >> *most* UARTS that are properly designed to sample near the centre of
>> >> each bit time can handle at least +/- 1/4 bit time error in a character.
>> >>   For a ten bit character thats near enough 2.5% and you can be sure
>> >> that you'll start getting errors somewhere before 5%.  Your incoming
>> >> signal is +/- 1.5% @ 15.625 KBaud.  Only way I see a problem is if you
>> >> are using the PIC internal oscillator and *not* trimming its frequency.
>>
>> >> If its running on a crystal as you propose,  *and* you are not doing any
>> >> other async. serial communication, spreadsheet the Microchip USART
>> >> divider calculation and pick an easily obtained crystal frequency that
>> >> gives less than 1% error at  15.625 KBaud.
>>
>> >> --
>> >> Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
>> >> ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
>> >> [at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:
>>
>> >Thanks for pointing out the second paragraph of page 5. Totally missed
>> >that when I went through the manual. I am planning to use an external
>> >oscillator of a 32 kHz crystal. If I were to use the suggested 15.625
>> >kbaud frequency, I think I would get within the acceptable range by
>> >dividing the crystal frequency by 2, which deviates from the suggested
>> >frequency by 2.4%. Would you suggest using a crystal that is even
>> >closer to the 15.625 kbaud frequency?
>>
>> You usually need a crystal at least 16 times the data rate to receive
>> accurately. The UART can only sample the middle of the data bit by
>> being much faster.
>>
>> IMHO, and without checking the PIC data sheet, with a 32kHz clock and
>> 15.625kHz data rate you will get aliasing which will cause a problem.
>
>From what you're saying, I would need at least a 256 kHz crystal to
>avoid aliasing issues while small enough so that I don't get the large
>errors from the divider as described in the PIC datasheet?

That's my opinion.

However the PIC data sheet should make this clear.