la_panza wrote:
> Klaus (DK) ha scritto:
>
> Hi , thanks for answering
>
>
>>On the other hand, please explain why your 13.56 (I guess it is MHz) is
>>so im****tant for you. What is the needed resolution of that clock? Any
>>other relevant information could be handy in order to help you.
>>Regards
>>Klaus
>
>
> I'm designing an ISO 15693 vicinity card reader, the unit is composed
> by a PIC16F and an analog front-end IC that drives the antenna and
> demodulates signals coming from the cards.
>
> The analog front-end IC has its own 13.56 crystal, the electromagnetic
> field generated by the antenna supplies power to the cards, but the
> 13.56 MHZ field is also used as master clock for the card's internal
> logic.
>
> All the card timings during data reception and transmission is based on
> a 18.88 microsecond time unit, (13.56 MHZ divided by 256). the PIC16F
> has the duty of composing data packets, send them with the right
> timing, as well as decode response packets coming from the cards.
>
> The PIC has a 20 MHZ quartz, it has other tasks and must be kept quite
> fast,
> Now, f I plan to use...say... Timer2 to generate the 18.88 Usec. timer
> tick, the value to set on Timer2 period register is 94.4; Since it's
> not possible to give such a value to a register, I fear that even a
> minimum timing error may propagate along the data frame and make the
> coding / decoding fail... Ideal solution would be to use the SAME clock
> supplied to the cards as a timer clock!
>
> Hope the explanation is clear enough...
>
> Thankyou for interest ....
>
> Adolfo (****denone, IT)
>
>
>
>
> The cards get power by inductively
>
>
>>la_panza skrev:
>>
>>>Hello all, I need to obtain a timer tick of 18.88 microseconds on a 20
>>>MHZ clocked PIC16F
>>>
>>>On the same board I also have a 13.56 clock, but for speed problems I
>>>neet to keep the processor clocked at 20 MHZ
>>>
>>>This 13.56 MHZ signal, divided by 256 is exactly the timer tick I want,
>>>so I thought to feed this signal as an external clock for Timer'0
>>>(T0ckl).
>>>
>>>On the datasheet though, I've read some warning and limitations about
>>>the use of Timer0 with external clock, I did not clearly understand if
>>>in a case like mine I'm allowed to do it...
>>>
>>>who can help please?
>>>
>
>
13.56 * 3 / 2 = 20.34
Overclocking the PIC this much should be possible, Substitute a PLL
based clock for the PIC's crystal, locked to 1.5 * the front end clock.
Your timer 2 period register could now be loaded with 96. (I haven't
checked the data sheet, I just scaled your values . . .)
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@[EMAIL PROTECTED]
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