38 kHz Crystal oscillator

[ljcox]

I want to run a PIC from a 38 kHz crystal. The data sheet for the 16F84 states that for low frequency crystals, it may be necessary to insert a series resistor.

Does anyone know how to determine the value of this resistor? The one I intend to use has a 1 uW max drive level and an ESR of 35 k Ohm.

So do I assume that the drive level is V^2 * 35 k ?

Len

 

[williB]

sorry no idea.. but i have allways had good luck taking an oscillator and dividing it down..
right now i am using a 5MHZ and am dividing it down to somwhere around 500kHZ..
with more counters i have taken it down to a heart beat .. lol

 

[Someone Electro]

It worked for me conecting it directly.But on an other PIC

 

[pittuck]

have you got a scope? make a quick program to toggle a pin every 10 instruction cycles and have a look on the scope to see if the timing it right/wrong.

BTW, is your application timing dependant? if so perhaps a specialist timing IC might be better???

 

[Nigel Goodwin]

I thought the possible requirement for a series resistor was for HIGH frequency crystals?.

I've not seen any used with 32KHz ones, usually all you need to do is set the fuses to LP (Low Power).

 

[upand_at_them]

You should be good with that watch crystal and the appropriate caps to ground. That's it. And like Nigel said, set the oscillator fuse to LP.

Mike

 

[eblc1388]

Hi Len,

See if this would helps. Rd(30pF) is for conventional crystal so Rd* would be for miniature crystals.

 

[ljcox]

You should be good with that watch crystal and the appropriate caps to ground. That's it. And like Nigel said, set the oscillator fuse to LP.

Mike

Thanks for the responses, but as I said in the original post, the 16F84 data sheet states that a series resistor is necessary, presumuably as too much drive power may shatter the xtal.

The post by L Chung tends to support this.

Len

 

[upand_at_them]

Thanks for the responses, but as I said in the original post, the 16F84 data sheet states that a series resistor is necessary, presumuably as too much drive power may shatter the xtal.

What page? I looked in the datasheet before I typed my response and didn't see anything to that effect. Just use the crystal with the necessary capacitors and then tell us if it doesn't work.

Mike

 

[eblc1388]

:cry:

 

[upand_at_them]

Microchip's own design for an RTC using a watch crystal (App Note 582) has no series resistor.

Mike

 

[ljcox]

What page? I looked in the datasheet before I typed my response and didn't see anything to that effect. Just use the crystal with the necessary capacitors and then tell us if it doesn't work.

Mike

Mike, you may be right. I have the Microchip 16F84A "preliminary" data sheet DS35007A.

Page 22 under the heading of 6.2.1 Oscillator types Figure 6-2 shows the series resistor RS. But on re-reading note 2 the resistor "may be required for AT strip cut crystals"

However, the crystal I have states a max drive of 1 uW. So I would like to check the drive power.

Len