Low-cost oscillator circuits

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Does anyone have any experience using ceramic resonators with AVR2313 or similar? I want a lowest-cost oscillator circuit of about 4 MHz for the AT90S2313-10SC and can't find any hardware design guidance for quartz crystal designs, let alone resonators.

Any help gratefully received!

& Atmel, if you're listening:

1. think about adding power-on reset circuits to AVRs
2. specify your parts fully for parameters such as oscillator functions
2. try producing some getting-started *hardware* application notes

Microchip are far ahead of you on low-cost microcontroller impelementations...

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Hi David,

The 2313 works great with ceramic resonators. The value of the caps is depending on the cer-resonator you are using. (eg muRata 4Mhz type: CSA4.00MG uses 30pF)
Resonator vendors usually specify the load for their own ceramic resonators.

1) All AVRs do have POR circuitry buildt in. e.g. see 2313 dsheet p 3-22. If you are refering to BOD, you are right that the 2313 and some of the very first AVRs did not have buildt in Brown-Out Detector. All newer ones do!

2) The AVR is very tolerant to the caps value. Using 27pF caps will work like a charm. Selecting an other values will work great too. Higer caps will give you better noise immunity, and higher power consumption, lower values will give lower power and noise margin. (as for all oscillators..)

3) Yes. you are right.
We are monitoring this forum on avrfreaks, and the types of questions that appear here, may very well influence future appnotes!

rgds
Arild
Atmel AVR support team.

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Thanks Arild,

I'll probably use a resonator with built-in caps as we're horribly short of space. The part I'm looking at is AVX PRBC-4.00HR.

The only other question is, do you need a series resistor into one of the AVR oscillator inputs and/or a load resistor across the resonator? I've looked at AVX & Murata websites & notice these components spec'd for parts like 4069s etc., but need accurate assurance for an application that will operate across the full industrial temperature range.

I've always used external POR circuits (DS1232 etc) & now can't think why - there must have been some reason when I started out with AVRs??!

Kind regards,
David.

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Pet peeve:

Crystal caps are determined by the crystal, and by parasitic loading in the circuit.
I have a writeup on this at www.dvanhorn.org, under microcontrollers, all, crystals.

They aren't hard to use at all, but there are a couple things you need to look at, instead of just dropping in some random crystal and caps, and hoping it works. The worst thing is, that your prototypes may all work! (But production may fail dismally).

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I've found that the 4MHz ceramic resonators sold by Maplin here in the UK, with 22pF load capacitors, work fine for me with a 90S2313-10.

These are marked as CSA400MG, and are Maplin order code DJ32K, at 49p each in quantities of 1.

Being a hobbyist, I'm just using stripboard for most of my designs*, so I was a little worried about getting anything over 1MHz to work reliably but I haven't had any problems so far. If it works at 4MHz on stripboard, it should be pretty stable.

Sean Ellis

* Yeah, I know: "bleah".