ER Clock input question

[Squibcakes]

Hi,

Does any body have any info or experience using the external resistor (ER) type oscillator.

All I know is that the recomendation is to use a value between 38k and 1 M ohm resistor, but NO data on what oscillator speeds can be achieved. (16F628)

While on the subject of oscillators.. how fast has anybody (overclocked) the Internal 4MHz oscillator before their PIC gives up?

Cheers

[Melanie]

I do recall I answered this very question only the other day (6 August 2002) - well it seems like only the other day... Search MeLabs archives "ER Oscillator on 16F628"...

Don't forget about that archive... it's pretty usefull... anyhow, the salient part I've replicated below...

--- snip ---

Simple to test though.... I just threw a few Resistors at a standard circuit, checked CLKOUT on RA6 (which is fosc/4) and measured...

Resistor = RA6osc. Vdd was 5v. PIC was 16F628-04

10K=1.83MHz
27K=1.07MHz
33K=875kHz
68K=470kHz
100K=344kHz
270K=144kHz
330K=120kHz
680K=62kHz
1M=42kHz

Remember RA6 is fosc/4. So it looks like 27k gives you 4MHz clock (a value which is below the recommended 38K).

Lowering Vdd causes an increase in fosc. Vdd of 4.0v had a +3.5% fosc with 10K, dropping to +1.1% fosc at 1M.

Also the clock pulse width with Resistors above 220K starts to become unequal. With a 1M Resistor for example, the Clock High (17.7uS) was almost three times that of the Low (5.9us).

--- snip ---

Actually, as you can see from the above Resistor table, the 10K Resistor was clocking at over 7MHz... so... get yourself a 50K pot and wind it down (slowly!) until the PIC falls over... then just throttle it back a fraction until the PIC just starts up and you have the optimum setting for your turbo budget pacemaker...

Melanie