Dual clock speeds on a PIC

[BrianT]

Thanks Tenaja and Skimask. Some more details might clarify.

The processor is an 18LF4620 TQFP. It spends most of its time asleep where it draws 2.6 uA with a 4 MHz xtal. It runs at about 4 mA plus the external signal conditioners on a 4 MHz xtal. I am hoping to reduce the 'on' current to much below 1 mA with the slower clock.

I have to accurately measure the frequency/rate of some events so RC oscillators are out of the question during the measure phase.

I will have ICSP programming brought out to a 6 pin header. I can load different code for the logging phase and for the data dump phase. The ICSP has access to the configuration fuses so the XT/HS changeover should be possible. I figure to push a high level (~1 Vpp)20 MHz clock over the top of the 100 KHz xtal through this header. Real estate is ultra cramped so extra gates for a clean clock changeover are out.

BrianT

[Archangel]

Hi Brian T,
Section 2.7 of the data sheet says you can do it, but I have had far too many beers right now to discover how . . .

[skimask]

The processor is an 18LF4620 TQFP. It spends most of its time asleep where it draws 2.6 uA with a 4 MHz xtal. It runs at about 4 mA plus the external signal conditioners on a 4 MHz xtal. I am hoping to reduce the 'on' current to much below 1 mA with the slower clock.

I have to accurately measure the frequency/rate of some events so RC oscillators are out of the question during the measure phase.

I will have ICSP programming brought out to a 6 pin header. I can load different code for the logging phase and for the data dump phase. The ICSP has access to the configuration fuses so the XT/HS changeover should be possible. I figure to push a high level (~1 Vpp)20 MHz clock over the top of the 100 KHz xtal through this header. Real estate is ultra cramped so extra gates for a clean clock changeover are out.

Well there ya go. Add a jumper on the header to swap OSC1 between the crystal and an external X Mhz signal source (or whatever......you've got the 4xPLL to play with) and reprogram the unit to dump the data. Done dumping, reprogram for recording. At 100khz, a bit of sloppiness on the OSC lines shouldn't hurt (I've had much uglier on a 10Mhz crystal mounted roughly 2 inches from an '877A on lines that weren't so straight, still worked).

But, in the end, I like using the internal oscillator better. 31.25khz at the slowest and you can feed the 8Mhz intosc into the 4xPLL for some high speed data dumping.

[paul borgmeier]

I have to accurately measure the frequency/rate of some events so RC oscillators are out of the question during the measure phase.

Another Approach? - The 18F4620 can be clocked by switching from an external XTAL on OSC1 and OSC2 to a 32kHz XTAL on TMR1. See section 3.3.2 of the data sheet for details (from the data sheet “This gives users the option of lower power consumption while still using a high-accuracy clock source.)

[tenaja]

An even faster way to save battery power--without even changing the bulk of your code--is to use a different PIC! The 18F4620 was NOT designed for long battery life. At 4MHz with INTOSC, it takes 1.3mA at 3.0v. There are PIC's that will do the same speed with only .58mA draw--well under 1/2 the power draw. With careful data sheet scrutiny, you should be able to find one that runs 1MHz with even less power.

You will almost double your battery life by going from 5.0 to 2.7v.

I have a friend who runs their systems for years on one battery... he has 8 or 9 voltage regulators in the system so each circuit is individually powered, and totally shut down when not in use. Yes, it is expensive, but it is very efficient.

You are trying to cram a V8 engine into a Toyota Echo so you can enter a stock car race... but there are much easier and better ways to do it. Your multiple oscillators just seems like more work than it'd be worth, considering all of the other options available. Especially when 18F's already allow you to run an external crystal or the internal osc.