Does there exist a uC of any kind where the clock source is selectable from code? With the PICs you have to choose the clock source at program time by burning fuses, and you are stuck with it. Yes the watchdog uses an internal RC but when it counts down the PIC wakes up with the programmed oscillator type. Here's my dilemma: I am using the clock generated by a genlock PLL to drive the PIC. When the video is gone, the PLL loses lock and its VCO output is then unknown to me. However the fact that HSYNC is gone is known, so I'd like to select an internal RC clock for the uC before the VCO goes out of spec. Yeah, it's not a great design, but it's a personal challenge to me and it's not a commercial product. Also (heh heh), since the PIC is being fed by the PLL, it is also acting as the feedback divider... I like this clever (imho) way saving of parts, and the clock is allowed to have jitter.
Check out the MSP430 series. Not only can the clock source be selected but you might be able to simply leave it on the digitally controlled oscillator (DCO) because it cannot stray too far from it's SW-selected adjustment range.
Im sure I came accros a PIC with a backup clock wich kicks in when the main clock stops, I may be mistaken I was just scanning quickly through quite a lot of datasheets looking for what I wanted, cant remember wich device it was in though.
The PLL oscillator will usualy go to max or min frequency on loss of lock, if you want to have a clock more accurate than this then you will probably need an external crystal.
Alternativly you could use one of the timers wich has a seperate input to do the dividing rather than count clock cycles in code (wich something you realy dont want to do unless you have no choice) and have a fixed clock.
It's in allot of them now. For example, the 12F683 (8 pin) and 16F88 (18 pin) have the new INTRC oscillator that is switchable on the fly (uncalibrated at 31kHz; calibrated (better than 2% over a large temperature range) at 125kHz, 250kHz, 500kHz, 1MHz, 2MHz, 4MHz and
8MHz). They also have the a fail-safe clock monitor that will auto switch on a detected clock failure. The OP needs to look again.
lock,
probably
INTRC on the PIC is actually better than 1% at room temperature. It is also tweakable on the fly by + or - 12% thru the OSCTUNE register.
Glad you like those oscillators. I redesigned those internal oscillators in 2003-2004 ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Hey that's great, thanks! Those features are not readily apparent if you just look at their comparison chart. I guess you have to find the right part first, because the 12F629 I picked doesn't do that good stuff. Boy things have improved since I messed around with the 12C508 back in the Cretaceous! All I need is for the PIC to be able to set the PLL to /4 mode so that the clock the PIC sees doesn't go beyond 20MHz when the lock is gone, shut down the video IC properly, then shut off the PLL entirely and wait for HSYNCs to start coming in again. Weeee! Too bad the weather is going to be beautiful on the weekend, or I'd actually read the datasheet!
Yeah, but they can be a bit confusing/overwhelming at first. OTOH, I suppose that's comes with all the flexibility. I had to read that part of the datasheet about ten times before it really started becoming clear as to how it all worked together. I forgot to mention the two-speed startup mode, pretty slick. Congratulations, it all actually appears to work, you should be proud on the relative lack of errata....being a PIC and all. ;-)
By any chance, did you design the T1OSC in the 12F683? I've been recently playing with that using a 32.768kHz crystal for timekeeping. It seems to oscillate faster the longer the interval between TMR1 roll-overs. >60ppm variation by just changing roll-over intervals (by preloading TMR1H with 0xC0, 0x80 and 0x00 for respective roll-over times of 1/2, 1 and 2 seconds before waking up) By toggling an I/O pin in the ISR and precisely measuring the period of the resulting square wave (1,
2 or 4 Hz), I could see that: the longer the sleep period, the shorter the interval to the next wake-up; i.e. T1OSC seemed to speed up as the length of the sleep interval increased. Here are my recently (last 10 minutes) measured results:
1Hz - period ~1000006 uS ( 6ppm slow )
2Hz - period ~1999950 uS ( 25ppm fast )
4Hz - period ~3999777 uS ( 56ppm fast )
Naturally, it appears that my measuring device is likely to be in error. Therefore, I have verified its functionality by dividing the 1Hz output using a binary counter chip and the results are precisely as they should be. Probably the most fascinating thing about this is that I don't really have to SLEEP. If I just loop in the main level without sleeping, I get the same exact results. This seems to rule out CPU noise as a source of interference. Now....I'd like to see someone explain that behavior.
I have played allot with the capacitance parallel to the crystal, from too little to oscillate all the way to wayyyyy too much. Nothing out of the ordinary occurs, just a bit of overall frequency pulling. Absolutely maddening, it's as if the T1OSC is affected somehow by the values contained in the TMR1H register.
"Why not have another external oscillator as a clock source and switch to it when HSYNC goes away? " I don't know, bullheadedness? I decided to make a minimal parts count design, not best performance or easiest to build... It's just fun to use the clock that's already being generated. As long as there's video, it'll work fine. Besides, with all these new PICs, why bother? My motto is "specify, don't design". Someone already went through all the trouble of figuring out these things and working out the glitches. If I can get that for 2$ by choosing the right part, great! BTW, aren't you supposed to be dead? ;)
I am only a ghost engineer... do the designs and leave ;-) I had nothing to do with the documentation.
Only problems I had were youngsters trying to sabotage my work and keep me from information I needed. Really pissed me off. About the worst NIH/job-protection situation I've ever experienced.
I don't know what they were called. I don't think I even have a data sheet.
Looking at the directories I find GRP1OSC, LPRC, and XTAL oscillators.
Looking at the schematics... to get all those "features" there are a maddening number of analog switches in there, from previous versions, that I had to adhere to.
One of those "fix it but don't change anything" projects :-(
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.