60Hz timebase clock

[astouffer]

Hello all, I am in the planning stages for a simple clock that uses numitron tubes for the display. That part is pretty much worked out. I decided against using a realtime clock chip as they do need an initial date setting and battery backup. My idea is to base it from the 60Hz power. For isolation and protection against spikes I plan to drive the LED side of an optic isolator using a small step down transformer. That should give nice clean 60Hz pulses on the output side. So far so good.

Now for the timekeeping part. How about preloading an 8 bit counter with 195 so it will overflow on the 60th pulse? Then the interrupt goes to the timekeeping code where each overflow is counted as a second. The time between these periods should be plenty for the necessary math and writing to the display.

Any comments appreciated.


Adam

[Demon]

If you have a logic probe like this one:
http://www.saleae.com/logic
then it simplifies testing radically.

You get to measure hertzes down to 4 decimal places, as well as view several communication protocols like I2C, SPI, 1-wire, CAN, etc. I used it to get LEDs blinking at 120Hz and I have to say it works great (wish I could afford to upgrade to Logic 16).

About the preloading, that's what I did using Darrel's template. Details here:
http://www.darreltaylor.com/DT_INTS-...rTemplate.html

The only limitation I found with the template is the 16 MHz cap, other than that it was easy to set up. It's too bad 'cause I was running the 18F44K22 at 64MHz until now. I wished I knew enough about interrupts to go without the template, but hey, it's the price I have to pay to be me.

Robert

[astouffer]

I don't have to measure the accuracy of my timebase, just count 60 pulses to give one second ticks. The power company makes small frequency adjustments over a 24 hour time so it always averages to 60Hz. Some quick testing with a transformer and a 4N25 optic isolator gave me a decent 60Hz square wave output. Not a 50% duty cycle but more like 60% on 40% off. Setting TMR0 to 195 and then having an interrupt when it rolls over looks like it will work.

[Demon]

... The power company makes small frequency adjustments over a 24 hour time so it always averages to 60Hz...

I didn't know they bothered.


If you're only interested in triggering an event every 60 Hz, have you looked at Darrel's interrupt I posted above? You can control the exact hertz you want a trigger without external parts. If you're not going faster than 16 MHz it's perfect for you.

Robert

[Amoque]

Well, since you promised to appreciate any comments...

I'd use an RTC anyway. Some of the DS series are cheap, do not require a backup battery (actually, even those that make the option available need not have it to run) and provide a 1 Hz output without setting the time explicitly. Not sure why you wouldn't want to have a battery backed up time at your disposal, but if you don't... [shrug] ... you don't have to use it.

There's also a good article here somewhere about making a very accurate 1 second timer with no other time base than an external crystal... that would be my second choice.

On the other hand, Robert's suggestions are valid - as are your own.

[astouffer]

Amoque: Using the 60Hz power for a source will give me more accuracy than an RTC chip and be cheaper. I actually built a large clock based on the link you posted. It worked great but requires long term tweaking for accuracy. Even a 20ppm crystal it still gained a minute or two every month. Here is me testing one of the digits

This code runs on a 16F684 and blinks an LED on pin 10 at 1Hz when fed a 60Hz input on pin 11. Of course the final version will be doing much more than blinking an led. I plan to use four IV-9 numitron tubes for the digits. http://hackadaycom.files.wordpress.c...2/numitron.jpg

@	__CONFIG _FOSC_INTOSCIO & _WDTE_OFF & _WDT_OFF & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _CPD_OFF & _BOD_OFF & _BOREN_OFF & _IESO_OFF & _FCMEN_OFF


OSCCON = %01110101			'Internal OSC 8MHz
DEFINE OSC 8				'8MHz
PORTA = 0					'All low
TRISA = %00000100			'Porta.2 input
ANSEL = 0					'No analog
WPUA = 0					'Weak pull up off
CMCON0 = %00000111			'Comparators off and I/O is digital
ADCON0 = 0					'A/D is off
CCP1CON = 0					'PWM and CCP off
OPTION_REG = %11111000		'Timer0 on
TMR0 = 195					'Preload so it rolls over on the 60th pulse
INTCON = %11100000			'INTCON.2 overflow bit

on interrupt goto blink

main:
	@ NOP
	goto main

disable

blink:
toggle portc.0
TMR0=195
INTCON.2 = 0
resume
enable

end

Almost forgot the basic schematic. Use a scope to see what resistor values give you the best output. My meter read 3mA going into a 4N25 optic isolator. http://i.imgur.com/fHWGJHr.jpg