PIC16F73 RTC help needed.

[Sam_40]

[ckielstra]

For a quick explanation see Temtronic's answer. For some unknown reason it is listed before your question.

Thing is that a logic input should never be left unconnected. Your options are: tie it high or low, and with or without a resistor. If the port is programmable to become an output then that's the cheapest solution.

Each choice has small pro's and con's. What you choose is personal preference and based on your project requirements.
A more detailed explanation can be found here: http://www.piclist.com/techref/logic/xtrapins.htm

Any resistor value of 1k to 50k will do.

[ckielstra]

The 32kHz clock is still nagging me as it conflicts with the speed required for LED multiplexing.

Now we know you are not stuck with the PIC16F73 I would choose one of the newer Extreme Low Power (XLP) series chips. Let it run on the internal 4MHz clock, this is cheap as it requires no extra components.
For an accurate day time clock use a 32768Hz crystal connected to the timer1 hardware ports as discussed before
Create your program so that the processor goes to sleep most of the time and is only awakened by a Timer1 hardware interrupt every second.

Before you start protesting that a 4 MHz clock will consume more power than your original 32kHz: Yes, but... since the processor is faster it will go back to power-saving sleep sooner too. This means that on average both clock speeds will consume about the same low battery capacity.

Advantage of the high speed clock is that your LED multiplexing will be much better looking.

On your earlier question about 4x7 segment driving I easily found some great discussions on the internet:
- A discussion on alternative methods for driving 4x7 LED segments

If you want to have some fun and save 7 resistors and 4 transistors here is a topic describing a modified 'Charlieplexing': http://www.berryjam.eu/2014/08/driving-4x7-led-with-4-resistors-only-kinda-charlieplexing/
Be sure to read the reactions on this topic though: http://www.reddit.com/r/electronics/comments/2czanv/cheapsimple_4x7_led_driver/
It requires a high clocked processor and the display will be driven at an average low 1mA, so test your display to be bright enough.

Finally, since you are going to program several event times you might consider a chip with EEPROM. The PIC16F73 doesn't have this but it would allow your customer to change the event times and store this in permanent memory.

[Sam_40]

ckielstra,
I would like to thank you for your very helpful input. I will consider all of your suggestions,
Best regards,

[Ttelmah]

and one little 'extra'. On making an interrupt from 32768Hz tick 'once per second'. In the interrupt you just set bit 15 in the timer register. This effectively advances the clock 32768 counts, so the next interrupt will be one second later. Has been described here in many versions over the years.

Worth also asking if you really need the 32KHz on the PIC.
Things like RTC chips draw less power than even the PIC at 32KHz. Depending on how often you must be awake, you could use one of these, program it to send you a 'tick' at some longer interval, and put the PIC completely to sleep, yet still have accurate 'time' available when the PIC wakes, by just reading this from the RTC.
It depends whether you really do need to wake every second, or whether you are just doing this so you can keep time.

[Sam_40]

Ttelmah,
Thanks for the reply, I do not want to add RTCC to this project. I know the RTCC would be more accurate, however for this project the LP crystal would be just fine. I think I have enough information to get creative with it

Regards,

[ckielstra]

[temtronic]

You should download Microchip Application Note AN606, Low Power Design Using PIC16/17.
A great basic 'how-to' article. You've got 3 different power requirements
1) sleep mode
2) LED On mode
3) Motor ON mode

You really need to 'do the math' to see how much power you need,especially for say 30,60 or 90 days. You'll be surprised !

As well as needing far less power, an RTC chip(with coin cell backup) means you do NOT have to reprogram time when main power fails( and it will...) OR if there's a glitch(motor stalls=high current surge= spike= PIC resets= NO 'RTC' anymore !) Also the RTC has battery backed RAM,can be used to store the 'on/off/duration' info.


Have you compared the cost of 4 x 7 segment LEDs + resistors + transistors + PCB layout + assembly versus a 2 x 16 LCD module? Currently I buy LCD 2 x 16 for $4, 4x20 for $6. LEDs draw a LOT of power. LCD module will give you more information to display!!

just 'food for thought'

Jay

[Sam_40]

ckielstra,