Built in RTC / SOSC on PIC24 - What's the secret?

[evan]

Hi all,
A new challenge, I am using PIC24FJ128GA310 which has the built in RTCC, and I have placed a 32.768kHz secondary oscillator and backup battery to run it.

I am running ex.rtcc with just the chip type changed, but my seconds don't count up, and I can see no waveform on the oscillator pins.

There is no entry in the help file for setup_rtc so I am a bit in the dark about whether it's configured correctly. Does this look like a configuration problem or a hardware problem, does anyone have this module working correctly?

I am using compiler Version 5.078, xxxxx

Thanks in advance as always!

[evan]

Update:
I changed the setup_rtc line to add RTC_CLOCK_INT, and now the clock is actually running! Albeit a bit fast.

OK, may I change my question to: please help me debug the secondary oscillator?

[temtronic]

If 'debug' you mean it's running fast, you can 'trim' the caps and xtal to get a more accurate speed, beware though that temp and humidty will affect the value a tad. Ideally have the SOSC buffered as the load of a scope probe can affect the speed as well....

Jay

[Ttelmah]

What is the specified Cl of your crystal?.
What loading capacitors have you got on your board?.
Running too fast suggests you do not have enough load capacitance on the crystal.

[evan]

To clarify: the RTCC is working with the internal oscillator, which I tried as a test. I don't want to use the internal oscillator, so I don't care that it's fast.

I can't get the secondary oscillator (the crystal) to work at all. I have checked the layout and it looks fine, in line with the datasheet guidelines, appropriate capacitors etc. Is there some trick to enabling it?

[temtronic]

While I don't use that PIC..
the 'trick' would be to select the correct FUSE and to enable the SOSC to use an external xtal/cap on the pins.

perhaps use the search button (top of the page) and look for 'SOSC' as a keyword. probably a few dozen hits. since you're not the 1st to ask, it's probably here or in the code library.

Jay

[evan]

[temtronic]

There has to be a CCS 'command' to instruct whatever 'timer' used to select what 'clock' signal to be the input. Scan the datasheet for the 'flow' diagram, it has to be there. Typically the 'timer' can get it's input signal from a xtal/caps on the SOSC pins, an external signal (say from a canned xtal osc) or an internal osc. So there has to be a 'configuration' SFR to do that.
As I said I don't use any 24PICs, don't have the datasheet but all PICs do share a lot of 'internal' stuff so I'm thinking I'm right.

[evan]

You are right - the RTCC can definitely get its clock source from the SOSC - the diagram is in the datasheet page 276: http://ww1.microchip.com/downloads/en/DeviceDoc/30009996g.pdf

And I have guessed that the way to select it for the RTCC is:
setup_rtc(RTC_ENABLE|RTC_CLOCK_SOSC|RTC_OUTPUT_CLOCK,0x00); //enables internal RTCC
(it is always a guess)

But regardless of that, the SOSC is not running, and I can't find whether something else is required to get it running or enable its pins or what have you.

[evan]

After more guessing, it turns out that the trick is to put "SOSC_SEL" in the #fuses line. Not SOSC.

Thanks Jay for the hint, but apart from that it was trial and error. I thought I was looking for SOSCEN, from the datasheet.

The assembler unlock sequence mentioned in other threads is *not* needed, at least for this processor version and chip.

Maybe this will help the next person...

[temtronic]

yeah, what CCS calls a register or pin or bit is not necessarily the exact same as what Microchip does. That's why you need to start with the Microchip datasheet, find the 'thing' you need to control THEN look at the device header file that CCS supplies for your PIC, to see what they call it.
I've seen in the past where CCS call the same bit different names between different PICs, sigh...so now I CONFIRM by scanning the device header file though I've settled on about 3 PICs,saves me a lot of headaches...well, some.....

Jay

[evan]

Thanks again for the help, I appreciate it. It just can be frustrating, you can see what you want to do from the datasheet but finding out how (or even if) CCS have implemented it is rarely obvious.
Referring to the device .h file is a must. I'm still in the dark on fuses though!

[newguy]

If you have the CCS IDE, View->Config Bits brings up a list of the fuses for your processor (the one used by your project). If you don't have the IDE, look for "fuses.txt" in the root directory of your compiler installation.

Between that and the processor's data sheet (the fuses are usually near the back somewhere), and you should be good.

[Ttelmah]

If you look at section 9.5.1 in the data sheet, you will see it says:

[jeremiah]