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Forum Index : Microcontroller and PC projects : PIC18F46K22 - personal experience
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vasi Guru  Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
I was very excited about this micro when it was available (if it works as expected, can be a reliable alternative to Sanguino board and that means that we can have a working 3D printer with PIC18F!!!). But then, people started screaming on Microchip forums about peripheral problems and my enthusiasm decreased. But now I'm about to find out myself. The AN1310 (modified behavior) serial bootloader works perfect every time - there are PC applications for it available for all three major operating systems. The I2C on MSSP1 works well on 100KHz and 400KHz bus speed but on 1MHz speed, after a few succeeded data exchanges, the firmware freezes. I'm testing it with the PCF8583 RTC (and JALv2 compiler with Jallib I2C master hardware libraries) but there is a problem: the 32KHz crystal is way off, as I get 2 hours advance on 3 hours run. It clocks way too fast! Maybe is the RTC at fault (I didn't looked at the datasheet to see if it sustains such a high I2C speed), and not the micro. So, this aspect remains to be solved another time. For me, it is important that the I2C works reliable at those two speeds. In fact, it will remain after night at 400KHz speed to see how it behaves on a long run. Man, those watches made on a ship in international waters... Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3196 |
I bet that the crystal is running on an overtone. I am not much of an expert on crystals but I understand that, if they are driven incorrectly, they will oscillate on a multiple of their proper frequency. It might be worth experimenting with the padding capacitors. Geoff Geoff Graham - http://geoffg.net |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
In FM transmitter work I do, they are often called harmonics, and are unwanted. 1st, 2nd and 3rd harmonics in the analog TV days were a real problem, as if not properly suppressed and filtered, they could be up around the TV band, and cause TV interference - the best and quickest way to get caught and shut down! 
I also have no idea about overtone crystals, but I DO remember reading about them during my electronics theory study. Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6102 |
The data sheet I have lists the max I2C clock as 100kHz. The date on my data sheet is 1997 so there is likely to be more up to date info available. Setting the Chrystal trimmer is usually done by measuring the 1Hz output on the interrupt pin. Values for the trimmer is likely to between 5 and 25 pF Any value around those limits may persuade the crystal to behave itself. Jim VK7JH MMedit MMBasic Help |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
The datasheet from 2010 is saying the same - 100KHz. It worked ok overnight at 400KHz but I understand this is above specifications and you don't usually do that. I restarted it at 0:00 and now at 13:50, the RTC shows 17:52 - that is how fast is running. I bought a while ago the right crystal and another PCF (this one is borrowed from the Polish board) along with a trimmer capacitor, but I can't find them... Anyway, this watch crystal served his purpose: helping me to port the PCF8583 libraries to JALv2 and to test I2C at different speeds. I have to build an I2C LCD to have at least 2 peripherals on I2C bus... Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 761 |
Morning Gents... One thing that might be causing the problem is the type of X-tal used... The ones used in watches are made to be driven from very low voltage oscillators. The ones made foe RTC chips have higher voltage drive and could easily overdrive the watch types causing the odd harmonics problem... |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
I've soldered a 22pF cap. on VDD and OSC1 pin and it seems to behave normally. I'm querying it at 500ms on 100KHz I2C speed and it appears to be good. I'll do a test tonight. Or maybe sooner if I modify the firmware to allow me to set the time/date on serial... _________ Edit: thanks guys, the capacitor cured the RTC problem. Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
Finally, I found the envelopes with the components! Now the AVR evaluation board can get her RTC back. But, I think I want to save the crystal for another project... Anyway, it would be there if something goes wrong again. 
There is an obvious difference in size between crystals, that's why I had my doubts at the beginning. Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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