RTC works in one board does not in other board

[mostafanfs]

I have a pretty weird problem with the LPC1768 RTC for couple of months. 

See I have two different boards with LPC1768 and both have external mounted 32768Hz crystal and the same program. One board is a custom designed board while the other is a starter board with almost nothing on it (just power and Jtag). 

Now the problem is that RTC works in starter board while it does not work in the other board. I mean it's obvious that it's not counting and incrementing time. 

LPC1768 is working fine and there is nothing wrong with it. The 32768Hz crystal is mounted almost near to the controller and the capacitors are mounted as well. I even soldered another board but it didn't work either !! 

That's why I decided to mount an external RTC like DS3231 but it has it's own problems and since I want to use Deep Sleep mode I figured to use RTC interrupt to wake up the controller in specific times (Is there other ways to do this?)

Now do you guys have some ideas why it's not working in some boards while does in others? 

[pavelhudecek]

I had the same problem: The cause was in swapped analog power supply pins. Everything worked, but ADC always read 0 and RTC did not run.

Note: For most types of MCU not be capacitors for low frequency RTC crystal.

[mostafanfs]

I've finally come to the conclusion that the problem is from MCU itself. I spent one day changing this with that and finally I soldered another MCU (that I got from somewhere else) and it worked with the same crystal and caps. 

I was really suspecting the MCU from the beginning !

Damn, I lost so much time

I didn't understand your note !

Note: For most types of MCU not be capacitors for low frequency RTC crystal.

[pavelhudecek · Accepted solution]

I didn't understand your note !

Note: For most types of MCU not be capacitors for low frequency RTC crystal.

I can not speak English very well and Google translator can not speak Czech very well:-)

Conventional high-frequency crystal oscillators require capacitors.

It was the same even with low-frequency crystals in circuits with simply logic ICs (4060 etc). But the new MCUs, currently in use, are equipped with special low power RTC oscillators for low-frequency crystals. In these oscillators capacitors are not required, conversely, may cause problems.

[mostafanfs]

I do not agree on this. First because they mentioned caps in documents second I tried without placing caps and it didn't work till I put the caps back then it worked. I mean it's not a general rule. 

Thank you anyway

[pavelhudecek]

I looked at it a little more closely and found that different LPC in this vary: Some capacitors require (eg 176x5x), and some do not (for example 11U6x). This is annoying. It means that it is in the design should always look whether a type of capacitors needed or not.

[mostafanfs]

Well by directing RTC clock to CLKOUT pin I figured there is no clock !

But I can direct other clock sources to this pin so I have no RTC clock. 

What could be wrong ? 

Could it be because of some sort of hardware fault ? I've soldered 10pF caps for crystal and crystal is located near the controller.