topic Re: The problem of the Kinetis RTC time in Kinetis Microcontrollers

The problem of the Kinetis RTC time

fukefeng — Wed, 02 Jul 2014 07:06:20 GMT

I use the kinetis MK70FN1M0VMJ15 MCU for more than two years，and we use the mqx system. but now we found that the RTC sometimes don't work well，it would reset to 1970 years. and most times it happened on the POWER ON. our schematic diagram is just same as the official schematic diagram(the only difference is the diode). The time is very important for us，does anyone found the same problem or has same suggestions for me. my schematic diagram is under blow：

the official schematic diagram is use the diode of BAT54C， and I use the 1N4148. Can this the problem? I measure the diode forward voltage is 0.3v, the VBATD voltage maybe enough for the mcu.

very thanks for give me some suggestion.

Re: The problem of the Kinetis RTC time

bobpaddock — Wed, 02 Jul 2014 12:12:46 GMT

Jack Ganssle has been doing a series on Coil Cell power at embedded.com :

http://www.embedded.com/electronics-blogs/21/Break-Points

The leakage of the capacitors could be a significant drain to the battery, as Jack discusses.

Also 1N4148 diodes are signal diodes not power diodes. At low currents, such as from a coin cell, this is usually not much of an issue, as long as there enough current to get it to conduct. Running the system power through it could be, an issue, no way to tell without the whole schematic. Taking more than ~75 mA through a 1N914/1N4148 is a bad idea.

Re: The problem of the Kinetis RTC time

fukefeng — Fri, 04 Jul 2014 00:42:13 GMT

this is my MCU power on VCC33 waveform

this is my MCU power off VCC33 waveform

Re: The problem of the Kinetis RTC time

fukefeng — Sat, 20 Sep 2014 01:49:07 GMT

thanks for answer me，maybe the problem is that the Coil Cell package is fit the pcb layout，some times the power of Coil Cell pin will touch the pcb GND