K64 reading RTC->TPR register

[nicolasguigo]

Using a K64FN1M0M12, we tried to log some events with a millisecond resolution. To do so, we use the TPR register from RTC peripheral.

millisec = base->TPR;
millisec = (millisec * 1000) / 32768;

We have a problem with some consecutive reads: the value read from the register can have a lower value than the previous read. To highlight the problem, we implemented a single task which only reads the TPR register every millisecond and prints the value on a console. After about 30 minutes, we have 3 occurrences of the issue. Each read gives the TPR value incremented by 32 or 33. Here are 2 occurrences of the problem with the values of RTC->TPR :

Millisecond	TPR read value	Is value correct?	TPR expected value
479	15711	YES	-
476	15616 (0x3D00)	NO	15755 (0x3D80)
481	15776	YES	-

Millisecond	TPR read value	Is value correct	TPR expected value
577	18911	YES	-
562	18432 (0x4800)	NO	18944 (0x4A00)
579	18976	YES	-

As you can notice, there is a one bit difference between the read value and the expected value. Is there something we do wrong? Is this a known issue? Is there a way to prevent this from happening other than reading the register multiple times?

[myke_predko]

@nicolasguigo 

I've run in the same problem and rather than use the RTC, I ended up using a PIT requesting an interrupt every 1ms (actually every 100us in my application) which avoids the issue.

myke

[bobpaddock]

http://www.ganssle.com/tem/tem355.html#article6 explains the general problem with multiple byte reads involving interrupts.  Then throw in that RTC is in a different clock domain...

It is important to note the distinction between simultaneous (the hardware ticking) and concurrent (the software reading the hardware with interrupts).

[bobpaddock · Accepted solution]

The general solution for reading multiple bytes across different clock domains is:

uint16_t t1,t2;

do{

 t1 = RTC->TPR

 t2 = RTC->TPR

}while( t1 != t2 );

t1 is now valid at this point.
 

[nicolasguigo]

Thank you for your reply. It solved the problem we had.