Part Number: MSP430F5529
I am developing a data logger that is designed to take a measurements from two different sensors (A and B) and store them. Sensor "A" records at 1 Hz and sensor "B" at 64Hz. The values are not tagged, they are simply stored as it is collected in a repeating pattern"ABBB...ABBB... etc.), so in order to decode the data the pattern must repeat without fail. The code uses the RT1PSIFG to generate the 64Hz interrupts for the data collection and storage. The timing for the 64Hz sensor is tight, but it works most of the time (see below).
The logger is designed to run for months at a time and the accuracy of the clock is a concern. So the 32768 Hz watch crystal is calibrated against a reference. The crystals generally run about 32 ppm slowly, so to compensate RTCALS is set to 0 and RTCCAL is set to 4 (+32 ppm of adjustment per the erratasheet). The accuracy is about right. No issue there.
The problem is that every 64 minutes, without fail, there is a glitch in the data where the 64Hz data is bad and it looks like one or more of the 64Hz measurements from sensor B is missing. The RTC Calibration is the #1 suspect because it is the only event that occurs on a 64 minute interval. I have read and re-read the user guide to understand the issue and try to find a solution but I don't quite understand and I need some help. The relevant section of the user guide reads:
Calibration is accomplished by periodically adjusting the RT1PS counter based on the RTCCALS andRTCCALx settings. In calendar mode, the RT0PS divides the nominial 37268-Hz low-frequency (LF)crystal clock input by 256. A 64-minute period has 32768 cycles/sec × 60 sec/min × 64 min = 125829120cycles. Therefore a –2-ppm reduction in frequency (down calibration) approximately equates to adding anadditional 256 cycles every 125829120 cycles (256/125829120 = 2.035 ppm). This is accomplished byholding the RT1PS counter for one additional clock of the RT0PS output within a 64-minute period.Similary, a +4-ppm increase in frequency (up calibration) approximately equates to removing 512 cyclesevery 125829120 cycle (512/125829120 = 4.069 ppm). This is accomplished by incrementing the RT1PScounter for two additional clocks of the RT0PS output within a 64-minute period. Each RTCCALxcalibration bit causes either 256 LF crystal clock cycles to be added every 64 minutes or 512 LF crystalclock cycles to be subtracted every 64 minutes, giving a frequency adjustment of approximately –2 ppm or+4 ppm, respectively.
If I understand this correctly, RT0PS is feeding the RT1PS at 128Hz, so the 64Hz interrupt is triggering for every 2 counts out of RT0PS. To adjust the clock, every 64 minutes, the RT1PS is either held for a number of RT0PS counts (creating a long "1/64th" of a second or it is pre-loaded with a value greater than 0 to speed up the clock (creating a short "1/64th" or a second). In my case, with a nominal crystal of that is 32 ppm slow, the RT1PS would be need to be preloaded with a value of 4 (or is it 8?) and that would then result in 2 missed interrupts. Or to put it another way, the "64Hz RT1PS interrupt" would only trigger 62 times during the "calibration second"? Is this interpretation correct? If not, what am I missing?
The minimum adjustment is -4/+8 ppm. So even if the 32768 crystal was exactly on target, there would still be a timing glitch every 64 minutes. Right? Is there any work around to assure that there are 64 interrupts in each and every second?
Thank you for the assistance.
In reply to Matthew Calvo74:
A resolution has not been found. The current state could be defined as confusion. By the way, It is not possible to switch to the MSP430FR5994 (or any other version) because this is a deployed product.
Here is my current thinking: We typically set RTCALS to 1 and RTCCAL is set to 4 which results in a correction of + 32 ppm. I have verified that this calibration works by comparing the clock to a reference with and without the RTC correction over a period of days/weeks. The clock adjustment works at a macro scale. But I don't think the exact adjustment method is adequately described in the user guide.
The +32 ppm shift equates to removing 4096 T0PS cycles per 64 minutes, or 16 increments of the RT1PS counter (per section 22.2.4, second paragraph). That would be 16/128ths or 8/64ths or 1/8th of a second. I had previously assumed that the adjustment occurred all at once and a jump of that magnitude would be really obvious. But a close read of the datasheet does not say that. Rather, it says the cycles are subtracted "every 64 minutes". It seems likely that the increments are spread out over the 64 minute interval, one 1/128ths of a second at a time until 1/8th of second of total adjustment has been made. I have two reasons for thinking this:
1. An older version of the hardware that uses a different, faster sensors, works at 64Hz. Significantly, it does not have "leap seconds" where one 1/64th of a second is missing (never mind 1/8th of a second). (If it did the data file pattern would not not hold and parsed file would be corrupt.) Presumably there is enough margin that the system can manage a short cycle (1/128th vs 1/64th) without dropping the data.
2. On the newer, slower, hardware that is more timing dependent, the data file errors show up as a pattern of errors every 64 minutes. It seems likely that this slower hardware can not manage the short cycles. Furthermore, the errors always occur on the same fraction of a second, even on different hardware, specifically 38.875 and 58.875 seconds into each minute. The number of errors is hardware specific and varies with the RTCCAL values. I am guessing that the RT1PS counter must be hard coded to incremented on the 38th and 58th second and the number of errors in the data file is proportional to the number of short cycles.
That is what I think is happening, but I tried to confirm it by capturing a short cycle on the oscilloscope. To do so, the code was modified to togging an output based on the RT1PS 1/64th interrupt. The scope is set to trigger on a short cycle (i.e. a cycle that is less than 1/64th of a second or 15.6 mS). I expect to see a cycle that is only 1/128th of a second long, but so far, I don't see any short cycles even after running for much more than 64 minutes, so the theory has not been proven, hence my confusion.
But the bottom line question is: Do positive RTC compensation values result in "missing" 1/64th interrupts or are there "short" 1/64th interrupts? The solution will depend on the answer.
Thank you for your response.
In reply to Nick Lowell1:
I am going to reach out to our Systems Engineering team who specialize in the RTC peripheral to see if they can shed some light on the issues you are seeing arise in your design and to inquire about some potential workarounds or solutions. I will reach back out to you when I hear back from them. Thank you for your patience!
Any word from the System Engineering team on the RTC behavior?
Good catch. That was a typo. RTCALS is set to 1. Complete register below:
BCD. Here is the entire RTC set of registers (trying to get ahead of the questions).
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