clockinterruptsmsp430timer
Basically that. The way I am doing it now is with the TimerA set to 1 second interrupts. But I think that it's very annoying. Are there any other ways to do it?
I want to basically set timers on that clock, like, shutdown until 40 seconds have passed…
The simplest method is to make your code pull some pin up before it executes the code you want to time, and pull it low after it has finished doing whatever. Then make the code loop (or use digital oscilloscope with memory in single-shot mode) and just scope then pin. The length of the pulse tells you how long it took to execute the piece of code plus one clock cycle from changing the pin state (I think it takes one cycle, not 100% sure).
A better crystal is one way to solve it, but it's going to cost. And even then you get maybe 10ppm, good (or bad) for 5 minutes after 1 year.
Temperature compensated crystals are good to get a predictable stability, but they don't solve the tolerance problem; if you have a 20ppm deviation from nominal frequency, you're sure you'll always have this deviation. BTW, this is also more expensive, and I haven't seen it for 32kHz crystals.
Software compensation is a cheap solution, and I've used it a few times. For production runs you'll have to measure the frequency and program the compensation value on a test jig.
You can also use a trimmer capacitor for the oscillator's load capacitor.
What I find the nicest solution, however, is to add an atomic clock receiver module (WWVB for North-America, DCF77 for Europe), and resync once a day or so. The picture shows an miniature DCF77 module from Conrad.
Best Answer
It's not perfectly clear to whether you want a real-time clock or a stopwatch (the 40 seconds you mention).
You could use an RTC IC (Real-Time Clock), like the NXP PCF8563. This one is available in several packages, including both the old DIL and a very small DFN.
But you probably don't need a separate RTC IC. It's typically used because it consumes very little power, and the rest of the circuit can power down while the RTC keeps running of a battery or supercap. The MSP430, however, is also a low-power device, and it has a low-frequency oscillator which can run on the same 32.768kHz crystal you would use for the RTC.
In one project I had the MSP430 running continuously on a 32kHz crystal, and yet it consumed less than 5\$\mu\$A. That's more than an RTC (the PCF8563 only needs 250nA), but it will be acceptable for many applications.
What's your problem with the 1s interrupt? If you want to make a real-time clock you'll need a time-cue anyhow, whether generated internally or coming from an external RTC. Upon interrupt you can perform required updates of seconds, minutes and hour counters, and wait for the next interrupt. You even could work with 10ths of a second, although with the 32.768kHz this will have a minor deviation. You would have four tenths of a second or 3277 clock ticks, followed by one tenth of 3278 ticks, to get exactly 1/2 second, so repeat this pattern a second time to complete one second.