Electronic – Breaking a 16-bit long int to write into eeprom

In the past I have noticed that some of the PIC datasheets do not correctly show the SPI CPOL and CPHA registers, there was some issues that when looking at the output on a scope, two of the four combinations were back to front from what was expected. So double check you are actually getting the expected waveform out of your PIC and that the waveform matches what is needed on the EEPROM.

I have also come across some A/D converters that worked intermittently when I had incorrect SPI settings on a HC12, drove me banana's trying to work it out, was one of my first projects as a professional engineer, eventually I worked it out. But I digress. So it is possible to get intermittent problems using SPI bus due to incorrectly set CPOL/CPHA. As it is a shift register after all, and the last bit on one character may be being read of the first of the next character. This type of problem may not be noticed straight away either.

So ensure you have the correct settings on a scope, and do not relay on just setting the registers of the PIC.

Two things come to my mind:

First, according to the data sheet, a erase-write-cycle takes at least 0.8ms, and up to 2.6ms. You say that you have an interrupt every 1ms, which may lead to a write operation. I have seen in the code that you disable interrupts for parts of the erase and for parts of the write function. But you still might get funny interleaving of the function calls. Maybe it helps when you disable interrupts for the whole sequence of erase and write?

Second - you might want to write while to power goes down, and the EEPROM write happens exactly in the moment when the supply voltage goes below the operating voltage. You can try to monitor the supply voltage, and refuse a write when it is below, lets say, 4.5V. This assumes that it stays long enough above 2.7V as the minimal operating voltage, and brown-out-detection is set to trigger only below that point.