Electronic – PIC16F EEPROM write errors

I'm developing on a PIC16F1936 using the MPLAB X IDE with the XC8 complier. I've been given this project where I need to "upgrade" the software of a PCB such that it can interface with an external tool via an RS232 link.

I have the link working and I have also checked that the date being transmitted is correct. The problem that I am having is with a specific function which calibrates the setpoints for the PCB. I can't say too much on what the exact function of the PCB so sorry for not posting everything. Also, I have inherited this a lot of the code on this project, such as the code I'm about to show you.

So the calibration function works like this: it is called once the tool sends the relevant command to the PCB. After this (depending on command sent), the function will read the value of the summed value of 40 values from the ADC and saves this to the EEPROM. This is where it gets really messed up! The calibration works perfectly well when I have to ZERO the PCB, the problem arises when I have to set MAX. Oh! And the code is identical, apart from the EEPROM address…

Here is the code for calibration:

if (zero == 1)
{
    WR = 1;                                         // Set EEPROM_WRITE flag
    EEPROM_WRITE(0x05,(AN1_sum >> 0x08));           // load MSbyte of AN1_sum into NV memory location 1
    EEPROM_WRITE(0x06,(AN1_sum & 0xff));            // load LSbyte of AN1_sum into NV memory location 2
    EEPROM_WRITE(0x07,(AN4_sum >> 0x08));           // load MSbyte of AN4_sum into NV memory location 3
    EEPROM_WRITE(0x08,(AN4_sum & 0xff));            // load LSbyte of AN4_sum into NV memory location 4

    zero1 = AN1_sum;                                // update volatile memory for zero1
    zero4 = AN4_sum;                                // update volatile memory for zero4
    WR = 1;                                         //Set EEPROM_WRITE flag
    EEPROM_WRITE(0x09, 2);                          //Write 2 to the EEPORM

    while(WR){                                      //Loop till WR flag is reset by EEPROM CONTROLLER
        MAL = 1;                                        //Light MAL until WR is reset
    }
    for(KILLTIME = 0; KILLTIME <= 200000; KILLTIME++);
    RESET();
    for(KILLTIME = 0; KILLTIME <= 200000; KILLTIME++);
}
else if (MAX == 1)
{
    WR = 1;                                         //Set EEPROM_WRITE flag
    EEPROM_WRITE(0x01,(AN1_sum >> 0x08));           // load MSbyte of AN1_sum into NV memory location 5
    EEPROM_WRITE(0x02,(AN1_sum & 0xff));            // load LSbyte of AN1_sum into NV memory location 6
    WR = 1;                                         //Set EEPROM_WRITE flag
    EEPROM_WRITE(0x03,(AN4_sum >> 0x08));           // load MSbyte of AN4_sum into NV memory location 7
    EEPROM_WRITE(0x04,(AN4_sum & 0xff));            // load LSbyte of AN4_sum into NV memory location 8

    MAX1 = AN1_sum;                                 // update volatile memory for MAX1
    MAX4 = AN4_sum;                                 // update volatile memory for MAX4

    WR = 1;                                         //Set EEPROM_WRITE flag
    EEPROM_WRITE(0x9, 3);                           //Write 3 to the EEPROM

    while(WR){                                      //Loop till WR flag is reset by EEPROM CONTROLLER
        MAL = 1;                                        //Light MAL until WR is reset
    }
    for(KILLTIME = 0; KILLTIME <= 200000; KILLTIME++);
    RESET();
    for(KILLTIME = 0; KILLTIME <= 200000; KILLTIME++);
}

After the reset, the PCB restarts and goes through this initialisation process:

MAX1 = EEPROM_READ(0x01);                                               
MAX1 = (MAX1 << 8) + EEPROM_READ(0x02);
MAX4 = EEPROM_READ(0x03);                                 
MAX4 = (MAX4 << 8) + EEPROM_READ(0x04); 
zero1 = EEPROM_READ(0x05);                                           
zero1 = (zero1 << 8) + EEPROM_READ(0x06);                          
zero4 = EEPROM_READ(0x07);                                 
zero4 = (zero4 << 8) + EEPROM_READ(0x08); 

Simple enough right? This is only time in the code where MAX and zero are configured.

I have no idea of why this isn't working MAX should be whatever value AN1 and AN4 are at the time of execution. I've swapped the EEPROM addresses around and changed the name of the variables to make sure that MAX and ZERO are not being overwritten elsewhere in the code. No avail. MAX only wants to be the correct value when I say MAX1 = x and MAX 4 = y. It can't be something with the values of AN1 and AN4 as they work fine for TARE no matter what their value is.

Any ideas?

Just to add, I have tried the following solutions:

1. I swapped the EEPROM addresses such that MAX writes to addresses. I mean that MAX is now being written to 0x05-0x08 and ZERO is written to 0x01-0x05. Still I have the same problem. The value from the sensor is correctly written to ZERO but not MAX.

2. I've played around with the WR flag by placing more and removing them completely. NO CHANGE.

3. I've tried storing the values of AN1_sum and AN4_sum in a temporary variable as soon as the calibration function starts and used those variables instead.

4. I've tried reversing the roles of ZERO and MAX. By this, I mean that I set ZERO to the maximum setpoint and MAX to the minimum. I found that ZERO sets accurately and the value it has can affect the value of the MAX. For example, I first set applied 45 to both channels and then set the MAX. After the calibration MAX had a value of 26. I then applied 198 to both channels and set ZERO. ZERO held a final value of 198, while MAX changed to 48!

I've known for some time that MAX is the culprit but I do not know why or how this is happening. I've gone through the rest of the code and checked to see if MAX is being affected in any way but it isn't.