16bit to 8bit to save data in eeprom and read it as a 16 bit value again

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Saving 16 bit data to eeprom by converting it to 8 bits modbus stm8l IC​

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Hello i am pretty new to modbus and this is my first program with it, I have several registers (21) 16bit register and some of them are NVM (non volatile memory) capable.
So, in order to save 16 bit register value to eeprom I am splitting the data into 8 bits such as
[INPUT_SAMPLE] //(basically this is a 16 bit register that samples out the adc between 200-2000 range) default val is 200

If(DUL==YES){
Stm8_eeprom_store_data (u_8 analog[input_samples >>8) ;

Stm8_eeprom_store_data (u_8 analog[input_samples) ;}

Stm_wdt // calling watchdog

I tried this code but it didn't help instead
if (DUL==YES){
Stm8_eeprom_store_data (((u_8)analog[input_samples>>8)+((u_8)analog[input_samples);
}

This code helped but the thing is, I can't store values greater than 255(8bit) my question is that do I need to convert it back to 16bit.
NOTE: I am using stm8l52c6 IC with modbus , the nvm is properly configured everything is working the only thing that's frustrating me is that I can't store val greater than 247-255 (8bit) . To read the configured registers I am using modbus tester & scada admin
I'd appreciate any quick and meaningful response from your end. Thank you in advance.
I was expecting it to store 16bit value but since my eeprom can only read 8 bit values that's why I had to split the 16 bit into 8bit but the thing is now it can store only 8 bit data if I try entering 16bit value for eg 2000; it would output some random value as 208 ...why is that so? Do I need to convert the 8 bit to 16 bit again or what

 

[Martaine2005]

I’m not clued up on this at all so maybe nonsense, but I remember cloning hard drives with the same situation. I could clone a 250Gb onto a 1Tb and only ‘see’ 250Gb. The rest of the available storage space was unavailable.So you might have to change the settings again. Or try different software?.

 

[Harald Kapp]

When you need to split a 16 bit word into 2 8 bit words (bytes) and vice versa, one way of doing this in C is this:

uint16 data; //defines a 16 bit datum

uint8 data_high; //defines a byte to store the upper 8 bits of "data"
uint8 data_low;  //defines a byte to store the lower 8 bits of "data"

//split data
data_low = (uint8) (data & 0x00FF); // mask out lower 8 bits
data_high = (uint8) (data >>8);   // shift to the right to access the upper 8 bits

// you can now store the upper and lower byte as separate bytes e.g. in the EEPROM
...
    
//re-assemble data
data = ((uint16) data_low) | ((uint16) (data_high)<<8)

data_low = (uint8) (data & 0xFF);
bitwise and of the 16 bit data with an 8 bit mask 0000 0000 1111 1111 which sets the upper 8 bits to zero, then casts the remaining lower 8 bits to an 8 bit value.

data_high = (uint8) (data >>8);
shifts the 16 bit data 8 positions to the right, then casts the result to an 8 bit value, leaving the former upper 8 bits as a single 8 bit value.

data = ((uint16) data_low) | ((uint16) (data_high)<<8);
casts the 8 bit data_low to a 16 bit value, then merges with the shifted data_high to create a 16 bit value.

Note that I assumes unsigned integers (uint) to avoid messing with the sign or 2's complement when using signed integers

 

[Martaine2005]

You are a wealth of knowledge Harald.