ADC Conversion - Combining ADRESH and ADRESL registers

When writing code for A/D conversion especially using PIC microcontroller, it is often required to read the digital value from the ADRESH and ADRESL registers. Depending on if the ADFM was set to right or left justified, the 10-bit digital value can be obtained.

Right justified is when the 6 MSBs of the ADRESH register are read as '0'
Left justified is when the 6 LSBs of the ADRESL register are read as '0'

For left justified form, 8 bits of the digital value are in the ADRESH register while the remaining 2 bits are in ADRESL register. It is possible to use only the 8-bits from the ADRESH register for left justification while not bothering about the 2 LSBs in the ADRESL register. However, 8-bit resolution of the A/D conversion will be obtained. To obtain a higher resolution (10-bit), the combination can be done by left shifting the contents of the ADRESH register by 2 then right shifting the contents of the ADRESL register by 6 and then perform an OR operation on the result as shown below:

unsigned int x;
unsigned int y;
unsigned int z;

x = ADRESH;
y = ADRESL;
z = (x << 2) | (y >>6);

z now holds a 10-bit number that represents the digital value of the A/D

If right justification is used (ADFM set to 1), a 10-bit A/D value can be obtained by left shifting the contents of the ADRESH register by 6, then right shifting the contents of the ADRESL register by 2, and then perform an OR operation on the result as shown below.

unsigned int x;
unsigned int y;
unsigned int z;

x = ADRESH;
y = ADRESL;
z = (x << 6) | (y >>2);