#Bit explanation

skybox63 · Joined: 30 Jun 2018 Posts: 17

Hello this is my first post.

I'm attempting to wrap my head around the theory behind #bit use.

Using an example I found illustrates my question:

newguy · Joined: 24 Jun 2004 Posts: 1909

You can refer to specific RAM locations, SFRs (special function registers), or just plain old variables. If you want to reference a particular SFR such as the port A tris, look at the #getenv directive. Very handy feature of the compiler which negates the need to look up the SFR's address in the data sheet.

Ttelmah · Joined: 11 Mar 2010 Posts: 19549

Step back.

Start by looking at #locate.

#locate allows you to 'locate' a variable at a specific memory location.
You create a varible, and can then place this at a particular location in memory.

Then there are the derivative functions of this:

#word
#byte
#bit

#word automatically creates an int16 variable, and locates this at a specific location.
#byte creates an int8 variable and locates this at a specific location
#bit creates an int1 varible and locates this at the specific location.

Each of these also has a 'secondary' operation, that if you create a variable 'first', and then use this name as the target, this variable is located at the specified location. Exactly like #locate.

The variables automatically created are all just normal variables. Exactly like any other. Existing variable names can also be used as locations.

So:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

To give more direct answers,

skybox63 · Joined: 30 Jun 2018 Posts: 17

Thanks everyone. Where I was getting hung-up is not understanding the correlation of the SFRs and the hex values used in the code.

Best
Roger

Ttelmah · Joined: 11 Mar 2010 Posts: 19549

As a comment, using hex values for the locations can be very dangerous. It is vital if you do this, that you document it really well. Something like:

//0xF81 is the address of the PortB SFR

Otherwise in a few months time you will find yourself having problems working out why the value was used... :(

Better now to use the getenv and locate to a named SFR.

One of the commonest mistakes is to get an address wrong. Using the getenv reduces the chance of this happening.

temtronic · Posted: Sun Jul 01, 2018 6:33 am

another comment.
When you read the datasheet, you'll see that most mfrs use hex notation for the 'memory map'.
Now that there are 1,000s of PIC to choose from ,it's GREAT that CCS added the getenv() function as it makes transporting code from one PIC to another series PIC easy( well...easier...)

Jay

skybox63 · Joined: 30 Jun 2018 Posts: 17

Appreciate the follow-up. All good points to consider.

Roger