The compiler extends the C/C++ language by adding the cregister keyword to allow high level language access to control registers. This keyword is available in normal mode, but not in strict ANSI/ISO mode (using the --strict_ansi compiler option). The alternate keyword, __cregister, provides the same functionality but is available in either strict ANSI/ISO mode or normal mode.
When you use the cregister keyword on an object, the compiler compares the name of the object to a list of standard control registers for the C28x (see Table 6-2). If the name matches, the compiler generates the code to reference the control register. If the name does not match, the compiler issues an error.
| Register | Description |
|---|---|
| IER | Interrupt enable register |
| IFR | Interrupt flag register |
The cregister keyword can be used only in file scope. The cregister keyword is not allowed on any declaration within the boundaries of a function. It can only be used on objects of type integer or pointer. The cregister keyword is not allowed on objects of any floating-point type or on any structure or union objects.
The cregister keyword does not imply that the object is volatile. If the control register being referenced is volatile (that is, can be modified by some external control), then the object must be declared with the volatile keyword also.
To use the control registers in Table 6-2, you must declare each register as follows. The c28x.h include file defines all the control registers through this syntax:
| extern cregister volatile unsigned intregister; |
Once you have declared the register, you can use the register name directly, though in a limited manner. IFR is read only and can be set only by using the | (OR) operation with an immediate. IFR can be cleared only by using the & (AND) operation with an immediate. For example:
IFR |= 0x4;
IFR &= 0x0800
The IER register also can be used in an assignment other than OR and AND. Since the C28x architecture has limited instructions to manipulate these registers, the compiler terminates with the following message if illegal use of these registers is found:
>>> Illegal use of control register
See Example 1 for an example that declares and uses control registers.