244/317
8 - C Language and the C Compiler
Some registers hold numeric values, like TAOC2HR or TAOC2LR; in such registers, reading or
writing a value is done using an assignment statement:
TAOC2HR = TimerAPeriod >> 8 ; /* Set new period as calculated in
the capture */
TAOC2LR = TimerAPeriod & 0xff ; /* interrupt service function. High
*MUST* be written first. */
or:
CurrentPeriod = TAOC2LR ;
Other registers, on the contrary, are made of a set of 8 independent bits that are grouped to
occupy only one address in the addressable space. This the case for example of configuration
registers like
TACR1, TACR2 and TASR. For these registers, a set of #define statements asso-
ciate the name of each bit (e.g.
OC1E) with its position within the register (here, 7).
Since the ST7 does not provide for individual bit addresses, to reach a single bit it is necessary
to give the name of the register that holds that bit, and the number of the bit in the register.
To test the state of a single bit, the following syntax is suggested:
if(TASR&(1<<ICF1 ) ) /* Determine which interrupt cause. */
where the expression ( 1 << ICF1 ) yields the value 0x80 (1 shifted left seven times); anding
the whole register with this value yields a non-zero result if the bit is true, a zero value other-
wise. This result is correctly tested using an
if statement, since C defines a boolean true as
anything different from zero.
To set a particular bit within a register, the following syntax works well:
TACR1 |=(1<<OLVL1 ) ; /* Validate pulse for next interrupt */
The register is ored with the 1, shifted by the appropriate number of bits; the result is written
back into the register.
To reset the bit, this syntax produces the reverse effect:
TACR1 &= ~( 1 << OLVL1 ) ; /* Invalidate pulse for next interrupt */
To Next Page
Loading...
