Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

Consider as an example a fuel tank with a capacity of 255 liters, with a sensor at the bottom of the tank indicating the remaining volume of fuel as a linear function of pressure. Assume that the sensor represents the capacity as a byte that can be accessed at Port B (see page 95), which we give the name FUEL. We wish to write a routine that will light an ‘empty’ light (at bit 0 at Port A) if the capacity is below 20 liters and ring an alarm buzzer (bit 1 at Port A) if below 5 liters. Both output peripherals are active on logic 0. This is how it could be coded:

Zequ 2 ; and bit2 is the zero flag

After each subtraction the Carry/borrow flag will be logic 1 (that is no borrow) if the datum in the Working register (the fuel reading) is higher or the same as the literal being subtracted – it is being compared with. The addlw -k instruction can be replaced by the more obvious sublw k.4 Remembering that this subtracts W from the constant, that is k − W, then the following Skip on Set (btfss) instructions must be replaced by Skip on Clear (btfsc) to give the same sense of magnitude.

The contents of the Working register can be tested for zero in the same way, that is addlw 0 or sublw 0. If W were zero then the outcome of this tstw type of instruction will set the Z flag. We have already seen that the instruction movf xx,f can be used in the same way as a tst f instruction to test File xx for zero. Note the use of the bcf (Bit Clear in File) instruction to clear the appropriate bit in Port A, which we assume to

4The Compare instruction of most MPU/MCUs is a subtract which sets the flags in the appropriate way, but which ‘throws away’ the di erence outcome; that is does not overwrite the operand. A type of non-destructive subtract.

122 The Quintessential PIC Microcontroller

ANDing an input with a 0 always gives a 0 output, whilst with a 1 does not change the logic value. For example:

which zeros the upper nybble of the Working register. ANDing is normally used to clear any bit or bits in the destination operand. Thus andlw b’00000011’5 clears the upper six bits in the Working register and leaves the lower two bits untouched. Of course, the bcf instruction can be used to clear a single bit in a file register.

Another use of ANDing is to check the state of any bit or bits in a datum; for example:

By ANDing the Working register with 11000000b, the outcome will be all zero only if both bits 7 and 6 of W are 0. In this case the Z flag will be set and the following Bit Test File Skip on Clear will not be taken and the program will transfer to ALL_ZERO. If a single bit in a file register is being tested for zero then the btfsc instruction (see Table 5.4) is a more e cient process.

The iorwf and iorlw instructions implement the Inclusive-OR operation in the same way as for AND. ORing with a 0 leaves the source bit unchanged whereas ORing with a 1 sets the bit to a 1 irrespectively. Thus ORing is normally used to set any group of bits in the destination operand. For example:

sets the two least significant bits to one.

The xorwf and xorlw instructions provide for the eXclusive-OR operation. You will recall from page 14 that XORing with a 0 leaves a data bit unchanged, whilst XORing with a 1 inverts (or toggles) that bit. Thus, for example if we wished to invert both bits 0 and 7 of W:

For example:

Another use for XOR is to isolate changes between two bit patterns. From the truth table on page 14 we see that only when the two inputs di er is the output 1. Consider as an example a program routine that continually monitors Port B to which has been connected eight switches. This routine is waiting until someone moves a switch.

5Notice the assembler notation used to represent numbers in binary, see page 223.

Two possible scenarios are:

The outcome in W reflects any changes. In the first case there are no di erences between the latest sample and the original switch settings put away in File 20h. In the second situation Switch 4 has just been thrown from 1 to 0. You can determine which switch changed by shifting the outcome (the change byte) right, counting until the residue is zero. You can also determine the type of change (0 → 1 or 1 → 0) by ANDing the change byte to the original switch settings in File 20h, i.e. andwf 20h,w. If the outcome at bit 4 is a 0, then the change must have been 0 → 1, and vice versa.

The PIC 12/14-bit cores have two instructions which can shift a datum byte in a file register one place left or right. Both rlf and rrf are known as Circular or Rotate instructions. These shift left or right respectively with the incoming bit injected in from the Carry flag and outgoing bit popped out into the same C flag. This circular action is emphasized in Fig. 3.7 on page 61.6

One use of the shifting operation is to bitwise examine a datum. Suppose you want to determine the position of the leftmost logic 1 bit in File 20h, with this information being put in the Working register. For example, if the pattern is:

This can be realised by continually shifting the pattern under investigation right, counting the number of times until the residue is zero. The coding given in Program 5.4 uses the Working register as a counter. The data byte in File 20h is successively shifted right and the count incremented. As the Carry flag is cleared each time before the shift, logic 0s are brought in from the left.7 Eventually the residue will become all zeros

6The PIC17CXXX series use the mnemonics rlcf and rrcf for Rotate Left/Right through Carry. The change allows for rlnc and rrnc for Rotate Left/Right Not through Carry.

7MPU/MCUs that have Logic Shift instructions always shift in 0s irrespective of the state of the C flag.