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THE ESSENCE OF THE PIC MICROCONTROLLER 125

Program 5.5 Triple-precision shifting to find the number of set bits.

COUNT_BIT clrw ; Zero the count

; WHILE data is not zero, shift right and increment counter

by testing each byte in turn for zero and going back to the top of the loop if any byte test gives a non-zero outcome.

Shifting can be used to multiply and divide data by powers of two. For example, to divide by eight shift left three times:

where we are assuming that the Carry flag is cleared before each shift. In general shifting n places right gives a 2n division and similarly to the left gives multiplication by the same factor – see page 11.

As an example consider that the byte in File 22h (called MULTIPLICAND) is to be multiplied by 3 to give a 2-byte product in File 24:5h (PRODUCT_H and PRODUCT_L respectively).

The implementation of Program 5.6 relies on factoring ×3 as ×2+×1. The former is implemented by shifting a 16-bit extension of the byte multipicand (upper byte zeroed) once left. The single byte multiplicand is then added to the 2-byte subproduct to give the desired outcome. Example 5.5 gives the more complex case of multiplying by ten.

Program Counter instructions

The instructions listed in Table 5.4 modify in some way the setting of the Program Counter. The most elementary of these is nop. No OPer-

126 The Quintessential PIC Microcontroller

Program 5.6 Multiplying by three.

ation does not alter the state of the system in any way, but the PC will increment as a consequence of the instruction code being fetched from the Instruction store. Thus its sole outcome is [PC] <- [PC] + 1. This takes one bus cycle, so its main use is to implement a short delay, 1 µs for a 4 MHz clock rate. For example, to pulse Port A’s pin low for 2 µs and then high we have:

with the assumption that bit 0 of Port A has been set up as an output (see page 95) and that bit 0 (pin RA0) was high before entering the routine.

The goto instruction is an absolute jump instruction allowing the program to transfer to the specified instruction anywhere in the Program store. The process has been described in Fig. 5.4.

The remaining four instructions can skip over the following command if some condition is met. The pair decfsz (DECrement File and Skip on Zero) and incfsz (INCrement File and Skip on Zero) augment the specified file contents and then if the outcome is zero the PC is further incremented. Strangely, the Z flag is not a ected by these instructions.

A typical use for these instructions is to count the number of passes through a loop. For example, suppose it is necessary to pulse Port A pin RA0 low 20 times.

THE ESSENCE OF THE PIC MICROCONTROLLER 127

Table 5.4: Program Counter instructions.

Notice the assembler notation d’20’ for decimal 20 – see page 223. This is equivalent to 14h but more readily understood by the programmer.

The btfsc (Bit Test File and Skip if Clear) and btfss (Bit Test File and Skip if Set) instructions have been used extensively in programs both here and in Chapter 3. Besides their obvious use in changing the program flow based on the state of a specified bit in any register file, they allow decisions to be make on the state of the various flags in the Status register. Thus in Program 5.5 the series of btfss STATUS,Z (or the more unreadable btfss 3,2) instructions enable the program loop to be exited when the Z flag is set, that is on a series of zero outcomes. Similarly btfsc STATUS,C allows the count action to be skipped over when the C flag is clear. Neither instruction a ects the flags.

All four skip instructions take two cycles to execute when the skip occurs but only one when the condition is not met. As described in Example 4.1 on page 99, the former situation needs to flush the instruction pipeline. This is to reflect the fact that the pre-fetched instruction code