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

132 The Quintessential PIC Microcontroller

The task list implemented in Program 5.11 splits the ×10 task into a ×8 + ×2 operation. Thus:

1.Multiply multiplicand by eight (shift left three times).

2.Multiply multiplicand by two (shift left once).

3.Add the two 16-bit partial products.

4.End.

Program 5.10 Average daily temperature.

;Task3: DO

;Task3A: Add Temp[i] to the double-byte grand sum

THE ESSENCE OF THE PIC MICROCONTROLLER 133

Program 5.11 multiplication by ten.

The coding copies the 1-byte multiplicand into the lower byte of the product to be. Clearing the upper byte extends the datum to 16 bits. Clearing the Carry flag and then shifting left three times gives the ×8 subproduct. As the upper byte is initially clear then the Carry flag is always zero after each double-byte shift, as it is going into the second ×2 shift routine. This is done on the extended multiplicand memory space and the resulting subproduct added to first datum. This addition is simplified as the shifted upper byte of the ×2 subproduct is small and so any carry from the lower byte addition is accounted for by simply incrementing the upper byte of this datum before the upper bytes are added. There will be no carry out from this latter addition.

134 The Quintessential PIC Microcontroller

Self-assessment questions

5.1 Can you deduce what function the following code fragment performs on the data byte in the Working register?

addwf FILE,w subwf FILE,w

5.2How could you extend Example 5.3 to give an outcome as packed BCD in File 21h? Hint: Consider making use of the swapf instruction.

5.3Develop Example 5.3 to give a 3-digit BCD outcome; removing the restriction that the original binary byte should be limited to decimal 99. The outcome is to be in File 21:2:3h.

5.4Extend Example 5.1 to decrement a quad-precision 32-bit word located at File 26:7:8:9h, most significant byte first.

5.5Example 5.4 evaluated the average of an array of hourly temperature samples by summing all bytes and then subtracting 24 until the residue dropped below zero. Write an extension to this program to round the average to the nearest integer; that is if the remainder is more than 12 then round up.

5.6Write a routine to multiply a byte in File 22h by 13. The 2-byte

product is to be located at File 23:4h. The memory map for this

is OVERFLOW F 21h MULTIPLICAND F 22h × 13 = PROD_H F 23h PROD_L F 24h

where File 21h is used to extend the single-byte multiplicand to a 16bit double byte datum. Note that this will require three shift and add processes.

5.7 A simple digital low-pass filter can be implemented using the algorithm:

Array[i] = S4n + Sn2−1 + Sn4−2

where Sn is the nth sample from an eight-bit analog to digital converter located at Port B.

Write a routine assuming that the three byte memory locations to store Sn, Sn−1 and Sn−2 are located at File 20:1:2h respectively. The outcome Array[i] is to be located at File 48h.

5.8 A certain television show has eight contestants which are evenly divided into Team A and Team B. Each member has a switch, giving logic 1 when pressed, which may all be read simultaneously by the

THE ESSENCE OF THE PIC MICROCONTROLLER 135

microcontroller at Port B. Team A switches appear on the lower four bits of the port.

Write a routine that will:

•Decide when a response to the question has been made – any switch closed.

•Determine the team identity that has responded, by clearing File 20h for Team A and setting it to any non-zero value to signify Team B.

•Ascertain which team member pressed his or her switch by putting the member number 0–3 in File 21h.