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

3. Stored Program Processing 65

the most significant addition becomes the highest bit of the outcome. For example FF FFh + FF FFh = 1 FF FFh (65, 535d + 65, 635d = 131, 070d).

F20h F21h

AUGEND_H AUGEND_L

F22h F23h

ADDEND_H ADDEND_L

Fig. 3.8 The process.

The overall process is diagrammatically shown in Fig. 3.8. However, given that we need to implement the process as a sequence of steps executable by the byte-sized instructions of Table 3.1 then the next step is to produce a task listing.

1.Add the low bytes of the augend and addend, generating the low byte of the sum and carry C1.

(c) The most-significant sum byte is the last carry-out

Fig. 3.9 Visualization of the task process.

68 The Quintessential PIC Microcontroller

borrow-out indicates that the last subtraction was not successful in that the divisor (i.e. ten) was larger than the residue it was subtracted from. That is the outcome is negative. The loop count on exit represents the number of successful subtractions and thus the quotient.

The coding in Program 3.6 closely follows the flow chart, with comment numbers referring to the statement boxes. The actual subtract constant ten is implemented by adding minus ten! This is because the more obvious sublw 10 instruction actually subtracts W from ten and not ten from W. Notice the mechanism for exiting the loop by testing and skipping if bit 0 in STATUS is clear after this subtraction.

Remembering that bit 0 in File 3 is the Carry flag doubling as the Borrow flag, then a borrow out is indicated if this bit is zero after the subtraction. On exit ten is added back on to compensate for the last unsuccessful subtraction and the outcome then gives the remainder.

Add ten to give remainder

End

Fig. 3.10 Division by repetitive subtracting.

70 The Quintessential PIC Microcontroller

C

0

PRODUCT_L

1 bcf CCR,C

C

2 rlf PRODUCT_L,f

PRODUCT_H

3 rlf PRODUCT_H,f

Fig. 3.11 Double-precision shifting.

the rlf (Rotate Left File) instruction twice, beginning with the lower byte. As well as shifting the byte PRODUCT_L left once, the most significant bit is moved into the Carry flag. Subsequently rotating the contents of the high byte PRODUCT_H moves this Carry in as the new least significant bit and shifts the high byte left as shown in Fig. 3.11. Of course the Carry flag needs to be cleared prior to this process. This 3-instruction double-precision routine is repeated three times to give the required ×8 function. Adding the original multiplicand, which is still in W gives the final ×9 function.

Example 3.5

The circuit diagram of Fig. 3.12 shows a 7-bit pseudo-random number generator (PRNG) based on a shift register with an Exclusive-OR gate feedback. Devise a routine to continually send these 127 binary random numbers to a port located at File 06. The routine must initialize the PRN to any non-zero value.

B6 B5 B4 B3 B2 B1 B0

C1 Sample rate

Fig. 3.12 A 7-bit pseudo-random number generator.

3. Stored Program Processing 71

Solution

A suitable task list is:

1.Initialize the number to 01.

2.DO forever.

(a)Rotate shift a copy of the number once left so that bits 5 & 6 are aligned.

(b)Bitwise XOR the number and its shifted copy.

(c)Rotate the outcome twice left to pop out bit 6 into the C flag, which will be B6 B5.

(d)Rotate the original number once left with C becoming the new bit 0.

Program 3.8 A 7-bit pseudo-random number generator.

The listing in Program 3.8 follows the task list fairly closely. The value of the number is temporarily saved in memory so that it can be processed without altering the original number down in W. This is done by specifying the destination to be the file, eg. rlf NUMBER,f. After rotating left once left, bit 5 in the shifted copy in NUMBER is aligned with the original bit 6 in W. After exclusive ORing the two, with the destination again being in File memory (xorwf NUMBER,f), bit 6 in NUMBER is now B5 B6. Shifting twice left puts this bit in the Carry flag. Finally rotating the original pseudo-random number in W moves that pattern once left with the new least-significant bit being the Carry flag, that is B5 B6 as specified in the diagram. The first 32 hexadecimal values output are:

02 04 08 10 20 41 83 06 0C 18 30 61 C2 85 0A 14