Question

Application Problem:

Answer the following questions at the bottom of the worksheet:

You are configuring a microcontroller (uC) to sample a signal connected to an input pin. Part of the initial setup requires that you clear (turn off) bits #17 and #2 in a 32-bit register, while leaving all other bits unchanged. To work with specific bits, we typically use a second number, called a mask, which has the bit positions we need to alter set to 1, and all other bits are 0. What sequence of binary operators could you use on the register and mask to get the desired result (bits 9 and 1 cleared)?  

Control Register:              10000001 00000000 00000000 00001010

Mask:                                  00000000 00000001 00000000 00000010

                                              __________________________________

Result:                                10000001 00000000 00000000 00001000

Hints:   Try a simplified set on paper, such as 1-bit numbers, to help you come up with ideas to solve the problem. Use the program you created in Part 3 to test your ideas on 32-bit numbers.

The “32 bit register” is your first integer (iNum1), and its value could be anything. Make sure your solution works regardless what iNum1’s value is.

Your second integer (iNum2) is the “mask”. Its value is all 0’s with the two key bits (#17 and #2) set to 1. Use the Windows calculator in Programmer Mode to convert binary 1 0000 0000 0000 0010 to decimal for iNum2.  

               You will need to apply more than one bitwise operator to solve this correctly

Part A.) Using the following binary operators, explain how the code can be developed to solve the problem?

>>	shift all bits right
<<	shift all bits left
&	AND
|	OR (inclusive)
^	Exclusive OR (XOR)

               ~             One’s compliment (flip all bits)

Part B.) Write the lines of code necessary to comple the process in C?

Answer 1

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Mask : Has the bit positions we want to alter set as 1

Bit positons that should remain unchanged as 0

We shall use Xor operation on register and mask to obtain desired result.

Xor operation is used to alter state of bit.

i.e If it is set(=1) it will clear(=0) it

and viceversa

(assume p shall be bit of register and q bit of mask)

Truth Table of xor

p q r

0 0 0

0 1 1

1 0 1

1 1 0

e.g

first 9 bits in a 32 bit register

register: 100111101

we want to alter 1st and 9th bit

so mask must be one(1) at bit position 1 and 9 will zero(0) otherwise

mask : 1 00000001

no when we perform xor bitwise on register and mask

register: 100111101

xor

mask : 1 00000001

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result: 000111100 (obtained using xor table)

the bits at position 1 and 9 are cleared