2021-09-08

1. Announcements

  • hw04 due Friday

Setup:

2. Class time

2.1. Operations on bits

…​instead of signed/unsigned integers

Bitwise operations:

  • AND (&)

  • OR (|)

  • XOR (^, careful!!). How do you do \(y^x\) then?

  • NOT (~)

Do you remember the difference between:

  • & and && ?

  • | and || ?

    MSB

    most significant Byte

    msb

    most significant bit

Bit shifting:

  • Arithmetic

  • Logical

Shift left is like multiply by 2!

Shift right is like divide by 2! What if the number is negative? (msb is 1)

-> modern compilers are smart enough to do this without your "help." If you do not immediately know what will happen if the most-significant-bit is a 1 and you shift right, then do not be tricky.

If you intend to multiply by 2, then just say x = foo * 2;. Be careful with integer division, though x = foo / 2 is relatively safe.

2.2. Activity

What does this code do?

Example from zyBook: 2.7 Bit access functions
// x: 8-bit value.   k: bit position to set, range is 0-7.   b: set bit to this, either 1 or 0
unsigned char SetBit(unsigned char x, unsigned char k, unsigned char b) {
   return (b ?  (x | (0x01 << k))  :  (x & ~(0x01 << k)) );
              //   Set bit to 1           Set bit to 0
}

-→ do you even remember what ? does or is called??

Your task with a partner:

  1. Paste this code into Notepad++

  2. Rewrite the comments to be reasonable and actually helpful

  3. Now that you are clear on what the code is supposed to do, rewrite the code to be easy to read.

  4. Paste your code into a text snippet in the #microcontrollers Slack channel so everyone can see (see demo)

2.3. Airbag control

(did not make it here during class time)

zy 2.3.4 Airbag system https://learn.zybooks.com/zybook/VALPOECE322Fall2021/chapter/2/section/3?content_resource_id=24426883

A car has a sensor that sets A to the passenger’s weight (e.g., if the passenger weighs 130 pounds, A7..A0 will equal 10000010). Write a RIM C program that enables the car’s airbag system (B0=1) if the passenger’s weight is 105 pounds or greater. Also, illuminate an "Airbag off" light (by setting B1=1) if weight > 5 pounds but weight < 105 pounds.

  • How do you verify that the code works correctly? This is different than compiling and loading without errors.

  • How many tests does this require?