1. 15-348 Embedded Systems Lecture 4 January 20, 2016

2. Road Map  What have we done  Some motivation about embedded systems  Gotten familiar with embedded hardware  What we plan on doing  Bit manipulation in C  Input and Output using I/O ports

3. Bits and Bytes – A Review  The unit of information in computers is a bit  8 bits make one byte  How do we represent a number using bits?

4. Data vs Values  We can use bits to represent  Values  Data  So what is the difference?  Values are numbers where we can do arithmetic  Values can be signed or unsigned  In Data representation, different bit groups will denote different values

5. What does that mean?  We can use 32 bits to represent my Salary  Sum of each bit multiplied by appropriate powers of 2 will determine how many dollars I actually get  I can add a bonus to my salary by adding a 32 bit Salary to a 32 bit bonus getting a 32 bit dollar amount (with 32 bits and my Salary, there will never be any overflow)  But what if the 32 bits represent a color?

6. Data representation  A color can be represented with 32 bits as:  0xAARRGGBB  AA is an 8 bit value representing alpha (opacity)  RR is an 8 bit value representing red component  GG is an 8 bit value representing green component  BB is an 8 bit value representing blue component  What does it mean to add two colors together?  0x0000FFFF + 0x000000FF = 000100FE  0x0000FF00 - 0x00000001 = 0000FEFF

7. How can we work with bits?  We want to work with individual bits of a variable instead of the value they represent  How do we clear a bit int error_Flags = getflags(); // now clear bit 5? error_Flags &= 0xDF; error_Flags &= ~(0x01 << 5);

8. Bit Manipulations  How do we set a bit? int error_Flags = getflags(); // now set bit 5? error_Flags |= 0x20;

9. Good Programming practice  Define a macro for set/clear bits #define BitNClear(arg,bit)(arg &= ~(1<<bit)) #define BitNSet(arg,bit) (arg |= (1<<bit))  Define constants for Flags #define IR_NOT_WORKING_FLAG5

10. Use descriptive code #define BitNSet(arg,bit) (arg |= (1<<bit)) #define LED1 5 int main() { int lights = 0; // turn LED 1 on BitNSet(lights,LED1); }

11. What do you think the following do? #define BitNflp(arg,bit) ((arg) ^= BitMsk(bit)) #define BitNtst(arg,bit) ((arg)&BitMsk(bit))

12. Using bit manipulation #define BitNSet(arg,bit)(arg |= (1<<bit)) #define LED1 5 #define BitNTst(arg,bit) (arg & (1<<bit)) int main(){ int lights = 0; // turn LED 1 on BitNSet(lights,LED1); if(BitNTst(lights,LED1)) printf("LED1 is on"); else printf("LED1 is off"); }

13. Defining Bits as part of a struct  Bit fields allow packing of data in a struct struct Lights_Status { unsigned int LeftTurnSignal:1; unsigned int RightTurnSignal:1; unsigned int HeadLights:1; unsigned int BrakeLights:1; unsigned int NumLightsOn:3; };

14. Using Bit fields struct Lights_Status carLights; carLights.LeftTurnSignal = 1; carLights.HeadLights = 1; carLights.NumLightsOn = 2;

15. Input/Output in “MC9S12C”  We will use pins PA[0-7], PB[0-7], and PP[0-1] as general purpose Input/Output pins. PA[0-7]  Pins(odd numbers only) 39-25 PB[0-7]  Pins(odd numbers only) 55-41 PP0  Pin 9 and PP1  Pin 11

16. Talk about the IO pins setup  Microcontrollers have physical pins that can be configured as input or output  Pins are grouped in ports  Each port has 8 pins  Port have names like A, B, C, etc.  Pins are numbered 0, 1, 2, …  A0, A1, A2, A3, B0, B1, …  A Pin can be configured to be input or output

17. Pin Architecture Microprocessor D D Q Q Write to port address Write to direction register Read from port address

18. Configuring the Pins  Each IO pin can be configured to be either input or output  Use Data Direction Register DDR to set the direction of the pin. 0 means input, 1 means output.  E.g. DDRB = 0x45  DDRB = 01000101  PB0, PB2, and PB6 are outputs  PB1, PB3, PB4, PB5, PB7 are inputs

19. Using output pins  Let’s start from DDRB = 0x45  If we want to read the value on PB3 if(PORTB_BIT3 == 1) // do something if input is 1 else // do something else if input is 0

20. Writing to IO pins  To write a 1 to PB0 PORTB_BIT0 = 1;  Following are defined for you:  DDRA_BIT[0-7]  DDRB_BIT[0-7]  PORTB_BIT[0-7]  PORTA_BIT[0-7]  Use, #include

21. Lets see an example  Pin PB4 is connected to one of the LEDs: int i ; DDRB_BIT4 = 1; PORTB_BIT4 = 1; for(i = 0; i <100; i++); PORTB_BIT4 = 0; What would happen in this case?

22. Another Example main( ) { DDRB = 0x00; DDRA = 0xFF; for(;;) { if(PORTB_BIT0 == 0) PORTA_BIT0 = 1; else PORTA_BIT0 = 0; }