Embedded Real Time Software Development - PowerPoint Presentation

Slide1

Embedded Real Time Software Development

Week

5

Mail Boxes / Binary Semaphores

Fixed Point ArithmeticSlide2

Today’s Agenda

Announcements

Homework

Mail Boxes

Mail Boxes as Binary Semaphores

Fixed Point ArithmeticSlide3

Announcements

Due date on “Intensity Task” homework assignment has been moved out one week to March 11.

The “Watchdog Timer” homework assignment is now optional. If turned in, the student may substitute that grade for the lowest of any homework or quiz grade.

There will be a quiz on fixed point arithmetic on March 4.

The mid-term exam will be on March 25

.

Homework due today will be accepted until 11:59 PM March 19Slide4

Homework Assignment

Start with OS

Blinky

project

Adds task that monitors Serial input

‘!’ received, print “Howdy, Darn it!”

Any other character, print “Howdy”

Requires modification of

uartstdio

.[

c|h

]Slide5

Modifications to uartstdio.h

Use the buffered (interrupt driven) version of the library code

Declare the semaphore we will be using laterSlide6

Modifications to uartstdio.c

1

Change include from “

util

/…” to “…”

Declare the semaphore we will be usingSlide7

Modifications to uartstdio.c

2

Create the Semaphore when we initialize the UARTSlide8

Modifications to uartstdio.c

3

Interrupt function needs to tell the kernel that we are servicing an interrupt.Slide9

Modifications to uartstdio.c

4

Put it here, because if the interrupt function is going to throw away the character, our function doesn’t need to know it was here

Make sure that the Semaphore was created, just in case we get an interrupt before the initialization is complete.Slide10

Modifications to uartstdio.c

5

Tell the kernel we are through processing the interrupt.Slide11

Modification to startup.c

Call the task initialization function.Slide12

howdy.c 1

Define the priority and stack size

Declare the stack and the task function

the ANSI declaration is here because

init_howdy

() is the only function that will ever refer to

howdy_task

()

howdy.h

(right) just needs to declare the

init_howdy

() function.Slide13

howdy.c 2Slide14

howdy.c 3

Wait for the semaphore with a 30 second timeout

Decide what to do based on the error code

OS_ERR_NONE – We got the semaphore normallySlide15

howdy.c

4

Had a timeout or unknown result code

Comment on line 125 because a “break;” would cause an unreachable code warning and no “break;” would cause a discussion in the code review as a standards violation.Slide16

Mail Boxes

Mechanism to send a single value (usually pointer sized) to a task

Receiving task can pend on the posting of a value

Can not post 0Slide17

Mailbox Functions

OSMboxCreate

()

Creates a new mailbox

OSMboxPost

()

Posts a value to a mailbox

OSMboxPend

()

Waits on a value to be placed in a mailbox, then empties the mailbox

OSMboxAccept

()

Same as Pend, but does not waitSlide18

OSMboxCreate()

OS_EVENT *

OSMboxCreate

(void *

msg

)

msg

is the initial value to go in the box, usually 0

Returns a pointer to the created mailbox or 0 if there are no more available Event Control Blocks

OS_EVENT *

my_mbox_ptr

;

…

my_mbox_ptr

=

OSMboxCreate

((void *)0);

…Slide19

OSMboxPost()

INT8U

OSMboxPost

(OS_EVENT *

pevent

, void *

msg

);

*

pevent

– pointer to the mailbox TCB

*

msg

– value to be posted, 0 doesn’t post anything

returns the following error codes:

OS_ERR_NONE, OS_ERR_FULL, OS_MBOX_FULL, OS_ERR_EVENT_TYPE, OS_ERR_PEVENT_NULL, OS_ERR_POST_NULL_PTR

OS_EVENT *

my_mbox_ptr

;

uint8_t

recvd_data

[100]

…

if (index ==

new_index

)

{

…

err =

OSMboxPost

(

my_mbox_ptr

,(void *)&

recvd_data

[index]);

…

}Slide20

OSMboxPend()

void *

OSMboxPend

(OS_EVENT *

pevent

, INT16U timeout, INT8U *err);

*

pevent

– Pointer to the mailbox TCB

timeout – Max time, in ticks, to pend. 0 waits forever

*err – Pointer to location that will hold the return code from the function

OS_ERR_NONE, OS_ERR_TIMEOUT, OS_ERR_EVENT_TYPE, OS_ERR_PEND_ISR,

OS_ERR_PEVENT_NULL

Returns the value in the mailbox, (void *)0, if the mailbox is empty

OS_EVENT *

my_mbox_ptr

;

UINT8

error_code

;

uint16_t *

next_value

;

…

next_value

=

OSMboxPend

(my_mbox_ptr,0,&error_code);

switch (

error_code

)

{

case OS_ERR_NONE:

return *

next_value

;

…

}

…Slide21

OSMboxAccept()

void *

OSMboxAccept

(OS_EVENT *

pevent

)

*

pevent

– Pointer to the mailbox TCB

Returns the value in the mailbox, (void *)0, if the mailbox is

empty

…

OS_EVENT

*

my_mbox_ptr

;

uint16_t

*

next_value

;

…

next_value

=

OSMboxAccept

(

my_mbox_ptr

);

if (

next_value

!= 0)

{

…

return

*

next_value

;

…

}

…Slide22

Mail Boxes as Binary Semaphores

You do not

have to

post a pointer, you can post any variable or constant that is the same size or smaller than a pointer.

If you “Post” a constant, the behavior of a mailbox is exactly the same as a binary semaphore would be.Slide23

BreakSlide24

Fixed Point Arithmetic

The Problem

Floating point math is slower than integer math (even with a Floating Point Unit)

Use of floating point math slows multi-threaded / multi-tasked systems even more (lots of BIG registers to push to save context)

We still need to deal with fractional values

The Solution

Fixed point arithmeticSlide25

What is a Fixed Point Number?

The meaning of an N-bit binary word depends entirely on its interpretation. For example, if I have the 8 bit binary value 01000011

Traditionally it could be interpreted as

Hex 43, Octal 103, Decimal 67, ASCII ‘C’

But what if I decided to interpret the value as 1/16

th

of the signed, binary count. We the value could then be interpreted as:

Binary 0100.0011, Hex 4.3, Octal 4.14, Decimal 4.1875

The notation describing the interpretation of this number is Q4.3, meaning 3 integer bits and 4 fractional bits (the high order bit is the sign).

We say that the “Scale” of our numbers is:

 Slide26

Whoa! What?

Assuming Q4.3 notation (scale = 16)….

Decimal 4.0 = 01000000 = 0x40… how?

Decimal 1.62

= 25.9, (25)=00011001

converting back

=1.56

Decimal -2.13

=-34.08, (-34)=11011110

=-2.125

Note the precision is

 Slide27

Huh?

Let’s think about this in decimal…

I tell you that every number I give you is 100 times the value I “mean” (scale=

) then if I say:

25, you interpret 0.25

314, you interpret 3.14

-7, you interpret – 0.07

3,583,771, you interpret 35,837.71

 Slide28

Why is this better, I understand floating point?

You only think you understand floating point

The math is faster than floating point because you can use integer arithmetic for everything

Addition and subtraction just work… using a scale of

:

7.16-3.05=4.11 and 716-305=411

Multiplication and Division require us to think about the scale:

3.71*0.4=1.484,

If our Scale is an even power of two the division above can be accomplished with shifts, which are cheap!

 Slide29

Facts to Ponder

Word size:

Range of values:

Resolution of values: for

Accuracy of values: for

Addition and subtraction require both operands to be the same scale

The result for addition and subtraction will always be the same scale

Multiplication and division the scale of the operands may be different

Multiplication, the scale of the result is the scale of the first operand multiplied by the scale of the second operand

Division, the scale of the result is the scale of the dividend divided by the scale of the divisor

“Saturation” is when a Fixed point number reaches the highest or lowest value in its range.

 Slide30

Tivaware

IQMath

Library

Based on 32 bit values

Supports Q30.1 to Q1.30

Type for Q30.1 is _iq30

Type for Q29.2 is _iq29

…

Type for Q1.30 is _iq1

Conversion

To String

To _

iq

?? From _

iq

??

Do not use floating point conversions

Multiplication / Division

Trig,

Sqrt

, Exponents, etc.

#include “

IQmath

/

IQmathlib.h

”Slide31

Tivaware IQ Functions

Division - _

IQNdiv

()

_iq16 numerator, denominator, result;

…

result = _IQ16div(

numerator,denominator

);

Multiplication - _

IQNmpy

()

_iq16

mpy1, mpy2, result;

…

result = _IQ16mpy(mpy1,mpy2

);

Addition and subtraction

_iq16 op1, op2, op3, result;

…

result = (op1 + op2) – op3

;

Output Format

_iq16 op1;

char

outstr

[16];

…

_IQ16toa(op1,”%2.05f”,outstr);

Convert from

IQn

to

IQm

#define

IQtoIQ

(Q1,Q2,A) ((_

iq

##Q1)(A) >> (Q1 – Q2))

_iq16 val1;

_iq24 val2;

…

val1 =

IQtoIQ

(16,24,val2

);Slide32

Creating a Constant

π

as Q24.7

Range is -8 to 7.999999996

Scale is 16,777,216

π

=

3.141592653 *

Scale =

52707178 (calculator)

#define Q24_SCALE ((int64_t)16777216)

#define PI_9DIGITS ((int64_t)3141592653)

#define TEN_EXP9 ((int64_t)1000000000)

#define PI_Q24 ((_iq24)((PI_9DIGITS*Q24_SCALE)/TEN_EXP9))Slide33

You have to design before writing code

Working with fixed point numbers you have to think about

Range

Precision

Order of evaluationSlide34

Range and Precision Example

Variable to hold distance in meters.

Assume Q10.21 (_iq10).

Range: >1300 miles

Resolution: ~0.1 inchesSlide35

Order of Evaluation (Range)

Assume C28.3 (_iq28)

Compute 2 * 4 / 5 = 1.6

2 = 0x2000 0000

4 = 0x4000 0000

5 = 0x5000 0000

2 * 4 results in 0x8000 0000, which is -1, OVERFLOW

4/5 results in 0xCCC CCCC, which is 0.79999…

2 * 0xCCC CCCC results in, 0x1999 9998, which is 1.59999…Slide36

Order of Evaluation (Precision)

Assume C4.27 (_iq4)

Compute (0.0625 / 64) * 1000 = 0.9765625

0.0625 = 0x0000 0001

64 = 0x0000 0400

1000 = 0x0000 0010

0.0625 / 64 = 0x0000 0000, UNDERFLOW

1000 / 64 = 0x0000 00FA

0.0625 * 0x0000 00FA = 0x0000 000F = 0.9375Slide37

Homework Assignment

Start with last week’s assignment.

Change blink rate of yellow led in proportion to the temperature reading. 68F (or less) blink at 2 Hz, 90F (or more) blink at 20 Hz.

Change messages from “Howdy” and “Howdy darn it!” to “Temp=??” and “Humidity=??” respectively, where ?? is the actual temperature or humidity.

Measure temperature and humidity once per second.Slide38

Homework StructureSlide39

I2C Port Setup

//

The I2C3 peripheral must be enabled before use.

ROM_SysCtlPeripheralEnable

(SYSCTL_PERIPH_I2C3);

ROM_SysCtlPeripheralEnable

(SYSCTL_PERIPH_GPIOD);

// Configure the pin

muxing

for I2C3 functions on port D0 and D1.

ROM_GPIOPinConfigure

(GPIO_PD0_I2C3SCL);

ROM_GPIOPinConfigure

(GPIO_PD1_I2C3SDA);

// Select the I2C function for these pins. This function will also

// configure the GPIO pins

pins

for I2C operation, setting them to

// open-drain operation with weak pull-ups.

GPIOPinTypeI2CSCL(GPIO_PORTD_BASE, GPIO_PIN_0);

ROM_GPIOPinTypeI2C(GPIO_PORTD_BASE, GPIO_PIN_1);

// Select the clock speed for the I2C bus to be 400kHz

ROM_I2CMasterInitExpClk(I2C3_BASE,SysCtlClockGet(),true);Slide40

I2C Port Commands

See SW-TM4C-DRL-UG-1.1.pdf

TivaWare

Peripheral Driver Library,

Section 13

Create and I2C interrupt function

Interrupts are generated when I2CMasterControl completes

Use post/pend to suspend task while I2C transfer is

occuring