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ELE271

Mon. April 7, 2014- Review LPM

- Morse Code Lab

- .ref and .def

- Multiplication, shift

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Principles of Low-Power Apps• Maximize the time in low-power modes.• Sleep as long as possible and use interrupts to wakeup.• Use the slowest clock while still meeting processing needs.• Switch on peripherals only when needed (ie, switch off

peripherals when not needed).• Use low-power integrated computer peripherals.

– Timers: Timer_A and Timer_B for PWM– A/D convertors, flash, LCD’s

• Use faster software algorithms / programming techniques– Calculated branches instead of flag polling.– Fast table look-ups instead of iterative calculations.– Use in-line code instead of frequent subroutine / function calls.– More single-cycle CPU register usage.

Low Power Modes

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How are these concepts applied to

microcontroller programming?

The following techniques are applied to low power Design:

- Use of Interrupts.

- Controlling the clock.

- Low Power design isn’t just for low power applications.

- Low Power/Interrupt-driven design helps with modular design.

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A method to reduce power consumption is to turn off some (or all) of the system clocks (and turning off the power to some circuits).

A device is said to be sleeping when in low-power mode; waking refers to returning to active mode.

MSP430 Clock ModesLow Power Modes

Average

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Processor Clock Speeds• Often, the most important factor for reducing power

consumption is slowing the clock down.– Faster clock = Higher performance, more power required– Slower clock = Lower performance, less power required

Processor Clocks

0.25 0.18 0.13 0.09 0.0650

50

100

150

200

250

300

Active Power

Leakage

Clock Speeds ()P

ow

er

(Wa

tts

)

Power Consumption

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MSP430 5xx Power Modes

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Reserved V SCG1 SCG0OSCOFF

CPUOFF GIE N Z C

Active Mode

LPM0

LPM3

LPM4

0

0

1

1

0

0

1

1

0

0

0

1

0

1

1

1

~ 250 µA

~ 35 µA

~ 0.8 µA

~ 0.1 µA

; enable interrupts / enter low-power mode 0 bis.w #LPM0|GIE,SR ; LPM0 w/interrupts

MSP430 Clock Settings (r2)Low Power Modes

SMCLK and ACLK Active

Sleep ModesOnly ACLK

Active No Clocks!

Can be combined

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Two problems with this:

- What if you want to stay in active mode after ISR?

- What if you want to keep ISR small (so you don’t turn off other interrupts)?

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Review of stack interaction with interrupts.

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PORT1_ISR

bic.w #LPM4,0(SP)

reti

A mildly tricky line of assembly language is required to clear the bits set for the low power mode

See Jimenez Ex. 7.4

How to come out of LPM in active mode.

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Morse Code Lab

- Interrupt-driven programming.

- Multiple source files (.ref,.def directives)

- Subroutines

- Timers

- Low Power Modes

- Advanced data structures (pointers)

- Exit an ISR in Active Mode (AM).

This lab will illustrate the following concepts:

Objective:

to program the device using assembly code to toggle the LED to send a Morse Code message “SOS” each time S1 is pressed.

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Assume one word.

Assume only letters.

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Morse code has a variable-length code for each character.

; letters--->A_ptr---->DOT,DASH,END ; A

; B_ptr---->DASH,DOT,DOT,DOT,END ; B

,etc…

Question: how to represent this in memory.

Solution: use pointers or “relocatable expressions” in assembly:

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Relocatable expressions

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DOT .equ 1

DASH .equ 3

letters:

.word A_ptr

.word B_ptr

,etc…

A_ptr: .byte DOT,DASH,END ; A

B_ptr: .byte DASH,DOT,DOT,DOT,END ; B

, etc…

A_ptr

0x5c00 0x5c10

0x5c10 01h

03h

00h

0x5c13 03h

01h

01h

01h

00h

dot

dash

end

Morse Code Data Structure

; letters--->A_ptr---->DOT,DASH,END ; A

; B_ptr---->DASH,DOT,DOT,DOT,END ; B

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Sleep;

Get address of .cstring message.

Get first character.

While character != 0 , do

normalize ASCII to 41h (ie, so “A”=1)

Get address of letters

Get first code ; code = dot or dash

While code != 0, do

Turn LED on

Sleep(1|3) ; 1unit=dot,3 units=dash

Turn LED off

Sleep(1) ; 1 unit

Get next code

End While

Sleep(3)

Read next character.

End while

Morse Code Algorithm

while ( n > 0 ) {

s1…

}

Loop:

cmp #0,r6

dec r6

jz End

s1…

jmp Loop

End:

Usually a “while” statement implies a “cmp”RTN to Assembly: The While Statement Part 2

“while” loops

cause jmp

backwards

Until condiion is met, then

Jmp forward.

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sleep

If(pushbutton) do

R4<-#message ; get address of message

R5<-(r4) ; get character pointed to by #message

while (r5 != 0), do ; keep getting another char unless zero

r5<-r5-41h ; normalize ASCII character to 40h (ie, A=0)

r5<-r5*2 ; multiply by 2 for word boundary

r7<-letters(r5) ; get first code for letter (dot or dash)

while(r7 != 0), do ; get next code unless end (0)

beep(r7) ; beep amount of code (1 or 2)

short_delay ; short delay between beeps

r5<-r5+1 ; increment address

r7<-letters(r5) ; get next code

endwhile

r4<-r4+2 ; increment character address

long_delay ; long delay between characters

Endwhile

End if

 

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Write an interrupt-driven program to toggle the LED based on the Morse Code.

 

• Learn how to use multiple source files with the .ref and .def directives.

 

• Learn how to use subroutines.

 

• Learn how use Timers.

 

• Learn how use Low Power Modes.

 

• Learn how use pointer data types.

 

• Learn how to come out of an ISR in active mode.