ELE271
Mon. April 7, 2014
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Review LPM
-
Morse Code Lab
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.ref and .def
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Multiplication, shift
1
Low Power Modes
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.
2
How are these concepts applied to
microcontroller programming?
- Low Power design isn’t just for low power applications.
- Low Power/Interrupt-driven design helps with modular design.
The following techniques are applied to low power Design:
- Use of Interrupts.
- Controlling the clock.
3
Low Power Modes
MSP430 Clock Modes
Average


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.
4
Processor Clocks
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
Power Consumption
Power (Watts)
300
250
200
Active Power
150
Leakage
100
50
0
0.25
0.18
0.13
0.09 0.065
Clock Speeds ()
5
6
MSP430 5xx Power Modes
7
Low Power Modes
MSP430 Clock Settings (r2)
SCG1
SCG0
OSC
OFF
CPU
OFF
Active Mode
0
0
0
0
~ 250 µA
LPM0
0
0
0
1
~ 35 µA
LPM3
1
1
0
1
~ 0.8 µA
LPM4
1
1
1
1
~ 0.1 µA
Reserved
SMCLK and
ACLK Active
Only ACLK
Active
;
V
No Clocks!
GIE
N
Z
C
Sleep Modes
enable interrupts / enter low-power mode 0
bis.w
#LPM0|GIE,SR
; LPM0 w/interrupts
Can be combined
8
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)?
9
Review of stack interaction with interrupts.
10
How to come out of LPM in active mode.
A mildly tricky line of assembly language is
required to clear the bits set for the low power
mode
PORT1_ISR
bic.w #LPM4,0(SP)
reti
See Jimenez Ex. 7.4
11
12
Morse Code Lab
Objective:
to program the device using assembly code to
toggle the LED to send a Morse Code message
“SOS” each time S1 is pressed.
This lab will illustrate the following concepts:
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Interrupt-driven programming.
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Multiple source files (.ref,.def directives)
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Subroutines
-
Timers
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Low Power Modes
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Advanced data structures (pointers)
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Exit an ISR in Active Mode (AM).
13
Assume one word.
Assume only letters.
14
Morse code has a variable-length code for each character.
Question: how to represent this in memory.
Solution: use pointers or “relocatable expressions” in assembly:
; letters--->A_ptr---->DOT,DASH,END
;A
;
;B
B_ptr---->DASH,DOT,DOT,DOT,END
,etc…
15
16
Relocatable expressions
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Morse Code Data Structure
A_ptr
DOT
.equ
1
DASH
.equ
3
0x5c00
0x5c10
0x5c10
01h
dot
03h
dash
end
letters:
00h
.word A_ptr
0x5c13
03h
.word B_ptr
01h
,etc…
01h
01h
00h
A_ptr:
.byte DOT,DASH,END
;A
B_ptr:
.byte DASH,DOT,DOT,DOT,END
;B
, etc…
; letters--->A_ptr---->DOT,DASH,END
;A
;
;B
B_ptr---->DASH,DOT,DOT,DOT,END
18
Morse Code Algorithm
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
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RTN to Assembly: The While Statement Part 2
Usually a “while” statement implies a “cmp”
“while” loops
cause jmp
while ( n > 0 ) {
Loop:
s1…
backwards
cmp #0,r6
}
dec
r6
jz
End
s1…
jmp
Loop
End:
Until condiion is met, then
Jmp forward.
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
21
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.
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