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S emb t9-arch_power (1)
- 1. Maths is not everything
Embedded Systems
4 - Hardware Architecture
CPU
Input/Output mechanisms
Memory
Buses and Aux I/O
Input/Output interfaces
RMR©2012
Power Management
- 3. CPU power consumption
Most modern CPUs are designed with
power consumption in mind to some
degree.
Power vs. energy:
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
heat depends on power consumption;
battery life depends on energy consumption.
- 4. CMOS power consumption
Voltage drops: power consumption
proportional to V2.
Toggling: more activity means more power.
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Leakage: basic circuit characteristics; can
be eliminated by disconnecting power.
- 5. CPU power-saving strategies
Reduce power supply voltage.
Run at lower clock frequency.
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Disable function units with control signals
when not in use.
Disconnect parts from power supply when
not in use.
- 6. Power management styles
Static power management: does not
depend on CPU activity.
Example: user-activated power-down mode.
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Dynamic power management: based on
CPU activity.
Example: disabling off function units.
- 7. Application: PowerPC 603 energy features
Provides doze, nap, sleep modes.
Dynamic power management features:
Uses static logic.
Can shut down unused execution units.
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Cache organized into subarrays to minimize amount
of active circuitry.
- 8. PowerPC 603 activity
Percentage of time units are idle for
SPEC integer/floating-point:
unit
Specint92
Specfp92
29%
17%
35%
17%
fixed-point
RMR©2012
28%
load/store
Maths is not everything
29%
I cache
© 2008 Wayne Wolf
D cache
38%
76%
floating-point
99%
30%
system register
89%
97%
- 9. Power-down costs
Going into a power-down mode costs:
time;
energy.
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Must determine if going into mode is
worthwhile.
Can model CPU power states with power
state machine.
- 10. Application: StrongARM SA-1100 power saving
Processor takes two supplies:
VDD is main 3.3V supply.
VDDX is 1.5V.
Three power modes:
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
Run: normal operation.
Idle: stops CPU clock, with logic still powered.
Sleep: shuts off most of chip activity; 3 steps, each
about 30 µs; wakeup takes > 10 ms.
- 11. SA-1100 power state machine
Prun = 400 mW
run
10 μs
160 ms
90 μs
10 μs
Maths is not everything
RMR©2012
© 2008 Wayne Wolf
idle
Pidle = 50 mW
90 μs
sleep
Psleep = 0.16 mW
- 12. Maths is not everything
MSP430
Power Management
RMR©2012
- 13. e on-demand
MSP430: achieving ultra-low power
Extended Ultra-low Power standby mode
Minimum active duty cycle
Performance on-demand
Maths is not everything
RMR©2012
13
- 21. MSP430: C coding tips
Use local variable as much as possible.
Local variables use CPU registers whereas global
variables use RAM.
Maths is not everything
RMR©2012
21
Use bit mask instead of bitfields for
unsigned int and unsigned char.
Use unsigned data types where possible
Use pointers to access structures and
unions
Use “static const” class to avoid run-time
copying of structures, unions, and arrays.
Avoid modulo
Count down “for” loops
- 22. MSP430: principles of ULP operation
Maximize the time in standby (LPM3)
Use interrupts to control program flow
Replace software functions with
peripheral hardware
Power manage internal peripherals
Power manage external devices
Device choice can make a difference
Maths is not everything
RMR©2012
22
Effective code is a must
Every unnecessary instruction executed is a portion
of the battery wasted that will never return.
