Low Power Applications

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Low Power Applications
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Low Power Terminology

• Many times it is hard to compare low power modes
  and terms lets review these for a few devices
• M16C
• High Speed Mode - CPU core is on and is clocked
  at f1
• Medium Speed Mode - CPU core is clocked by f1
  divided by 2,4,8 or 16.
• Ring Oscillator Mode Ring oscillator provides
  CPU and peripheral clocks.
• Ring Oscillator Low Power Dissipation Mode Ring
  oscillator provides CPU and peripheral clocks.
  Main oscillator is turned off
• Low Speed Mode sub-clock (32 kHz typically)
  provides CPU clock. Main clock provides
  peripheral clocks
• Low Power Dissipation Mode - sub-clock provides
  CPU clock. Main clock is turned off
• Wait Mode - CPU core is not clocked (typically
  specification has 1 timer running from sub-clock)
• Stop Mode Oscillators are all shut off
• Terms
• CPU clock Bclk f1 clock speed of high
  speed oscillator or high speed oscillator PLL
  multiplier

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Low Power Terminology

• H8
• High Speed Mode - CPU core and peripherals are
  clocked at high speed
• Medium Speed Mode - CPU core and peripherals are
  clocked at medium speed
• Sub-active Mode System clock stopped. CPU
  running from sub-clock. Most peripherals can run
  from sub-clock
• Sleep Mode - System clock oscillator is running,
  CPU core is not clocked. Timers operate from
  system clock.
• Sub-sleep Mode System clock oscillator is off.
  Sub-clock is running. CPU core is not clocked.
  Timers operate from sub-clock
• Watch Mode Same as sub-sleep except IO not
  operating but retained, WDT operation and timer
  can change operating state
• Software Standby Mode Clock oscillators off.
  RAM and registers retained, ports are halted.
  NMI,IRQ and WKP interrupts are functioning
• Hardware Standby Mode Only RAM is retained.
  Requires release of hardware STBY pin and Reset
  to exit

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Low Power Terminology

• TI
• Active Mode - MCLK is enabled from any source
• ACLK can be used for MCLK
• LPM0 CPU clock (MCLK) is disabled. SMCLK or
  MCLK ACLK on. Electrical specification
  typically given with on Timer_B clocked at 1 Mhz
  
• LPM1 MCLK and DCO oscillator are disabled.
  SMCLK and ACLK are enabled. DC generator
  disabled if DCO not used in active mode
• LPM2 - MCLK, SMCLK and DCO are disabled. DC
  generator and ACLK active.
• LPM3 MCLK, SMCLK , DCO and DC generator are
  disabled. Only ACLK active. Electrical
  specification given for Timer_B clocked by ACLK
  (32 kHz)
• LPM4 CPU and all clocks disabled
• Terms -
• MCLK - CPU clock SMCLK Peripheral
  clock
• ACLK Sub-clock DCO - Digitally
  controlled oscillator with RC type
  characteristics

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Some new terms

• I am going to define a few terms for discussion
• These are modes that are application type modes
• High Speed Active - MCU core and peripheral
  clocks actively running. MCU and peripheral
  clocks near 1MHz or greater
• Low Speed Active MCU core and peripheral clocks
  actively running from a sub-clock. High speed
  oscillator is off
• RTC Mode - MCU core is not clocked. A timer is
  running to maintain some level of real-time clock
  function which can be used to wake-up the device
• Standby Mode - All oscillators are off. MCU
  will require some external signal to bring it out
  of low-power mode

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Low Power Mode ComparisonTypical Values
CPU

• 25C/85C
• Running from RAM/Flash

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Low Power Mode ComparisonMax Values
CPU
Derived from data sheet Calculated by
proportion
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Battery Types and Capacities

• Typical Battery Capacities
• Standard Alkaline
• AAA 1250 mAh
• AA 2700 mAh
• 1.5V nominal
• mAh rating typically given for 25 mA discharge to
  0.8V
• Ratings also typically show capacity for 1.0V and
  1.2V cutoff
• Lithium Coin
• Ampacity ranges from 30 600 mAh
• Standard 2032 cell is 225 mAh rated to 2.0V
• Nominal cell voltage is 3V
• Usually rated for 10K loads or less
• Cylindrical Lithium (photo battery)
• Ampacity ranges from 800 -1500 mAh
• 3V or 6V ratings
• Very good for higher pulse loads

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Standby ModeApplications
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Energy Consumed in Standby Mode

• Calculate the energy consumed in standby mode for
  various conditions
• 1 year 100 standby mode
• 0.1uA .876 mAh
• 0.7 uA 6.1 mAh
• 1.0 uA 8.76 mAh
• 5 years 100 standby mode
• 0.1 uA 4.38 mAh
• 0.7 uA 30.5 mAh
• 1.0 uA 43.3 mAh - This represents less than
  2 of AA capacity, yet in five years an alkaline
  cell battery would have lost close to 20
  capacity due to self-discharge (under ideal
  conditions, in real conditions the self-discharge
  is much higher than this)

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RTC Mode Applications
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RTC Mode Applications

• Three sources of current consumption in an
  application involving RTC mode
• RTC Current (Watch or Wait Mode Current)-
  Current consumed while the device is in the low
  power state
• Startup or Transition Current This is the
  current consumed between the interrupt waking up
  the device and Active Mode operation. Directly
  related to oscillator startup and stabilization
  times
• Active Current - Current consumed while
  operating at some high speed clock level

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RTC Example

• Consider an application that has the following
  requirements
• Real time clock mode
• Wake up once per second measure an analog
  parameter, compare it, perform some control
  functions
• AA batteries used (Assume 2000 mAh)
• 1 Year and 5 year life calculations
• For discussion assume active state takes 5 mSec
  at 1 Mhz CPU clock speed
• This is 5k CPU clock cycles
• Battery Life will be determined by 3 states
• Active State - 5 mSec/ sec
• Startup State
• RTC State
• Calculate battery life by
• Battery mAh/(.005Active current SU Energy/ 1
  Sec RTC current time)

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RTC Mode Consumption Comparison

• The MCU is in this mode the majority of the
  time.
• Table below compares the energy consumption for
  various RTC current levels

2000 mAh assumed for battery ampacity
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Oscillation Startup
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Oscillation Startup

• Requirement is to start-up once a second from RTC
  mode.
• In one year this is 31,536,00 startups
• 5 year life (157,680,00 startups)
• Oscillation startup times
• TIMSP430 - 6 uS
• H8/38602 - 20 uS
• M16C/28 - The M16C response is
• 18 clock cycles (interrupt response) 1/32kHz
  clock
• Oscillator stabilization time lt300 uS
• This equates to approximately 600uS _at_ 400 uA plus
  300 uS at 1 mA

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Startup Current Consumption1 year 1 time per
second
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Startup Current Consumption5 years 1 time per
second
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Startup Current Consumption

• Why is the contribution so negligible?
• Consider an application that would run at 1 mA
  for 1 year
• 1mA 8760 hours 8760 mAh
• However if this only runs for 1 millisecond
• 8760 mAh .001 duty cycle 8.76 mAh
• If there are multiple oscillation startups in a
  second this can become significant

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Energy Consumed During Start Up

• 1 year starting up 1 time per second
• M16C 4.7 mAh
• H8 0.2 mAh
• TI 0.03 mAh
• 5 years starting up 1 time per second
• M16C 23.5 mAh
• H8 1.0 mAh
• TI 0.15 mAh

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Active Energy in Application Example

• Battery Capacity consumed
• Energy consumed in 1 years Hrs/year Active
  current mA Duty Cycle years
• Capacity is energy consumed/Battery Capacity
  100
• This calculation assumes the ADC is left on
  during the active state

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Application Example Active Energy
Assume ADC duty cycle is low (AD turned off
after conversions)
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An Analysis of RTC Mode Applications with
Peripherals
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Communicating RF Smoke Detector

• Requirements
• gt 1 year using AA batteries
• Transmission time is 2.3 milliseconds
• Must receive an alarm signal from any other
  detector within 10 seconds
• Solution
• M16C/28 waking up once every 4.9 seconds for 15
  mSec (0.3 duty cycle)
• This allows at least one missed transmission
  while still alarming within 10 Sec
• At least 8 MHz operation was required to meet
  communication timing requirement from MCU to
  Radio

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Application 2 RF Smoke DetectorPower Consumption
Power consumption for one year low power time
low power current receive time receive
current M16C (.997 .042 mA .003 43 mA)
8760 hours 1497 mAh TI MSP430 (.997 .0416 mA
.003 40.4 mA) 8760 hours 1425 mAh (TI
Numbers due not include additional MCU run
currents due to SIO channel being on)
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SLP Current Consumption Calculator
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Current Calculator
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Current Calculator
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