AVR powersave implementation

A recent battery powered project required to save as much power as possible. The then used CPU was of the Mega32L type. From the datasheet 2503i-AVR-04/06, page 288, the power supply current is [mA]:

Active Vcc=3V, 1MHz, Mega32L	typ 1.1	..
Active Vcc=3V, 4MHz, Mega32L	typ 3.8	max 5
Active Vcc=5V, 8MHz, Mega32	typ 12	max 15
Idle Vcc=3V, 1MHz, Mega32L	typ 0.35	..
Idle Vcc=3V, 4MHz, Mega32L	typ 1.2	max 2.5
Idle Vcc=5V, 8MHz, Mega32	typ 5.5	max 8
Power down, Vcc=3V, WDT enabled	typ < 0.01	max 0.02
Power down, Vcc=3V, WDT disabled	typ < 0.001	max 0.01

for comparison the Tiny26, (document Rev 1477G-AVR-03/05)

Active Vcc=3V, 1MHz, Tiny26L	typ 0.7	..
Active Vcc=3V, 4MHz, Tiny26L	typ 2.5	max 6
Active Vcc=5V, 8MHz, Tiny26	typ 8	max 15
Idle Vcc=3V, 1MHz, Tiny26L	typ 0.18	..
Idle Vcc=3V, 4MHz, Tiny26L	typ 0.75	max 2
Idle Vcc=5V, 8MHz, Tiny26	typ 3.5	max 7
Power down, Vcc=3V, WDT enabled	typ 7.7uA	max 15uA
Power down, Vcc=3V, WDT disabled	typ 0.3uA	max 3uA

for comparison the Mega324P, (document Rev 8011E-AVR-04/07)

Active Vcc=2V, 1MHz, Mega324P	typ 0.5	max 0.5
Active Vcc=3V, 4MHz, Mega324P	typ 2.5	max 2.7
Active Vcc=5V, 8MHz, Mega324P	typ 8.7	max 9
Idle Vcc=2V, 1MHz, Mega324P	typ 0.1	max 0.15
Idle Vcc=3V, 4MHz, Mega324P	typ 0.6	max 0.7
Idle Vcc=5V, 8MHz, Mega324P	typ 2.5	max 4
Power save 32kHz, Vcc=1.8V	typ 0.5uA
Power save 32kHz, Vcc=3V	typ 0.6uA
Power down, Vcc=3V, WDT enabled	typ 4.3uA	max 8uA
Power down, Vcc=3V, WDT disabled	typ 0.2uA	max 2uA

for comparison the Mega64, (document Rev 2490K-AVR-04/06)

Active Vcc=3V, 1MHz, Mega64L		max 5
Active Vcc=5V, 8MHz, Mega64		max 20
Idle Vcc=3V, 4MHz, Mega64L		max 2
Idle Vcc=5V, 8MHz, Mega64		max 12
Power down, Vcc=3V, WDT enabled	typ 10uA	max 20uA
Power down, Vcc=3V, WDT disabled	typ 4uA	max 10uA

The list of requirement could include :

continously operating or being switched on/off by a switch
burst mode operation with very low duty cycle
being awakened by an external event or a timer

The battery life depends on the current consumption as

battery type	lifetime @ 100uA	lifetime @ 1mA	lifetime @ 10mA
Lithium Vanadiumpentoxyd 30mAh 2% p.a.	300h or 12.5 days	N/A - exceeds spec	N/A - exceeds spec
Lithium CR2032 225mAh shelf life 5 years	2250h or 93 days or 3 months	225h or 9 days	22h or 1 day
LiPolymer 500mAh	5000h or 208 days or 7 months	500h or 21 days	50h or 2 days
LiPolymer 3Ah	30000h or 1250 days or 3.4 years	3000h or 125 days or 4 months	300h or 12.5 days

Note, that the self depletion of the battery is not included in the numbers.

The following measures are applied to save current.

The L type takes less current under identical conditions
reduce the voltage from 5V to 3V, the cpu appears approximately as resistor and thus halving the voltage halves the current. When powered through a lowpower switcher, the power consumption is then reduced to a quarter.

If possible have a low power switcher to convert excess voltage into current. Here is a small selection

type	supply current	frequency [kHz]
LT1934, 250mA stepdown	12uA burst	unknown
LTC3405A, 300mA sync stepdown	20uA sleep 300uA active	1500
LT3406, 600mA stepdown	20uA sleep 300uA active	1500
LT3440, 600mA sync buck-boost	25uA burst 600uA active	300-2000
TPS62056, 3.3V 800mA sync stepdown	12uA burst 1.5uA shutdown	850
TPS62102, 0.8 to 8V 500mA sync stepdown	220uA burst 1uA shutdown	1000
TPS62202, 1.8V 300mA sync stepdown	15uA burst 0.1uA shutdown	1000
as reference MC34063A, 1.5A stepdown	max 4.0mA	100
as reference LP2951, 150mA linear LDO	75uA quiescent @ 100uA load 8mA quiescent @ 100mA load	N/A

reduce the clock frequency. The current is roughly proportional to the clock frequency.
remove any polling loops from the application, and unless a realtime kernel was specifically written for power save operation, it won't be useable as it is a complex way of polling. Interrupts and statemachines are one way to go. See my page on a tick implementation
rearrange the code such that there is a central wait for a wakeup event.
select one or more powersave states from page 34, "active clock domains and wakeup sources". The most power can be saved when the cpu is controlled by some external hardware and thus can enter the power down state. The wake up can then be done with an external interrupt or a TWI adress match.
The external hardware could also be an RTC with alarm output.
When the power has to be saved fully under cpu control, the lowest states are then achieved with a 32kHz quartz running on it special oscillator. They are Standby and Extended Standby. When the 32kHz is not available, then some internal clock has to be used for the wakeup. This then is the idle state.
add the sleep statement to the main loop and do some debugging with an unused pin to verify the length of the powersave state during runtime.

The powerdown dream

The powerdown state is the one taking the least power. It however is rather difficult to fully exploit. The cpu taking a mere 10uA, a single 300kOhm pullup somewhere on the pcb already doubles this current. Further is the wakeup not really immediate. The oscillator takes its time to stable oscillation. Either 4ms or 65ms depending on some bits in the clock source select. For a system that has to do very little every few minutes/hours/days, an external RTC with alarm output to be connected to the external interrupt of the controller together with the power down mode would be that way to go. Some RTCs

RTC	RAM	interface	supply	case	additional
DS1302	31 bytes	3-wire	2.0 to 5.5V 300nA @2V	DIP8, SO8
DS1305	96 bytes	3-wire	2.0 to 5.5V 300nA @2V	DIP16, TSSOP20	dual interrupt on time match, programmable trickle charge battery
DS1306	96 bytes	SPI & 3-wire	2.0 to 5.5V 400nA @2V	DIP16, TSSOP20	1Hz & 32768Hz output, dual interrupt on time match, programmable trickle charge battery
DS1307	56 bytes	I^2C	2.0 to 5.5V 500nA @2V	DIP8, SO8	programmable squarewave 1Hz, 4k, 8k & 32kHz out
DS1337	7 bytes	I^2C	1.8 to 5.5V 425nA @ < 1.8V	DIP8, SO8	programmable squarewave 1Hz, 4k, 8k & 32kHz out, 2 programmable Alarms
DS12887	114 bytes	parallel muxed bus	4.5 to 5.5V battery included 10 years lifetime	DIP24	programmable squarewave 2Hz to 8kHz out, interrupt on time match

The power save mode

This is an easily doable solution when at design time, the 32kHz Quarz is planned to be on the timerpins. Timer2 running on this 32kHz can be used to wake up periodically. When the prescaler is set to 1:1024 and the 8 bit Timer2 is naturally overflowing every 256 cycles, then the wakeup is every 8 seconds.

The extended standby mode

Identical to the power save mode, here the oscillator keeps running and a wake takes just 6 clock cycles.

Further

Creating a low power project involves considerations on many parts, the controller just being one of them. Just adding low power in the final phase of the project is too late. Yes, some power consumption measurements are missing yet.

Outlook

The new picopower models running from just 1.8V allow to lower the supply while sleeping.

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last updated : 9.dec.06, or perhaps later

Copyright (99,2006) Ing.Büro R.Tschaggelar