Re: [Contiki-developers] Tmote sky low-power modes

[Joakim E. <jo...@si...>]

Hello Jonathan,

The reason that we did not use the SCG1 flag before was that it cause 
the processor into a mode that turns all clocks off making UART 
communication over the USB impossible. The latest version of Contiki has 
a fix for that on the Sky platform and will go down in LPM3 when it is 
"sleeping".

The reason for the high energy consumption is probably:
1 - the ADC systems is configured to sample several sensors frequently 
(I have not measured what effect this actually has, but it will consume 
some power for sure).

and

2 - other external components such as the flash memory are on, the 
analog sensors (light), possibly the "digital" sensors (SHT11).


The small blips 64 times a second is likely when the timer system 
updates the clock (if you have CLOCK_CONF_SECOND set to 64 in 
contiki-conf.h). This is going from LPM1 (or 3) and into active mode 
during the interrupt.


I have never tried LPM4 wakeup but the button-sensor should wake the 
application up from LPM4, and possibly sending in data on the USB might 
also work with the latest fix.

Best regards,
-- Joakim Eriksson, SICS

Jonathan Lewis wrote:
> Hi guys,
> 
> To set the scene, I am part of a team implementing a high-level
> component-based language called Insense for use on resource constrained
> devices such as the Tmote Sky nodes. 
> 
> 	http://www-systems.cs.st-andrews.ac.uk/wiki/Dias/InsenseLanguage
> 
> Thanks to Contiki our work has been fairly straight forward in not having to
> deal with very low-level issues, well done ... ;-)
> 
> 
> Recently I've been playing about with a Scheduler component that we want to
> provide as part of our language and we would ideally like to incorporate
> some power-management features. So I started playing around with low-power
> modes under Contiki.
> 
> As far as I can tell, sending the processor to sleep when nothing is to be
> done using the 
> 
> 		_BIS_SR(GIE | SCG0 | SCG1 | CPUOFF); 
> 
> 	(not sure why Contiki leaves out the SCG1 flag,
> 	seems to work fine when it is added, but that is
> 	not my main concern here, although I am intrigued)
> 
> statement in the contiki-sky-main.c scheduler loop saves roughly 1mA when
> there is nothing to do. I get a pretty constant usage of around 2.5-2.6mA
> without the sleep statement and roughly 1.5-1.6mA with the sleep statement
> in place (excluding current spikes due to radio listen as set in xmac.c).
> Even sending the processor into LPM4 doesn't make a huge difference, maybe
> takes off another 0.05mA.
> 
> To clarify how I measure current: I am using an oscilloscope to measure the
> voltage drop over a small resistor that is in series with the battery supply
> to the device. I have experimented with 10 and 100 Ohm resistors, both show
> the same 1.5-1.6mA results.
> 
> 
> 1) My main query is why we don't seem to get anything near the low μA usage
> as outlined in the tmote datasheet (values shown below). Maybe I'm reading
> the figures in the datasheet wrong or the figures in the datasheet are
> somewhat optimistic, maybe some other parts of the tmote can be switched off
> but aren't, maybe ... ?  I was wondering if anyone had had similar
> experiences and/or has an explanation.
> 
> 2) It is also odd that I can hardly detect the wake-sleep cycles. I can just
> about see small blips (of less that 100μA) roughly every 15ms (64 times a
> second) within the fairly constant 1.5mA (1.6mA for tmote with sensors)
> trace. I am no electronics expert, but my initial thought for this one is
> that I'm running into some sort of resolution issue in that the scope isn't
> picking up the change in voltage drop over the resistor for very small times
> (when the processor wakes up for some micro-seconds and goes back to sleep
> as nothing needs doing).
> 
> 3) Has anyone experimented with waking the tmote up from LPM4? If anyone has
> any info here that would be great.
> 
> 4) Apart from setting XMAC_CONF_OFF_TIME at compile time, is there any way
> (without hacking Contiki or duplicating code) of altering the CC2420 power
> cycle at runtime?
> 
> 
> If anyone can shed any light on the above, that would be great. The values
> taken from the tmote datasheet are below.
> 
> Cheers
> 
> 	Jon
> 
> -----------------------------------------------------------------
> 
> According to the tmote datasheet we should be able to expect around the
> following for the device as a whole (I think)
> 
> 						NOM	MAX	UNIT
> Current Consumption: MCU on, Radio RX		21.8	23	mA
> Current Consumption: MCU on, Radio TX 	19.5	21	mA
> Current Consumption: MCU on, Radio off		1800	2400	μA
> Current Consumption: MCU idle, Radio off	54.5	1200	μA
> Current Consumption: MCU standby		5.1	21.0	μA
> 
> -----------------------------------------------------------------
> 
> And for the MSP430 with Vcc = 3V
> 						NOM	MAX	UNIT
> Active current at 1MHz 				500	600	μA
> Sleep current in LPM3, 32.768kHz active		2.6	3.0	μA
> 
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