AVR Freaks

Those 32kHz crystals are ... very low power.

I had a casio wristwatch once. I lost it after 7 or 8 years :( but it still ran on the original battery (small coin cell).

 

Good low power design however covers the whole circut. A leaky elco or the wrong lineair voltage regulator or a single misplaced resitor can ruin your battery life.

 

Also experiment with different oscillator speeds. It is often better to run the AVR on a relative high clock frequency ( 8MHz RC oscillator) with the goal to put it to sleep faster.

Jeelabs has done a bunch of blogs about reducing power consumption.

On what kind of battery do you want to run your clock?

 

RC oscillator is "not advised" for uart or other timing sensitive stuff.

You can overcome that by callibrating the RC oscillator regularly from the 32kHz crystal.

(watch out for wearing out the callibration bytes Edit: Brainfart. See #17. )

Running the main AVR clock from a high frequency oscillator requires long startup times which is bad for battery life.

(Are there AVR's witth a PLL which runs from the 32kHz oscillator ? ).

 

You can also deduce Crystal oscillator power consumption from the diffence in power supply current between the different sleep modes.

For lower power a common solution is to run on the lower power int RC but use a 32.768k on T2 clockedasync and used to calibrate. This has the further advantage of not wasting the SUT every time the device starts and it waits for resonance.

Best I've found is Microchip DSC6000 at 1.3mA typical (24MHz, 1.8V, 4V abs max) (Note: CMOS power is proportional to V squared and frequency)

Some Linear Technology silicon oscillators have reasonable current consumption :

• LTC6900, 2mA typ at 5V and 10MHz
• LTC1799, 3.3mA typ at 5V and 20MHz
• LTC6905, approx 5mA typ at 5V and 20MHz
• LTC6930, approx 0.25mA typ at 5V and 8MHz

 

http://www.microchip.com/wwwproducts/en/DSC6000

http://www.linear.com/products/silicon_oscillators

 

 

SiTime have some "ultra-low power" oscillators:

 

The SiT1533 is an ultra-small and ultra-low power 32.768 kHz oscillator optimized for mobile and other battery-powered applications. The SiT1533 is pin-compatible and footprint-compatible to existing 2012 XTALs when using the SiTime solder-pad layout (SPL). And unlike standard oscillators, the SiT1533 features NanoDrive™, a factory-programmable output that reduces the voltage swing to minimize power. The 1.2 V to 3.63 V operating supply voltage range makes it an ideal solution for mobile applications that incorporate a low-voltage, battery-backup source, such as a coin cell or supercap.

 

 

https://www.sitime.com/products/...

 

We need to clarify some things here:

 

1. Crystals, all crystals, of any frequency, "consume" power. The consumed power is smaller as the crystal Q increases. Thus, ceramic resonators, with relatively low Q (in the low thousands, probably) take more power than a crystal (with Q in the 10-thousands to 100-thousands or more).

 

2. When the power consumption of a crystal OSCILLATOR is quoted in an AVR spec sheet, the OSCILLATOR power includes the power consumed by the crystal. That said, the power consumed by the crystal is a very small fraction of the total oscillator power, for either a crystal or a ceramic resonator.

 

Power consumption by an EXTERNAL OSCILLATOR is a totally different issue and is a red-herring if you are talking about microcontroller built-in oscillators.

 

Jim

If you want to go low power it's probably best to use a internal RC oscillator for your AVR and keep it almost always in the best sleep mode you can afford.

You have to use an external RTC chip for the timekeeping then.

You can use the alarm function of that chip to wakup the AVR if you want.

 

Real low power design is a challenge though...