MC9S08LC60: 32.768Khz External Xtal+1sec RTI+KBI at 5 Inputs+STOP3. Current consumption?

[BasePointer]

Hi,

I'm new at freescale and thinking of developing a project with MC9S08LC60 that is not released yet. I have some doubts before starting. My project should consume very low power at the battery.

MCU will run at ~2.7V VDD.

It manages all RTC functionality with a deviation 0.5 sec/day.

It works with a low power external xtal 32.768Khz.

It has 5 inputs to wake-up in sleep.

It enters STOP3 mode and wakes every second.

It should start-up as fast as possible and than sleep again after some code executions.

No LVD, No LCD, No ADC while in stop3 mode.

There is no external hardware consumes current.

Under these conditions, Can you estimate the approximate current consumption of my application?

Thank you.

[paul]

Hi again,

The controller was set to its default oscillator mode. The 3 uA was pretty close to average . On the meter, there was a slight jump in current when the micro would wake-up.

In my design, when the micro awoke, it would increment an internal time keeping clock, and test some port values, then return to stop3. The "on" time was minimal.

I also forgot to mention that this was the total consumption for the micro, RTC and one hex inverter. I thought that was pretty good!

Paul

[paul]

I built a project very similar to what you described, using the 9S08QG8 . I used a RTC IC/32Khz Xtal, and had the RTC output trigger an interupt every second from the RTC square wave o/p (all 3.0V). The MCU would do some work then goto stop3 mode until the next interuption(every second). At first, I could not get the consumption any lower than approx 85 uA........ This was not acceptable for my application. After further investigation, I discovered that the majority of the consumption was from the internal pullup  where the RTC square wave signal connects to the MCU. With the 1 sec square wave, the signal was low 50% of the time, using excessive current.

My solution was to disable the internal pull, use an external 470K pullup and using a a clamping diode/series capacitor to generate a short pulse to trigger the interupt........ Consumption dropped to approx 3uA. If you need to know how I setup the pulse network, I dont mind sharing it.

BTW.... make sure you tie unused i/p hi or low.... make sure the o/p's are not sourcing or sinking any loads. There was also stuff with the SOPT register .... turn off the BDM mode you will not be able to test the low power consumption until BDM is disabled and the circuit is running solo. Also there were some settings with the LVR that had to be turned off. Also make sure the internal clock is disabled. It took me many-many hours to finally get it down to 3 uA. I had overlooked many of the above requirements, so I hope to help someone avoid these problems.

Hope this answers your question..... Happy New Year.

[BasePointer]

Hi paul,

3 uA is a good point. What about your start-up time when the MCU wakes up in stop3 mode? Did you use PLL when the MCU is working?  After a waking up, the MCU must enter sopt3 mode again in a few hundred microseconds for my application. This is also important parameter for average current consumption. Is 3uA your instant or average current consumption?

Regards.