Yes, I agree with the statement that a microcontroller requires more power. But I prepared some numbers and the results are interesting. TCxxx requires only up to 70uA @ 3.3V 1 MHz. Very good. Now, we can elect some ultra-low power microcontroller of the ARM Corte-M family. For example, a bit old ST's STM32L100C6 that has 32KB of flash ROM and 4 KB of RAM, 2 I2C, DAC, ADC, USB, SPI, UART, RTC, LCD DRIVER, TOUCH SENSING, ... http://www.st.com/web/en/catalog/mmc/FM141/SC1544 -Run mode @0.5MHz, 150 uA -Assuming a well designed firmware, you can sleep the micro and awake for scanning the keyboard. I estimated around a rough 500 instructions for scanning completely the kbd. -This is around 2 ms of work, including awake and return to sleep. -Now, assume that you scan the kbd at 50 Hz rate. That is, every 20 ms. -The remaining 18 ms (20-2), you can maintain the microcontroller in low-power mode. -Low-power mode: 9 uA -In average the requirements are: (2*150) + (18*9) = 23 uA These numbers are only for the CPU. You need to add the current drain of the powered peripherals: I2C, GPIO, ... so I estimated around 200 uA of draining in the worst case. Excellent for the kind of power supply of a smartphone. But you can do better thing. Put the micro in Stop mode and awake it when the keyboard is opened: -Stop mode: 0.6 uA far better than the TCA.
In any case, I think the ST's micro is not adequate here, it'a a hell soldering it by hand.