Micro selection, part 4

This week I will discuss power savings modes in micros. You will need to thoroughly research the modes you want to use AND what the on-chip peripherals are capable of doing in these modes. On a firmware project I recently worked on, there was a single sentence with HUGE implications buried on page 358 of a 550+ page datasheet that said “During Sleep mode, all clocks to the EUSART are suspended.” Because of this, the micro was unable to be put in a sleep mode while communicating with a wireless radio module. More significantly at the product feature level, a data transmission to the device from another device in the system would not wake-up the micro, the micro would have to wake-up periodically to poll other devices to check if they had a message for it.

Low Power Modes

Modern micros provide a wide variety of power saving modes, from a simple run/idle/sleep selection to extreme control over individual circuits and on-chip peripherals. This can get very confusing because you can run into terms like “idle”, “nap”, “snooze”, “sleep”, “hibernate” and “deep sleep”. There are no standards regarding low-power states and the power levels and functionality available for a given mode name may vary widely from micro to micro.

There are several things you need to pay close attention to regarding power saving modes:

  • Make sure in the low power modes you want to implement that the on-chip peripherals you need are functional. For example, in some micros the UARTs are fully operational while the micro is sleeping, others like I mentioned above require the micro to be running for the UART to be operational. As discussed in a previous post, pay special attention to the clock chains in the micro you select. If the peripheral clock is the same as or is simply divided down from the CPU clock the micro will probably have to be running while any peripherals are functioning. You may still be able to put the micro in an “idle” mode which means it won’t be executing code but it’s internal clocks are still running and consuming power.
  • One popular family of small micros powers down its main registers and internal SRAM to achieve sub 1uA deep-sleep mode currents. On this micro, only the instruction pointer and two special registers for program state information are maintained while in deep-sleep. Other than these two registers, waking up from deep sleep isn’t much different than coming out of reset. One very important difference is any variables that the compiler caused to be automatically initialized on reset or are initialized in your reset code won’t be initialized when coming out of the deep sleep mode. Your wake-up code must handle initializing these variables and any on-chip peripherals, delaying the firmware’s response to the event that triggered the wake-up.
  • Pay close attention to what conditions bring a micro out of its low power states. In light and medium sleep modes, any internal or external interrupt will generally wake the micro. The lowest current modes usually have the fewest options for waking the micro. Some micros allow GPIO pins to be configured to wake the micro on a per-pin or per-port basis while some micros will only wake on transitions on external interrupt pins or a specific “wake” pin. For the micro mentioned earlier, the only conditions that will wake the part from deep-sleep are an alarm interrupt from its Real Time Clock or a specific external interrupt pin.
  • Micros with an on-chip voltage regulator for generating the micro core voltage will often have the core voltage on the device pins for filtering. If you select a micro that turns off or reduces its core voltage in a low-power mode, be careful with the amount of capacitance on these core voltage pins. These pins typically only require a filter capacitor so the smallest capacitor possible should be used since it will be charged and discharged every time the low-power mode is entered or exited.

This wraps up the discussion on micro selection. As you can tell, selecting the micro for your application will either enable a good low power design or set you up for failure in your power saving efforts. Unfortunately there is no “right” micro for all low power applications. For a low power design, the micro selection must be a collaborative effort between the hardware and firmware engineers working on the project. Without knowing the specifics of what the firmware needs to do, it is impossible for a hardware engineer to select the right micro for the job.


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