New Low Power Run and Low Power Wait Modes. What’s new about them?
July 30, 2007Hi, folks,
If you are one of our microcontroller customers, hopefully by now you have heard about our newest MCU family, the Flexis(TM) QE series, which unites our 8-bit S08 MCUs with our new higher-performance 32-bit ColdFire V1 MCUs. We have introduced a few new features on this family intended to help you extend battery life. Two new features I’d like to discuss in detail are the new low power run and low power wait modes.
Issue: the need to run slow
In my first posting, I discussed using the high clock frequencies to reduce average system current by reducing the amount of time spent in higher power run mode and maximizing time spent in the lowest power stop modes. However, this approach may not always be practical. For example, you may need to run a peripheral that doesn’t run in one of the stop modes, such as a timer or UART.
All of our S08 and ColdFire MCUs have on-chip voltage regulators that are active anytime the MCU is in run or wait modes. Therefore the MCUs have a DC offset current that never goes away in these modes. As a result, the slower the operating frequency, the less efficient the MIPs per uA becomes.
Why can’t all peripherals run in stop mode?
There are a couple of reasons many peripherals cannot run in the stop modes. In these modes, the system bus clock is disabled to save power. The peripherals that can run in stop mode either do not require a clock (e.g., analog comparator) or have a built-in clock source (e.g., analog-to-digital converter). The changes to enable the system bus clock in stop modes are non-trivial.
Another reason is that the voltage regulator goes into a low power “loose” regulation mode when one of the stop modes is entered. In this loose regulation state, the regulator can only supply enough current to power a few modules. The current required to run all modules would drain the regulator causing voltage drops that could fall below the minimum operating voltage.
Solution: low power run and wait modes
To resolve these issues, our designers created a new voltage regulator that can provide enough current in its loose regulation mode to power all peripherals at a limited frequency. However, we still had the issue with the bus clock being off in stop modes.
Instead of redesigning our clock architecture to allow the bus clock to run in stop modes, we decided to create two new modes: low power run and low power wait. As you might guess from the names, these modes are simply the standard run and wait modes with the voltage regulator in loose regulation. By taking this approach, we are able to offer full functionality of our peripherals in a very low power state. In addition, the low power run mode allows code execution at a much more efficient MIPs per uA at low frequencies.
Using low power run and wait modes
Using the new low power modes requires configuring just a couple of modules: the clock source (ICS on the QE family) and the low voltage detect (LVD). The clock source must be configured to be running off of an external clock or the crystal oscillator. The reason for this is that the internal clock sources for the S08 and V1 MCUs require a current source that needs the regulator to be in full regulation mode. Therefore, the internal clocks must be disabled. Also, the CPU clock frequency must be limited to 250 kHz to ensure that the regulator is not overloaded if all modules were to be enabled.
The LVD module must also be disabled to enter the low power modes since it requires a bandgap voltage reference that is only available when the regulator is in full regulation. Now we did pull a little trick here. The LVD module can either be disabled by clearing the LVD enable bit (LVDE) or the module can be enabled as long as the LVD stop enable bit (LVDSE) is clear. In this second case, when the MCU is in standard run or wait modes, the LVD is active. When low power run or wait is entered, the LVD will automatically shut down and then restart when the low power mode is exited.
Not a one-off solution
I hope you found this discussion useful and even more importantly, I hope you find these new low power modes useful. These new modes will be standard on our low voltage S08 and ColdFire V1 MCUs moving forward, so it is beneficial for us to make sure our customers are aware of them and understand how the modes can benefit your low power applications.
-Scott
