Freescale Energy-Efficient Applications Blog

Battery Life Calculator for Freescale Microcontrolers

scottpape — Tue, 23 Oct 2007 13:21:23 +0000

We get many questions from customers asking what the IDD of a particular MCU is at a specific voltage/frequency/temperature combination. Obviously we cannot take data on all of MCUs across all possible combinations of operating conditions. So to help customers answer questions about power consumption under various conditions we created a battery life calculator.

How does it work?

This battery life calculator is a small executable program that allows the user to enter the operating conditions (voltage/frequency/temperature) along with battery capacity, the percentage of time the application spends in each of the MCUs modes (run, wait, stop3, stop2 and stop1), and a periodic wakeup interval. The calculator takes this information and determines the average current the MCU is consuming and an estimation of the resulting battery life.

MC9S08GB/GT Battery-life Calculator. Click to enlarge.

How does it calculate battery life?

The calculator was created by measuring currents on a few typical samples under many different conditions. The data results were analyzed and equations created for each mode of the MCU (i.e. run, wait, stop3, etc.) that fit the data as much as possible. The results of each equation are averaged together based on the percentages and wakeup interval the user provides. This resulting average current is then used to calculate the battery life based on the provided values of the battery capacity and an estimate of the battery self-discharge current for the type of battery. Capacities and self-discharge currents are provided for some common alkaline and lithium batteries.

Which devices are covered?

Currently, we only have a calculator for the MC9S08GB/GT 8-bit MCU family, but we are working on versions for the new QE128 Flexis Duo. The MC9S08GB/GT can be downloaded free of charge from:

http://www.freescale.com/files/microcontrollers/software/app_software/HCS08BATTCALC.zip?fpsp=1

-Scott

What Controls the Power of a Platform?

cchun — Tue, 16 Oct 2007 09:00:45 +0000

The Confusing Platform Definition

The term “platform” means different things to different people. At one level a platform is a processor core with its associated memories. At a higher level it may be a processor core with a few peripheral modules. At an even higher level it can be a collection of components connected together with software to create an end product, which is the definition I’m using for this discussion.

Voltage Domains

There are typically multiple voltage domains in a platform, each catering to the differing requirements of the platform component, such as basebands, application processors, memories and displays. For high current consuming components, a switching regulator is desired for efficient power conversion. So do the regulators control the power of a platform? No … the regulators only source the power.

Hardware Components

The hardware components in a platform are typically designed using low power techniques, such as clock gating, reduced power modes and state retention power gating. A great deal of thought is put into the component architectures to ensure that the low-power techniques can be used and the transitions between low-power modes are smooth. So do the hardware components control the power of a platform? No … the hardware components only provide means to effectively use the power.

So what controls the power of a platform?

The answer revolves around the word “control.” The software controls the behavior and operation of the platform, and ultimately, the platform’s power. The software can either utilize the platform’s low power techniques, or it can ignore those techniques and operate at the highest voltage and frequency of the hardware. In both cases, the use case will be performed, but one will be more power efficient than the other.

When putting together a power-sensitive product, software control for power minimization is a critical optimization step that is often skipped due to time-to-market constraints. Ideally, the hardware and software should be developed interactively to create a platform with the lowest power consumption. However, in most cases, this is not possible since off-the-shelf components are used. So, it is important to select components that have the right low-power hardware capabilities and software that is written to take advantage of all the low-power hardware in the components.

– Chris

Flash in battery-powered applications

scottpape — Tue, 09 Oct 2007 14:48:30 +0000

One advantage Freescale microcontrollers have over many of our competitors is that our flash memories can be programmed and erased at low voltages. On most low voltage 8-bit S08 and 32-bit ColdFire® MCUs (the ones specified to run from 1.8V to 3.6V), the flash memories can be written and erased across the entire operating voltage range.

This provides several advantages for battery-powered applications:
· No worries about battery level for user-programmable applications,
· No worries about battery level for firmware updates
· Ability to save the current RAM/register states when the battery gets close to its end of life.

User-programmable applications
It is fairly easy to see the advantage of programmability across battery-life in a user programmable application. A programmable remote control is a good example of such an application. With no limitations on battery voltage, users can add or change functions on their remotes whenever needed.

If the MCU places limitations on programming voltage, then the user has to change to fresh batteries when adding the new DVD player to the master remote. Alternatively, the remote has to maintain the settings in RAM, meaning the user has a limited amount of time to replace batteries when they are drained.

Firmware updates
Let’s take the example of a monitored home security system that includes battery-powered wireless sensors throughout the house. Suppose the manufacturer of the system has a software upgrade to improve the security of the wireless transmissions. If the MCUs used in the battery-powered sensors cannot re-write the flash across the entire battery voltage range, then the company that installed the system either has to contact the homeowners to instruct them to install fresh batteries in every sensor or send a technician out each home to perform the update.

Using MCU’s that can rewrite flash across the entire battery voltage range, the upgrade can be made without concern for the current voltage output of the batteries. The manufacturer can therefore provide a remote way to upgrade the firmware through the monitoring system without having to contact the homeowner, making the process much more efficient and reliable.

Saving the current state
The last advantage I want to discuss is saving the current state of the system. Some smoke detectors and carbon monoxide detectors will keep a running count of their sensor readings in RAM to keep track of the highest readings for the last hour, day, week, etc.

With a voltage-limited flash, these values can only be saved into flash when the detectors have relatively fresh batteries or when AC line powered. If the power goes out or the batteries aren’t replaced before going dead, the RAM contents will be lost.

With a flash that programs across operating voltage, the RAM contents can be saved as the batteries get near end-of-life or if the power goes out and backup power is available from a capacitive storage.

-Scott

Effective Low Power Modes

cchun — Tue, 02 Oct 2007 09:00:16 +0000

What is a low-power mode?

A power mode is basically an operating point of a component. For instance, one operating point may have the core running along with a few peripherals running – this operating point may be called “ACTIVE.” Another operating point may have the core clock gated but a few peripherals running – this may be called “IDLE.” Similarly, a third operating point may have the core and peripherals all clock gated – this may be called “DISABLED.” Other operating points may be defined and are typically named RUN, WAIT, STOP, DOZE, DEEP SLEEP, etc.

The definition of a “low”-power mode is a power mode that is less than ACTIVE or RUN. These low-power modes are used to match the performance of the hardware with the use-case requirements.

What makes a low power mode effective?

There are two features of a low power mode that make it effective:

Capability in the mode
Entry/exit latency of the mode

The capability of a low-power mode is determined by the modules that may be active and the frequency at which they operate in that mode. If a specific use case can be performed at a minimum frequency with a small subset of the modules active, the low-power mode may be effective.

The second quality measurement of a low power mode is the entry/exit latencies. The transition times into and out of the low-power mode will dictate when they are useful. In some cases, very short latencies are required, in which only “shallow” low-power modes are useful, such as WAIT or DOZE modes. In other cases, longer entry/exit times are acceptable, in which the “deeper” low power modes are useful, such as DEEP SLEEP.

The most effective low-power modes keep only required modules clocked and have the appropriate entry/exit latency times to meet the use case. The use-case software should gate off the clocks of the unused modules and use the lowest power modes that it can, given the entry/exit latency constraints of the use case.

– Chris

ColdFire vs. S08 in Low Power Applications

scottpape — Tue, 25 Sep 2007 20:14:00 +0000

Hopefully by now you’ve heard of Freescale’s first family Flexis(TM) microcontrollers. These are families of 8-bit S08 MCU’s paired with families of pin and peripheral compatible 32-bit ColdFire(TM) V1 MCUs. The first Flexis MCUs, the QE family, are also some of the lowest power MCUs Freescale has produced.

Since the Flexis QE family is designed for minimum power consumption, the question arises, “Which one is better for a low power design?” Let’s take a look…

Specs
If we look at just the raw supply current specs for the S08 QE vs the V1 QE, we see that the S08 typically consumes half as much power in run and low power run modes, but is only slightly lower in the stop3 mode and is equal to the V1 in the lowest power stop2 mode.

Figure 1: Supply current comparison of the V1 QE128 and the S08 QE128

CPU Workloads
So based solely on these specs, you would probably favor the lower power S08 QE. However, we need to look at how much work the CPUs must perform while in run mode. The 32-bit V1 CPU has far more horsepower than the 8-bit S08. For one thing, the V1 executes instructions at the CPU frequency and the S08 executes instructions at the bus frequency, which is 1/2 of the CPU frequency. In addition, the V1 has far more CPU registers and, of course, utilizes 32-bit instructions/operands for more efficient data processing.

In a previous post (Clock Management for Low Power Application, July 13th), I discussed the concept of running fast to spend as much time in the lowest power mode possible. With this philosophy, you can easily see how a more powerful CPU will complete any calculations faster and therefore be able to spend more time in the lower power stop modes, thereby lowering the overall MCU average power consumption.

Same Peripherals
So, the V1 QE should be lower power on average than the S08, right? Well, not necessarily. Both the S08 QE and V1 QE use the exact same peripherals. Therefore, the V1 doesn’t get a performance advantage when a peripheral’s operation is the only thing preventing a return to a low power mode. For instance, if an IIC transmission is the only task left before entering stop2 mode, the IIC doesn’t run any faster on V1 just because the CPU is more powerful. Both MCUs will have to wait the same amount of time for the transmission to complete.

Also, these peripherals were designed for 8-bit CPUs. So the S08 is very efficient when it comes to setting up and using the on-chip peripherals. The V1 does have 8-bit instructions to improve its efficiency with these peripherals, but this does limit its performance advantage over the S08. So when much of the CPU’s time is spent accessing peripherals versus pure data manipulation, the V1’s energy use per task is not much different than the S08’s.

Which to Choose?
As you can see, there is not a simple answer to choosing the right CPU core for obtaining the lowest power. In general, if the application requires a fair amount of number crunching, particularly 16-bit and 32-bit data, then the V1 will probably be the best choice. Conversely, if the application requires the CPU to spend more time configuring and waiting on peripherals than number crunching, then the S08 will likely be the best choice.

Of course, if you think you made the wrong choice, the compatibility of the Flexis MCUs makes switching to the other core a piece of cake!

-Scott

Low Power Component Architecture

cchun — Tue, 18 Sep 2007 09:00:59 +0000

Keep It Simple …

In the progression of things, circuits are used to build modules and modules are used to build components. Component architecture is how the modules are connected to perform a specific function. The best way to save power on a component is to build onto the component only what is needed to do the required functions. All extraneous options add area and complexity, which in turn usually increases the component’s power consumption.

Once the minimum set of modules is determined, then it’s important to know how the modules are connected together to allow the data to flow through the system. Basically, getting the most work done per clock cycle is more important than raw MHz. By using direct memory access (DMA) modules and cross-bar switches, moving data on a component can be made more efficient. DMA’s can save significant power by allowing data on the component to move from one location to another without the assistance of the large general purpose core(s). Cross-bar switches enable multiple data paths to simultaneously send data from one module to another, thus generating more work per clock cycle.

Turn It Off …

For leakage control, some form of power gating is normally used on a per-module basis. Power gating techniques include:

State retention power gating
Save and restore power gating
Basic power gating

Along with architecting the module connectivity for low power, components can also be designed to operate in different power modes. Power modes can have names such as RUN, RUN-HIGH, WAIT, DOZE, HIBERNATE, STOP, SLEEP, DEEP SLEEP, OFF, etc. Each component will have its own set of power modes. They are defined for a component with the intention that software can match the application’s requirements to the capabilities of the hardware.

When selecting a component for a system, you need to know if it has all the necessary modules to perform the required functions without a lot of extra “bells and whistles” that your system does not need. You should also understand how the architecture allows data to be moved around within the chip. And finally, you should find out about the power modes and whether or not they will be useful for your application(s).

– Chris

Brownout protection in low power designs

scottpape — Tue, 11 Sep 2007 14:00:54 +0000

A frequent question that comes up about low power designs using our microcontrollers has to do with brownout detection. I’d like to use this week’s posting to address the MCU features that help protect these devices from brownouts.

What happens in a brownout?

There are two main problems a brownout can cause. First, if the VDD reaches a point where the logic is still operating properly but the program memory (usually flash) does not read correctly, a runaway code condition can occur, and it continues until the VDD rises back to a safe operating range and a reset occurs. In this case, the danger is only when the CPU is actively executing code. If the MCU is in a low-power mode where the memory is not being accessed, the runaway code condition can’t occur.

Second, when the VDD falls below the digital logic’s safe operating level, brownouts can corrupt the RAM and registers content, which, if undetected, can result in unexpected operation of the application. In this case, even if the device is in a low-power mode, the corrupted data can still cause problems when the MCU wakes from the low-power state, even after the VDD has returned to a safe operating level.

Features for protecting against brownouts

Freescale MCUs have two primary features to protect against brownouts: a low voltage detector (LVD) and a power-on reset (POR). The MCUs also have system protection features (watchdog, illegal address and illegal opcode resets) that help recover from improper operation should a brownout occur.

Low voltage detection
The LVD can protect against both brownout cases listed above. LVD module features vary somewhat between devices, but in all cases the LVD can be configured to generate a reset when the VDD falls below the LVD trip point. The trip point is chosen to cover the MCU’s appropriate safe operating point.

Now one of the drawbacks of the LVD is that it consumes extra current when operating. On the 8-bit S08 and 32-bit ColdFire® V1 MCUs it requires the internal voltage regulator to be running, which consumes roughly 70 uA of current. Therefore, you typically want to disable the LVD when in the low power stop modes. These LVDs have the option to automatically disable detection when entering stop mode and re-enable detection upon exit from stop mode.

So this provides protection when the MCU is in run mode, but how do we protect against brownout in stop modes?

Power-on reset

Many of the 8-bit HC08 and S08 MCUs and all of the ColdFire V1 MCUs have an active POR versus a passive POR used on older devices. This active POR sets the POR trip point at the level where the digital logic is operational. Therefore, if the VDD falls below the point where RAM and registers can be corrupted, a POR occurs and holds the device in reset until the VDD recovers and the device can be re-initialized.

Because the POR doesn’t release until the chip logic is functional, the LVD circuit will be operational once the POR releases reset. Since most MCUs will enable the LVD when a POR occurs, the MCU is further protected until VDD reaches a safe operating point for the program memory.

Unlike the LVD, the active POR consumes very little power, only about 100-200 nA. Also, unlike the LVD, the active POR is not optional. It is always on, even in the low power stop modes, so there is no lapse in coverage.

So with the always-on protection of the POR and use of the optional LVD reset, you can have excellent protection against brownout conditions, even in low power modes, without significant impact to your battery life.

-Scott

Low Power Circuit Techniques

cchun — Tue, 04 Sep 2007 09:00:15 +0000

Work Takes Energy

It takes energy to perform work, whether it is physical work, such as digging a ditch, or electrical work like playing an MP3 file. The tools may be different for physical work as compared to electrical work, but the idea is that the tools will allow the work to be done more efficiently using the least amount of energy. For instance, you could dig a ditch using a teaspoon, but it would take a lot more energy than if you used a shovel. The tools used to do electrical work are circuits. Some circuits are designed to be more efficient than others, even though they may perform the same function. We are interested in designing circuits that are energy efficient when performing the necessary work.

Working Efficiently

There are multiple ways to design efficient circuits, but it boils down to controlling active currents, static currents and leakage currents. Active current is frequency dependent, leakage current is temperature dependent and static current tends not to be either frequency or temperature dependent. There are several techniques to address each one of these types of currents, including, but not limited to:Active currents:

Reduced voltage
Dynamic voltage and frequency scaling
Accelerators
Asynchronous circuits

Static currents:

Reduced voltage
Better bias current circuits
Power gating

Leakage:

Reduced voltage
State retention power gating
Save and restore power gating
Basic power gating

When selecting a component for your system, identifying and understanding the power control techniques implemented on the component will help you make the right selection for your end product. Each of these techniques will be described in future posts to help you understand what they are and how they may (or may not) impact your end system. Please let me know if there are other topics that interest you.

– Chris

System Hardware Strategies to Extend Battery Life

bongosai — Tue, 28 Aug 2007 20:09:16 +0000

Last time, I talked about software strategies for reducing power. This time, I’d like to talk about hardware strategies. From a hardware standpoint, controlling power to peripherals, both internal to the MCU and external, can save considerable power. It is straightforward enough to disable on-chip peripherals using the MCU’s control registers. But it may not be as obvious to disable peripherals external to the MCU. Using general I/O pins, power can be controlled to external circuits.

Power discreet and IC components

Now discreet components such as LEDs and photosensors may be obvious candidates for controlling with I/O, but just about any circuit can be controlled in a similar fashion. If the component requires more current than the MCU can provide directly, buffers can be used as power switches to these circuits. In some cases, simply connecting several I/O pins in parallel can provide enough current.

Use hardware modules for monitoring input

Another hardware/software trick is to use modules with interrupts to monitor external signals versus polling the input. If you just need to observe a rising or falling edge, use an interrupt pin. If you need to know a relative time the edge occurred, use a timer input capture. Both timers and pin interrupts can operate in several low power modes.

For analog signals, a comparator combined with an ADC channel can create a very efficient monitor. The Freescale MCU’s comparator uses a fraction of the current of the ADC module. Configure the comparator to run in a low power mode and trip at a threshold voltage. When the MCU wakes, the ADC can measure the same signal for a more accurate reading. Several Freescale MCUs even have comparators and ADC channels on the same pins to make this simple.

Configure I/O pins for lowest power

The MCU’s I/O pins themselves can be a source of excess current if not configured properly. Unused pins need to be terminated to prevent a floating input from creating a high current path. This is frequently overlooked when using an MCU that comes in a variety of packages.

We tend to forget that the pins available in the highest pin count version are still on the silicon in the lower pin count versions. Any input pin left floating will become a frustrating source of excessive current, many times only under certain circumstances, such as temperature variations. In these cases, an internal pull-up should be enabled, or, if the pin is an I/O, it can be configured as an output (data is irrelevant if it is driving an open circuit).

Packaging and Power

cchun — Tue, 21 Aug 2007 09:00:16 +0000

Parasitics Everywhere

When you mention a product’s “power consumption,” packaging is not normally the first thing that comes to mind. Usually you’ll conjure up images of battery life or the heat dissipation or the extreme capability of the product. However, packaging plays a key role in a product’s power consumption rate. For this discussion, we will talk about the packaging of multiple components to create a product, such as a mobile phone.

Traditional mobile phones have three main components: a power management IC (PMIC), a baseband modem (BB) and one or more RF transceivers. Information is transferred between these components by signals that flow from one component to another on a printed circuit board (PCB). The PCB has parasitic, or unwanted, capacitances and impedances that the output circuits must drive. If the output circuits are not capable of putting out enough power, the signals between components can be corrupted, which can cause a system failure, such as a dropped call.

The larger the distance between two components, the more parasitic capacitance and impedance the output circuits must drive. So, to reduce the output circuits’ power consumption, the components should be placed as close to one another as possible. This has the additional benefit of making the product smaller. If taken to the extreme, the components can be placed so close that the packages touch one another. The next step is to merge the packages and put both die into the same package. Taking one step further, the two die within a package can be moved closer to each other until they touch, with the final step being both die integrated together onto a single die. Some companies are doing exactly this in the mobile phone arena with their “single chip radios.”

Single Chip vs. Single Package

The one major drawback of single chip radios is that the integration of the various components introduces compromises in the circuit designs, which can lead to less efficient designs that may consume more power. Integrating all components into a single chip also has the unwanted side-affect of making the product inflexible to change.

Freescale is pioneering a new packaging concept called redistributed chip-scale packaging, or RCP. RCP moves the die as close together as practical while still allowing each component to be fabricated in the process that is best optimized for its functionality. For instance, high power circuits like those found on PMICs can be fabricated in a SmartMOS(tm) process that is tailored to source high power. Other examples are fully digital baseband modems and application processors that can take advantage of the smallest geometry processes to make them as small as possible.

RCP not only allows for each function to be fabricated in the most optimum process, but also it allows the flexibility to mix and match components to meet the quickly changing demands of the consumer marketplace. With product lifecycles dropping to less than a year in some instances, this flexibility is very important for new product developers to stay competitive.

Selecting components in the right packages is important to your end product it affects not only the size of your product but also the power consumed.

– Chris