Portable system designers are continually struggling to add more innovative features and at the same time minimise power consumption. Thus, designers must integrate system power management components, which add higher levels of intelligence and system communication to improve overall efficiency and extend system runtime.
They must find a way to add power management control for new functions without increasing board complexity. To achieve that, they must limit the use of pins on power management devices as well as extend their product's feature set without using up all the GPIO pins on their microcontroller.
There are three basic types of interfaces for power management functions, which designers can choose from: a parallel bus; a serial bus; a single-wire bus.
Parallel approach
From a hardware and software viewpoint, this is the simplest method. It is comprised of independent parallel control lines and provides a separate GPIO line to each enable line of power components.
Although highly effective, it also presents specific expandability limitations:
* It requires more pins on the microcontroller and peripheral components than any other interface option.
* It occupies significantly more PCB real estate to route individual lines - a primary concern in space-critical portable systems.
* Clamshell handsets and PDAs use a flex PCB to connect the motherboard PCB with a daughterboard. Many of the functions on the daughterboard require individual control lines and running a large number of lines across the flex PCB drives up cost and negatively impacts system reliability.
Serial approach
The I2C and SMBus have been proven to be the most popular serial interfaces in portable designs. Multiple slave devices of a single bus are supported by these serial architectures that simplify the communication between peripherals.
Tradeoffs are inevitable between these architectures, serial buses use fewer wires than the parallel buses and this space gain comes at the cost of speed:
* 500 Kbps throughput rates or above are uncommon.
* The designer is forced to look at extensive hardware solutions to solve the complex timing mechanisms used by the serial architecture.
* Minimum of one line each for data, clock and ground is required which is unacceptable in space-constrained devices.
Single-wire approach
To lower cost and to simplify the design, single-wire interfaces are used to supply control and signalling. AnalogicTech's S2Cwire single-wire bus offers relaxed timing requirements. With the microcontroller programmed to run fast or slow, a local clock or precise master is not needed for the slave device. This offers easier hardware and software implementation. The S2Cwire architecture of counting clock edges increments a counter which steps through a ROM build into the slave device to perform various programming functions.
Data read-back and addressability options that is offered on an I2C, is however, not offered on the S2Cwire. However, for example, in portable system lighting (white or RGB LEDs), designers can eliminate the PWM control signal and programmable logarithmic brightness settings can be provided instead. This in turn offers superior control over the brightness spectrum via software.
Entirely new approaches are offered by these single-wire interfaces. For instance, a single-wire interface with multiple independent load switches and a group of I/O expander load switches will allow engineers to expand the I/O capabilities with minimal cost and effort. Previously a discrete MOSFET and multiple control lines solution were used, now five functions can be controlled with a single-wire in a device 2 x 2 mm, while consuming just a few microamps of current.
Conclusion
To meet the ongoing challenge of adding new functions as well as continuing to squeeze the physical size of their systems, designers have to explore all of their interface options to find the most power efficient and cost effective implementation possible.
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