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I2C devices for controlling LEDs

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article image I2C general purpose I/O expanders.

PHILIPS Electronics has introduced two new series of I2C general purpose I/O expanders, the PCA953x and PCA955x.

The devices have been optimised for controlling LED dimming and blinking in products ranging from mobile phones to servers in computing, communication and networking applications.

The two, four, eight and 16 bit devices represent an innovation in I2C capabilities, providing designers an easier way to build systems with more dimming or blinking LEDs than previously possible using just basic general purpose I/Os (GPIO) or microcontrollers (MCUs).

The PCA953x LED dimmers have a frequency range of 160Hz to once every 1.6sec, with a duty cycle range of completely off to 99.4% on, delivering both dimming and blinking of LEDs.

The PCA955x LED blinkers have a frequency range of 40Hz to once every 6.4sec with a duty cycle range of 0.6% off to completely on, so designers can set, for example, a 4sec blink rate with the LED on for 1sec and then off for 3sec.

Asian manufacturers of applications such as cell phones and servers are increasingly requiring multiple blinking LEDs for eye-catching keypad lighting applications, as well as practical purposes such as status indication.

The new PCA953x LED dimmers and PCA955x LED blinkers provide more system flexibility by off-loading the LED power consumption and eliminating the programming of the MCU.

"The Philips' PCA955x family is exactly what we've been looking for to control the maintenance and control bus LEDs in our next generation server platform," said Wally Tuten, Senior Engineer, X-series server development of IBM.

"With these new parts from Philips, customers and field repair technicians can trouble-shoot our systems without having to be in the normal operating mode."

For example in IBM's current product offerings, the memory card can be removed from the system where the DIMM LEDs are obstructed from view from the top of the system.

Using a "super-cap" as a battery to maintain power continuity, the last settings in the PCA955x enable specific control of the individual LED's blink rates as well as on/off status, making it much easier to trace and locate specific faults.

With this capability, secondary or tertiary LEDs can be placed on the different cards and components to assist in rapid trouble shooting, even after disassembly or with the system ac power off.

"This approach delivers a simpler, but more robust system design and shortens development time, so IBM can get its products to market faster and provide better quality debug aids for the customer," Tuten said.

The I2C devices have an internal oscillator with four 8-bit (256 values) internal registers. The internal registers set two programmable blink rates, removing the need for the MCU to use one of its timers to send commands to individual LEDs to repeatedly turn on and off, or “blink”.

The internal oscillator also reduces I2C bus traffic compared with using a standard GPIO to blink or dim the LED, freeing up the I2C bus for additional traffic and control of additional devices.

Once programmed, the internal oscillator enables the I2C bus to be disconnected from the PCA953x or PCA955x with the LED continuing to be dimmed or blink. This is not possible with normal GPIOs.

"The I2C /SMBus has become the de facto standard serial bus for maintenance, control and configuration in most electronic platforms ranging from computing applications to networking and communications," said Pierre-Yves Lesaicherre, general manager, interface products business line, Philips Semiconductors.

"With these new families of PCA955x and PCA953x I2C LED blinkers and dimmers, we are introducing significant innovation in a mature technology, enabling electronics manufacturers to more simply blink and/or dim LEDs in their systems, while freeing up the microcontroller and the I2C bus for more efficient operation of the system."

Output drive strength is 25mA per bit at 5V operation and 100mA for 8-bit groups, which is much more than the amount of current typically allowed by a MCU.

The devices operate between 2.3V and 5.5V and up to a 400kHz I2C bus speed. Any I/O pins not used as outputs can be configured and used as normal general-purpose inputs.

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