24mar10SiLabs......
Silicon Labs has introduced a wireless microcontroller family that can communicate across 3km, powered by a coin cell.
"There are 12 Si10xx devices, with 64kbyte to 8k of flash; +20dBm to +13dBm of output power; and with and without a dc-dc boost converter," director of marketing Mike Salas told EW.
Within the 7x5mm package are two die, an 8bit 8051-based controller and an RF transceiver, designed at the firm's labs in Austin and Hungry respectively.
The CPU is pipe-lined, executing 70% of instructions in one or two system clocks cycles.
The RF chip has been selected from the in-house EZRadioPRO range and is rated for operation between 240 and 960MHz, with the receiver sensitive to -121dBm, giving a 141dB link budget when used with the 20dBm transmitter.
"The 13dBm version is a little bit cheaper and saves power," said Salas.
RF power consumption is 18.5mA receiving, 85mA transmitting at +20dBm, and data rates from 0.123bit/s to 256kbit/s are possible.
According to him, 3km open field range is possible at 4.8kbit/s with a 2% raw error rate is permitted, rising to 6km if 20% can be tolerated.
For operation in congested environments where standing waves from reflections are likely to be a problem, automatic two antenna receiver diversity is included in the package.
"The antenna diversity runs in radio, switched during the preamble, so there is no overhead in the controller," said Salas. "With two antennas available you can claw back 10dB."
Like just about every maker of wireless microcontroller, Silicon Labs has been working on power efficiency.
And like just about every chip maker worrying about power consumption at the moment, it has chosen a 0.18µm process.
"The family is the industry's most power-efficient single-chip wireless MCU solution, offering the lowest active-mode current consumption at 160µA/MHz," claimed the firm.
In sleep mode the devices consume 615nA with the 32.768kHz real-time clock (RTC) running, which drops to 315nA with the RTC running from a 2% accurate internal low frequency oscillator.
Deep sleep is 65nA with the brown-out detector turned on, and when this is turned off, "they can operate on as little as 25nA with full RAM retention," said Salas.
To this, the RF contributes 15nA, said Salas.
Minimising wasted time, the core wakes up in as little as 2µs.
Operating voltage is 1.8V to 3.6V without the switched capacitor dc-dc boost converter, and 0.9V to 3.6V for the versions with a converter.
The converter resides on the MCU chip and powers the transceiver with some left over for external loads.
"The dc-dc converter supplies power needed for periods of RF transmission and reception with efficiencies of up to 90 percent," said the firm.
Intended applications are battery-operated home automation systems like lighting and air conditioning, smart meters, in-home utility monitors and security systems.
Antenna diversity
When communicating between antennas sticking out of an open plain, there is no need for antenna diversity.
However, practical environments are never like this.
Nearby reflective objects cause multi-path effects - including complete signal nulling in certain situations.
If the receive antenna is in a null, a second receive antenna positioned a quarter of a wavelength away will almost certainly get a good signal.
Three such antennas improve the situation even further.
This is the basis of antenna diversity.
As most antennas are directional, antennas at different orientations can reception when devices are positioned arbitrarily in 3D space.
Two antennas positioned at 90 degrees to each other, and a quarter of a wavelength apart, help to get over both multi-path and orientation issues.
Add another one or two, and it will be close to impossible to find an arrangement within reasonable range where reception is impossible.
Some receivers, like the Silicon Labs device on this page, chose one of the available receive antennas by simply measuring received signal strength.
Others are far more sophisticated, combining signals from all receive antennas using phase shifting to get the most out of the system.
Multi-path interference between two transceivers is not the same in both directions, so the best receive antenna in a given situation is not necessarily the best to transmit from.
In a simple system, transmitters may as well chose one antenna at random, then stick to it, while the receivers chose between the available antennas often enough to allow for movement in the environment.