Machine Design

I did it MiWi

A free, small-footprint communication protocol from Microchip Technology targets developers who don't need the interoperability of a ZigBee protocol but still want to use IEEE 802.15.4 transceivers for inexpensive peer-to-peer networks.

The MRF24J40 contains a complete IEEE 802.15.4 transceiver for ZigBee or MiWi applications. As evident in this block diagram, the chip contains MAC and PHY layers, a 128-bit AES encryption engine, and an SPI interface for communicating with the host controller.

Dubbed MiWi, the protocol was devised to work with Microchip's first RF transceiver, the MRF24J40 — a 2.4-GHz transceiver developed for ZigBee and MiWi protocols.

The MiWi wireless protocol works with star, mesh, or peer-to-peer networks. While billed as an alternative to ZigBee, it doesn't replace the protocol. MiWi permits only eight coordinator nodes with up to 127 child nodes per coordinator for a total of 1,024 nodes. Another limitation is that it allows only four hops between nodes per message.

MiWi comes out of a feeling among some companies that the ZigBee protocol has grown too large and complex for many applications; and that the ZigBee certification procedures are too costly and cumbersome for smaller systems. In contrast, MiWI needs no certification. The protocol stack is free when used with Microchip's PIC microcontrollers and MRF24J40 transceiver.

The MRF24J40 is a complete IEEE 802.15.4 radio that supports both Zigbee and MiWi protocols. Housed in a 40-pin leadless QFN 6 × 6 mm package, the transceiver typically draws 18 mA in receive and 22 mA transmitting. Consumption drops to 2 µA during sleep mode. The chip contains a 32.768 kHz clock oscillator for timekeeping and a 20-MHz main oscillator. The main oscillator has a reference output pin that can drive the host PIC or dsPIC controller, eliminating the need for additional oscillator components. Communication between the paired radio chip and MCU is via a four-wire SPI interface.

The chip contains an embedded 128-bit Advanced Encryption Standard security engine that supports both inline and stand-alone modes for encryption and decryption. Building communication security into the transceiver unburdens the host processor.

The media access controller (MAC) is also built in. It works with either ZigBee or MiWi protocols and can retransmit packets.

Microchip offers a demonstration kit that includes Zigbee and MiWi-protocol software stacks and two radio cards. Each card consists of a PIC18-controller motherboard with an MRF24J40 RF daughter card for swapping in future controller or RF advances.

The Zena Wireless Network Analyzer also comes with each development kit. Zena works with a PC to create a simple graphic interface for configuring ZigBee and MiWi protocol stacks. The network-configuration display window shows wireless traffic in real time as it travels from one node to another. Developers can save sessions to a file for later review.

Microchip Technology Inc.,
(480) 792-7200,

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