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This document is an introduction to the use of Bluetooth for embedded control applications.? Although there are many resources that discuss Bluetooth for wireless headphones, keyboards, mice, and similar uses, this document is confined to the advantages and potential pitfalls of using Bluetooth for embedded control.
?In general, Bluetooth can be very useful for certain embedded applications, but applications that require either very high data reliability or guaranteed data availability will have to be designed very carefully since these are both weaknesses in Bluetooth.
Bluetooth is a radio communication system that is intended to replace any cable in any short-range communication link.?? It easily meets this goal in non-demanding applications such as connecting a keyboard to a PC or headphones to a cell phone, but will have great difficulty replacing an EIA-485 link with more than eight nodes.? Several dozen potential applications have been identified by the Bluetooth SIG, and are supported with specific protocols.? One of the intended applications is the replacement of legacy EIA-232 links.? Bluetooth works well for EIA-232 applications that don?t push the envelope of the EIA-232 specification.
Bluetooth uses the ISM band starting at 2.4GHz.? Bluetooth divides this band into 79 sub-bands and hops among them as described below.
Bluetooth devices connect to each other in a network called a piconet.? A piconet may consist of from two to eight devices.? In a piconet, one device is always the master, and the remaining devices are all always slaves.? Master-slave roles are never exchanged because the message and channel timing are derived from the clock and Bluetooth address of the master.? A device may participate in more than one piconet at a time, and may be a master in one piconet and a slave in another piconet at the same time.? A device may not be the master in more than one piconet at the same time.
Since Bluetooth shares its bandwidth with many competing services, a piconet attempts to reduce interference by hopping from one frequency to another at frequent intervals.? Since there may be more than one Bluetooth piconet in an area at the same time, the hop pattern that a piconet uses is partially determined by the unique BT address of the master device. The Bluetooth devices themselves negotiate this frequency hopping; the user or designer does not have any role.
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