Configuring USART parameters
The URL for USART access can uniquely be extended with additional parameters to configure the USART if required. For example:
InputStream inputStream = Connector.openInputStream( "serial://usart?baudrate=57600&databits=7&parity=even&stopbits=1.5");
databits may be 5, 6, 7 or 8, parity can be none, even, odd, space or mark, and stop bits can be 1, 1.5 or 2. You need specified only those options you wish to change. Because a call of this nature will disrupt any ongoing communications, this kind of call can only be made while there are no existing output or input streams connected to "serial://usart".
As we said above, System.out and System.err are by default connected to "serial://" and by implication to "serial://usart". Thus, if it is required to adjust the USART settings in the fashion shown above, then System.out and System.err must be diverted like this:
Isolate currentIsolate = Isolate.currentIsolate(); currentIsolate.clearOut(); currentIsolate.clearErr();
currentIsolate.addOut("serial://usb"); // not required but useful for development currentIsolate.addErr("serial://usb"); // not required but useful for development OutputStream serialOutput = Connector.openOutputStream("serial://usart?baudrate=9600");
Note that none of this is required if the default settings for the USART are acceptable (these are baudrate=115,200, databits=8, parity=none, stopbits=1).
Limitations on the use of the USART
When the Sun SPOT boots it will send some characters over the USART using the default settings unless USB is connected and in use by a host application. These characters are necessary to allow the SPOT to talk to the SPOT Client program (the program that underpins many of the ant commands) via a RS232 connection. Your application should take steps to ignore these characters.
USART input uses a 512 byte input ring buffer, loaded using an interrupt handler. There is no protection for overflow of the ring buffer. It is up to the application to ensure that data is read quickly enough, given the baud rate and rate of transmission.
Transmission is not interrupt driven: each transmit request is a synchronous call to VM native code. Whilst transmitting the SPOT cannot do anything else, so it is wise to use the highest baud rate compatible with reliable reception.
The radio communication library
About the radio stack
The classes for the radio stack above the Mac layer are in corresponding sources can be found in multihoplib_source.jar.
J2ME uses a Generic Connection Framework (GCF) to create connections (such as HTTP, datagram, or streams) and perform I/O. The current version of the Sun SPOT SDK uses the GCF to provide radio communication between SPOTs, routed via multiple hops if necessary, using a choice of two protocols.
The radiostream protocol provides reliable, buffered,
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