C H A P T E R 3 - S E R I A L I N T E R F A C E
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H A R D W A R E F L O W C O N T R O L
Flow control refers to the control of data flow between the host and the MDR29x. It is the method used to
handle data in the transmit/receive buffer of the MDR29x interface, and determines how data flow between
the host and the MDR29x is throttled. Often in serial communication, one device is capable of sending data
much faster than the other can receive. Flow control allows the slower device to tell the faster device to
pause and resume data transmission. (Flow control CTS and RTS are used by the MDR29x and its Host,
locally - NOT over the air. Therefore, one MDR29x cannot tell the other to slow down or speed up as is
mentioned above in the paragraph).
When the RTS Enable option is selected on the Configuration Utility, the transceivers use hardware flow
control to regulate data flow. While using hardware flow control, the transceiver that is ready to receive
data sends a Clear To Send signal to its host (or the device it is connected to). On the other hand, when a
transceiver has something it wants to send to its host, it checks the state of Ready To Send and if it is logic
low, will send data to its host. If RTS is logic high, it will not send data to its host. These signals are sent
apart from the data itself on separate wires. Note: CTS is always enabled by default. RS-485
Interface does not support Hardware flow control.
Can I implement a design using just Txd, Rxd and Gnd (Three-wire Interface)?
Yes. However, it is strongly recommended that your hardware monitor the CTS pin of the
radio. CTS is taken High by the radio when its interface buffer is getting full. Your
hardware should stop sending at this point to avoid a buffer overrun (and subsequent loss
of data).
You can perform a successful design without monitoring CTS. However, you need to take
into account the amount of latency the radio adds to the system, any additional latency
caused by Transmit Retries or Broadcast Attempts, how often you send data, non-delivery
network timeouts and interface data rate. Polled type networks, where the Server host
requests data from the Client host and the Client host responds, are good candidates for
avoiding the use of CTS. This is because no one transceiver can monopolize the RF link.
Asynchronous type networks, where any radio can send to another radio at any point in
time, are much more difficult to implement without the use of CTS.
DESIGN TIP
