Linx Technologies TRM-915-R250 - Manual (Page 16)

– – – – 26 27
Using the Command Response (CMD_RSP) Line
The CMD_RSP line is normally high, but the module lowers this line when
responding to a UART command. This indicates to an external processor
that the data on the TXD line is a response to a command and not data
received over-the-air.
The module outputs received RF data immediately following the command
response. The CMD_RSP line does rise before resuming RF data, but
some processors cannot react quickly enough to this signal and may not
able to separate the command responses from RF data.
The regCMDHALT register controls the behavior of the TXD line when the
CMD line is low and the external processor is configuring the module.
If this register is set to 0x01 and the CMD line is low, the module stops
outputting the RF data and internally buffers it. Once the CMD line is raised,
the buffered RF data is output on the TXD line. This allows the external
processor to have separate configuration times and data times instead of
potentially having to handle both at once.
The CMD Line
The CMD line is used to inform the module where incoming UART data
should be routed. When the line is high or left floating, all incoming UART
data is treated as payload data and is routed to the transmitter to be sent
over the air. If the CMD line is low, the incoming UART data is routed to
the command parser for processing. Since the module’s controller looks
at UART data one byte at a time, the CMD line must be held low for the
entire duration of the command plus a 20µs margin for processing. Leaving
the line low for additional time (for example, until the ACK byte is received
by the application) does not adversely affect the module. If RF packets are
received while the CMD line is active, they are still processed and output on
the module’s UART. Figure 19 shows this timing.
The CMD line is also used during the module startup process to determine
whether or not to reload the non-volatile registers with factory defaults. The
module startup process is executed when the module is powered on from
an off state or is issued a software or hardware reset. When the module
goes through this startup process, it checks the state of the CMD line. If it
is low, the module clears the non-volatile registers and re-populates them
with factory default values. It is important to ensure that CMD is held high
or left floating during power-up under normal conditions.
Possible reset sources that could cause the module to reboot are power
supply brown-out, power supply instability and noise present on the RESET
line, noise/voltage spikes on digital I/O lines, issuing a reset command
through the command interface, and toggling the RESET line when not in
deep sleep.
RXD
CMD
0xFF ... B1 B0
≥20µs
Figure 19: 250 Series Transceiver CMD Line Timing