TRM-915-R250 datasheet(16/39 Pages) LINX

16 / 39 page

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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

...B1B0

≥20µs

Figure 19: 250 Series Transceiver CMD Line Timing

TRM-915-R250 Datasheet(PDF) 16 Page - Linx Technologies

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