TRM-915-R250-CFT datasheet(7/39 Pages) LINX

7 / 39 page

–

If acknowledgements are enabled for assured delivery, then once the

packet is sent the module looks for an ACK from the other side. If the

ACK is not received, a retry is performed and the transmission is sent

again. If the number of transmission retries exceeds the value in the

regMAXTXRETRY register, an exception (EX_NORFACK) is raised.

Once the packet is sent, the transmitter deactivates but remains tuned to

the current channel until its hop time expires. If another packet is queued

for transmission, the module transmits this packet once the CSMA

mechanism allows access to the channel. Once the hop timer expires, the

module hops to the next channel.

Synchronization is lost whenever there is no more data to transfer and the

module has detected two consecutive hop indices without data present.

The module then returns to scan mode.

If another unit is transmitting when the module is ready to transmit a

packet, the module receives that data before attempting to transmit its

data. If the UART receive buffer gets full, the CTS line goes high to prevent

the host UART from over-running the receive buffer.

The CSMA mechanism introduces a variable delay to the transmission if

it detects that the channel is occupied. This delay is the sum of a random

period and a weighted period that is dependent on the number of times

that the module has tried and failed to access the channel. For applications

that guarantee that only one module is transmitting at any given time, the

CSMA mechanism can be turned off to avoid this delay.

Module Operation

The module employs a Frequency Hopping Spread Spectrum (FHSS)

algorithm. It has 32 channels spaced on 750kHz boundaries with a guard

band on either side. These channels are pseudo-randomly arranged into

six unique hopping tables comprised of 26 channels. The order of these

tables is chosen so that cross-correlation is minimized, allowing multiple

networks to operate in proximity with minimal interference.

When the module is not actively transmitting or receiving packets, it is in a

scan state. It cycles through the channels in the hop sequence looking for

a synchronizing packet. If it detects a preamble, it pauses to wait for the

start code and packet header. If the packet is addressed to it, the module

processes the packet and outputs the payload on the UART. If the packet

is not addressed to the module or the start code and header fail their

checks, the module resumes scanning for another packet.

When data is input on the RXD line for transmission, the module fills

a buffer. Once the UART has buffered enough data to send (either

regUARTMTU bytes input or regTXTO has expired between bytes on the

RXD line), it transmits the data. The protocol engine makes a best-effort

attempt to keep the data in at least regUARTMTU-sized packets, but splits

the data based on the remaining dwell time before hopping. New data is

not sent within the last 5% of the hop sequence, but data which is already

in the process of being sent is processed normally.

The module prefixes the data with a packet header and postfixes the data

with a 16-bit CRC. The 16-bit CRC error checking can be disabled to allow

the host application to do its own error checking.

Initially, the transmission of the packet begins on a random hop index within

the current hop sequence, and follows the hop sequence thereafter until

synchronization is lost. The module uses a Carrier-Sense-Multiple-Access

(CSMA) protocol to determine if another module is already transmitting on

the selected channel. If the channel is occupied then the module waits for it

to clear before transmitting its data.

Once the module gains access to the channel, if it is not already

synchronized, it assigns itself master status, and sends a synchronizing

preamble. Following a hop, the module that sends the first transmission

assigns itself master status, sends a synchronizing preamble, and

communications resume.

TRM-915-R250-CFT Scheda tecnica(PDF) 7 Page - Linx Technologies

TRM-915-R250-CFT Scheda tecnica(HTML) 7 Page - Linx Technologies

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Scheda tecnica Scarica

Link URL