TRM-915-R250 datasheet(14/39 Pages) LINX

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Voltage Supply Rise Time

The power supply rise time is extremely important. It must rise from ground

to 2.7V in less than 1ms. If this specification cannot be met, an external

reset supervisor circuit must be used to hold the module in reset until the

power supply stabilizes. Failure to ensure adequate power supply rise time

can result in loss of important module configuration information.

Using the Buffer Empty (BE) Line

The BE line indicates the state of the module’s UART buffer. When the

module receives data in the RXD line and the CMD line is high, the BE line

is lowered until all data in the buffer has been processed by the protocol

engine. If acknowledgement is not enabled, the BE line is raised as soon as

the protocol engine processes the outgoing packets. If acknowledgement

is enabled, the buffer is not updated until either the data transmissions

are acknowledged by the remote end or delivery fails after the maximum

number of retries. When the BE line returns high, the EX line may be

sampled, or the regEXCEPTION register polled to determine if an error

occurred during transmission.

Using the Exception (EX) Line

The EX line indicates whether or not a module exception has occurred.

The line is normally low, but it is raised if an exception occurs that passes

masking. When the regEXCEPTION register is read, the exception is

cleared and the EX line returns low. If more than one exception occurs

before the regEXCEPTION register is read, the old exception is overwritten

by the new one. Please see the Exception Engine section for more details.

Using the Processing Incoming Packet (PR_PKT) Line

The PR_PKT line indicates whether the protocol engine has determined

there to be valid or potentially valid data incoming. The line is normally low

(not processing). When awake and not transmitting, the protocol engine

is constantly searching for incoming data. When scoring indicates that a

potential packet is inbound, this line is raised until either the scoring falls

below a given threshold or the complete packet is received. It is possible

that the packet scoring will fall below the threshold during reception,

causing the line to be lowered. Such an instance can occur when the

module hops to a channel late in the transmitter’s extended preamble.

Since there aren’t a large number of valid bits to score, the line may fall

during the packet start sequence. Once this sequence arrives, the PR_PKT

signal rises and latches for the duration of the packet reception.

Receive Signal Strength Indication (RSSI)

The RSSI line outputs an analog voltage that is proportional to the signal

strength present on the channel at the time. In normal operation, the

module is hopping rapidly from channel to channel. In this case, the

RSSI value varies greatly and does not provide much useful information.

However, it can be used to keep a module awake by sampling the RSSI

line to determine if the module is processing a packet before putting it to

sleep.

The 250 Series module has an internal digital RSSI indication of the

immediate ambient environment and of the last good packet received.

Additionally, the PR_PKT line can be used to indicate the state of the

protocol engine.

RSSI level is dependent on the power of the signal received at the antenna

port and the mode the LNA is in. regLNAMODE controls the mode of the

internal LNA. Figure 16 shows typical traces of RSSI voltage versus signal

strength.

500

1000

1500

2000

2500

-102 -98-94 -90-86 -82-78 -74-70 -66-62 -58-54 -50-46 -42-38 -34-30 -26-22

RF IN (dBm)

High Sens

Mid IIP3

High IIP3

Auto Gain

Figure 16: 250 Series Transceiver P

IN vs RSSI Voltage

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