AX5243 datasheet(21/42 Pages) ONSEMI

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AX5243

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HDLC packets are delimited with flag sequences of

content 0x7E.

In AX5243 the meaning of address and control is user

defined. The Frame Check Sequence (FCS) can be

programmed to be CRC−CCITT, CRC−16 or CRC−32.

The receiver checks the CRC, the result can be retrieved

from the FIFO, the CRC is appended to the received data.

In Wireless M−Bus Mode, the packet structure is given in

the following table.

NOTE: Wireless M−Bus mode follows EN13757−4

Table 20. WIRELESS M−BUS PACKET STRUCTURE

Preamble

FCS

Optional Data Block

(optionally repeated with FCS)

FCS

variable

8 bit

16 bit

48 bit

16 bit

8 − 96 bit

16 bit

For details on implementing a HDLC communication as

well as Wireless M−Bus please use the AX−RadioLab

software and see the AX5243 Programming Manual.

Raw Modes

In Raw mode, the AX5243 does not perform any packet

delimiting or byte synchronization. It simply serializes

transmit bytes and de−serializes the received bit−stream and

groups it into bytes. This mode is ideal for implementing

legacy protocols in software.

Raw mode with preamble match is similar to raw mode.

In this mode, however, the receiver does not receive

anything until it detects a user programmable bit pattern

(called the preamble) in the receive bit−stream. When it

detects the preamble, it aligns the de−serialization to it.

The preamble can be between 4 and 32 bits long.

RX AGC and RSSI

AX5243 features three receiver signal strength indicators

(RSSI):

1. RSSI before the digital IF channel filter.

The gain of the receiver is adjusted in order to

keep the analog IF filter output level inside the

working range of the ADC and demodulator. The

value of the AGC and can be used as an RSSI. The

step size of this RSSI is 0.625 dB. The value can

be used as soon as the RF frequency generation

sub−system has been programmed.

2. RSSI behind the digital IF channel filter.

The register RSSI contains the current value of the

RSSI behind the digital IF channel filter. The step

size of this RSSI is 1 dB.

3. RSSI behind the digital IF channel filter high

accuracy.

The demodulator also provides amplitude

information in the TRK_AMPLITUDE register.

By combining both the AGCCOUNTER and the

TRK_AMPLITUDE registers, a high resolution

(better than 0.1 dB) RSSI value can be computed

at the expense of a few arithmetic operations on

the micro−controller. The AX−RadioLab Software

calculates the necessary register settings for best

performance and details can be found in the

AX5243 Programming Manual.

Modulator

Depending on the transmitter settings the modulator

generates various inputs for the PA:

Table 21. MODULATIONS

Modulation

Bit = 0

Bit = 1

Main Lobe Bandwidth

Max. Bitrate

ASK

PA off

PA on

BW = BITRATE

125 kBit/s

FSK/MSK/GFSK/GMSK

Df = −fdeviation

Df = +fdeviation

BW = (1 + h) ⋅BITRATE

125 kBit/s

PSK

DF = 0°

DF = 180°

BW = BITRATE

125 kBit/s

= modulation index. It is the ratio of the deviation

compared to the bit−rate;

fdeviation = 0.5⋅h⋅BITRATE, AX5243 can

demodulate signals with h < 32.

ASK = amplitude shift keying

FSK = frequency shift keying

MSK= minimum shift keying; MSK is a special case of

FSK, where h = 0.5, and therefore

fdeviation = 0.25⋅BITRATE; the advantage of MSK

over FSK is that it can be demodulated more

robustly.

PSK = phase shift keying

All modulation schemes, except 4−FSK, are binary.

Amplitude can be shaped using a raised cosine waveform.

Amplitude shaping will also be performed for constant

amplitude modulation ((G)FSK, (G)MSK) for ramping up

and down the PA. Amplitude shaping should always be

enabled.

Frequency shaping can either be hard (FSK, MSK), or

Gaussian (GMSK, GFSK), with selectable BT = 0.3 or

BT = 0.5.

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