TMS37157 datasheet(39/47 Pages) TI

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TMS37157

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SWRS083A – SEPTEMBER 2009 – REVISED NOVEMBER 2009

Modulation Frequency Check

During LF transmissions a FSK signal is transmitted. The resonance frequency of the trimmed antenna circuit

(fL) represents a low bit and high bits are represented by a lower frequency (fH), which is achieved by switching

in a Modulation Capacitor in parallel with the antenna resonance circuit. This frequency can be measured in the

same way as the normal resonance frequency, but using Probe Test Mode 0x16 instead of 0x18.

CRC Calculation

A Cyclic Redundancy Check (CRC) generator is used in the TMS37157 during receipt and transmission of data

to generate a 16-Bit Block Check Character (BCC), applying the CRC-CCITT algorithm as shown in Figure 51.

The CRC generator consists of 16 shift register cells with 3 exclusive OR (Xor) Gates. The first Xor gate (X16)

combines the input of the CRC generator with the output of the shift register (LSB first) and feeds back to the

input of the shift register. The other two Xor gates combine certain cell outputs (X12, X5) with the output of the

first Xor Gate and feed into the next cell input.

The CRC Generator is initialized with the value 0x3791 as shown in Figure 50).

Figure 50. Initial CRC Value 0x3791

Figure 51. CRC Generator Block Schematic

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TMS37157 Datasheet(PDF) 39 Page - Texas Instruments