ST25R3920B datasheet(39/158 Pages) STMICROELECTRONICS

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DS13890 Rev 4

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ST25R3920B

Application information

157

4.2.12

Active wave shaping

To use the specific wave shaping functionality of ST25R3920B, the logic must be enabled in

the auxiliary modulation setting as well as the corresponding AWS registers. Additionally,

the external VDD_AM capacitor must be selected in a range of 10-50 nF.

Note:

Contrary to the recommendations for the ST25R3920, do not insert a 2.2 μF capacitor on

the VDD_AM pin when using AWS.

To generally enable the ST25R3920B specific AWS:

•

set 0x28h in register SpaceB (Auxiliary modulation setting register) to 0x94h

•

set 0x2Eh in register SpaceB (AWS configuration) to 0x08h

The following registers need to be adjusted depending on whether OOK (for instance

NFC-A) or ASK (for instance NFC-B, NFC-F) modulation is transmitted.

OOK:

•

set am_mod<0:3> in TX driver register to 0xFXh, which is the lowest VDD_AM level

during modulation (82 % modulation index)

•

clear am_sym and set en_modsink in AWS Config 2 register by writing 0x1Xh, which

activates nonsymmetrical shape and strong en_modsink during OOK.

ASK:

•

set am_mod<0:3> in TX driver register to the required modulation index, which could

be 0x4Xh or 12% for the usual case, for instance NFC-B.

•

set am_sym and clear en_modsink in AWS Config 2 register by writing 0x2Xh, which

activates symmetrical shape and weak en_modsink during ASK.

The shaping of the modulation pulse is done through am_filt speed and by setting the

switching time of the clamp between VDD_RF and VDD_AM.

The following picture gives a graphical representation of the wave shaping mechanism and

involved register bits.

Figure 9. AWS mechanism

MS69269V2

r_trim

am_filt

am_sym

en_modsink

VDD_TX

am_mod

clamp

VDD_DR

VDD_TX

modsw

RFO

mods d_res

VDD_AM

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