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AS3910

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

8.14 Receiving 4Bit Tag Response Frame

Mifare Ultralight tag uses 4 bit response frame to indicate ACK and NACK. The AS3910 framing expects that response frame is composed of

bytes (except in case of anticollision frame) and it rejects a 4 bit response frame as an error (error due to receive data coding). By setting the bit

1 (frm4l) of register #0B, a 4 bit response frame is correctly processed and put in FIFO (MSB bits of first byte). In case of setting this bit a

standard frame response is not processed correctly.

Response to Mifare Ultralight WRITE command is either ACK or NACK. So in case of this command bit frm4 has to be set to distinguish between

the two possible 4 bit responses.

Response to READ command is either a standard frame, in case command is correctly processed, or a NACK, in case of an error. In this case

the bit frm4 can not be used, interrupt due to receive data coding should be interpreted as a NACK.

8.15 AM Modulation Depth: Definition and Calibration

The AM modulation of the transmitted carrier is used for communication reader to transponder in two configuration cases:

The ISO-14443B mode is configured in the ISO Mode Definition Register (#01)

The Transparent mode with AM modulation [direct command Transparent Mode, the bit 6(am) of the Configuration Register 5 (#05) is set to

1]

In other cases the OOK modulation is used.

The AM modulation depth can be automatically adjusted by setting the Modulation Depth Definition Register (#10) and sending the direct

command Calibrate Modulation Depth. This procedure is patent pending. There is also an alternative possibility where the command Calibrate

Modulation Depth is not used and the modulated level is defined by writing the Antenna driver AM Modulated Level Definition Register (#12).

8.15.1 AM Modulation Depth Definition Using Direct Command Calibrate Modulation Depth

Before sending the direct command Calibrate Modulation Depth the Modulation Depth Definition Register (#10) has to be configured in the

following way:

The bit 7 (am_s) has to be set to 0 to chose definition by the command Calibrate Modulation Depth

The bits 6 to 1 (mod5 to mod0) define target AM modulation depth.

form of the modulation index. The modulation index is defined by formula (a-b)/(a+b) where ‘a’ is amplitude of the non-modulated carrier and ‘b’

is the amplitude of the modulated carrier.

The modulation index specification is different for different standards. The ISO-14443B modulation index is typically 10% with allowed range

from 8% to 14%, range from 10 to 30% is defined in the ISO-15693 and 8% to 30% in the Felica™.

The bits mod5 to mod0 are used to calculate the amplitude of the modulated level. The non-modulated level which was before measured by the

A/D converter and stored in an 8 bit register is divided by a binary number in range from 1 to 1.98. The bits mod5 to mod0 define binary decimals

of this number.

Example:

In case of the modulation index 10% the modulated level amplitude is 1.2222 times lower then the non-modulated level. 1.2222 converted to

binary and truncated to 6 decimals is 1.001110. So in order to define the modulation index 10% the bits mod5 to mod0 have to be set to 001110.

The table below depicts setting of the mod bits for some often used modulation indexes.

Table 37. Setting mod bits for Modulation Indexes

modulation index

[%]

a/b

[dec]

a/b

[bin]

mod5………mod0

8

1,1739

1,001011

001011

10

1,2222

1,001110

001110

14

1,3256

1,010100

010100

20

1,5000

1,100000

100000

30

1,8571

1,110111

110111

33

1,9843

1,111111

111111