ST25R3916 datasheet(37/157 Pages) STMICROELECTRONICS

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DS12484 Rev 4

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ST25R3916/7

Application information

156

Wake-up (WU) timer

This timer is primarily used in the Wake-up mode, it can be used by sending the direct

command Start Wake-up Timer. This command is accepted in any operation mode except

Wake-up mode. When this command is sent the RC oscillator, which is used as clock

source for wake-up timer is started, timeout is defined by setting the Wake-up timer control

The WU timer is used in the Power-down mode, in which other timers cannot be used

because the crystal oscillator, which is the clock source for the other timers, is not running.

Note that the tolerance of wake-up timer timeout is defined by tolerance of the RC oscillator.

In NFCIP-1 passive target mode the WU timer is used for time out the temporary device

enable after the initial peer field on was detected.

4.2.8

A/D converter

The ST25R3916/7 contains an 8-bit successive approximation A/D converter. Inputs can be

multiplexed from different sources to be used in several direct commands and adjustment

procedures. The result of the last conversion is stored in the A/D converter output register.

Typical conversion time is 224/fc (16.5 µs).

The A/D converter has two operating modes, absolute and relative.



In absolute mode the low reference is 0 V and the high reference is 2 V. This means

that A/D converter input range is from 0 to 2V, 00h code means input is 0 V or lower,

FFh means that input is 2 V - 1 LSB or higher, LSB being 7.8125 mV.



In relative mode low reference is 1/11 of VDD_A and high reference is 10/11 of VDD_A,

so the input range is from 1/11 VDD_A to 10/11 VDD_A.

Relative mode is only used in phase measurement (phase detector output is proportional to

power supply). In all other cases absolute mode is used.

4.2.9

Phase and amplitude detector

This block is used to provide inputs to the A/D converter to perform measurements of

amplitude and phase, expected by direct commands Measure amplitude and Measure

phase.

Phase detector

The phase detector observes phase difference between the transmitter output signals

(RFO1 and RFO2) and the receiver input signals RFI1 and RFI2, proportional to the signal

on the antenna LC tank. These signals are first passed by digitizing comparators. Digitized

signals are processed by a phase detector with a strong low-pass filter characteristics to get

the average phase difference. The phase detector output is inversely proportional to the

phase difference between the two inputs. The 90° phase shift results in VDD_A/2 output

voltage, if both inputs are in phase the output voltage is VDD_A, if they are in opposite phase

the output voltage is 0 V. During execution of direct command Measure phase this output is

multiplexed to the A/D converter input (A/D converter is in relative mode during the

execution of this command). Since the A/D converter range is from 1/11 VDD_A to 10/11

VDD_A the actual phase detector range is from 17º to 163º. Figure 9 and Figure 10 show the

two inputs and output of phase detector in case of 90º and 135º phase shift, respectively.

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