ST25R3911B datasheet(15/116 Pages) STMICROELECTRONICS

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Wake-up timer

Wake timer is primarily used in the wake-up mode (see Section 1.2.5 Wake-up mode). Additionally it can be

used by sending a 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 used as clock source for wake-up timer is

started, timeout is defined by setting in the Wake-up timer control register. When the timer expires, an IRQ with

the I_wt flag in the Error and wake-up interrupt register is sent.

wake-up timer is useful in the Low Power operation mode, in which other timers cannot be used (in the Low

Power operation mode the crystal oscillator, which is clock source for the other timers, is not running).

Note:

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

1.2.8

A/D converter

The ST25R3911B contains an 8-bit successive approximation A/D converter. Inputs to the A/D converter can be

multiplexed from different sources to be used in several direct commands and adjustment procedures. The result

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

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 2 V, 00h code means input is 0 V or lower, FFh means that input is 2 V - 1 LSB or higher

(LSB is 7.8125 mV).

•

In relative mode low reference is 1/6 of VSP_A and high reference is 5/6 of VSP_A, so the input range is from

1/6 to 5/6 VSP_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.

1.2.9

Phase and amplitude detector

This block is used to provide input to A/D converter to perform measurements of amplitude and phase, expected

by direct commands Measure Amplitude and Measure Phase. Several phase and amplitude measurements are

also performed by direct commands Calibrate Modulation Depth and Calibrate Antenna.

Phase detector

The phase detector is observing phase difference between the transmitter output signals (RFO1 and RFO2) and

the receiver input signals RFI1 and RFI2, which are proportional to the signal on the antenna LC tank. These

signals are first elaborated by digitizing comparators, then digitized signals are processed by a phase detector

with a strong low pass filter to get average phase difference.

The phase detector output is inversely proportional to the phase difference between the two inputs. The 90°

phase shift results in VSP_A/2 output voltage, in case both inputs are in phase output voltage is VSP_A, in case

they are in opposite phase output voltage is 0 V. During execution of direct command Measure Phase this output

is multiplexed to A/D converter input (A/D converter is in relative mode during execution of command Measure

Phase). Since the A/D converter range is from 1/6 to 5/6 VSP_A the actual phase detector range is from 30º to

150º.

Figure 6 and Figure 7 show the two inputs and the output of phase detector, respectively, in case of 90º and 135º

shifts.

ST25R3911B

Application information

DS11793 - Rev 6

page 15/116

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