Application description

AN3255

6/24

Doc ID 17784 Rev 1

1.2

Short-range contactless communication principles

1.2.1

CR14 contactless coupler

The CR14 is the main component of the RFID reader PCB. It interfaces with the following

components:

●

The contactless tags

The data frames exchanged with the tags are compliant with ISO14443 type-B radio

frequency protocol. Data are stored in the CR14 input/output Frame registers (see

Table 1: CR14 control registers).

●

The CR14 is organized as 4 functional blocks:

●

The I²C bus controller

It handles the serial connection with the STM8S105C6 application host, and controls

specification.

●

The RAM buffer

The RAM buffer is bidirectional. It stores all the request frame bytes to be transmitted to

the tag, plus all the received bytes sent back by the tag on the answer frame.

●

The transmitter

It powers the tag by generating a 13,56 MHz signal on an external antenna. The

resulting field is 10% modulated using ASK (amplitude shift keying) modulation to

transmit data.

●

The receiver

It demodulates the signal generated on the antenna by the load variation of the tag.

The resulting signal is decoded by an 847 KHz BPSK (binary phase shift keying) sub-

carrier decoder.

The CR14 generates an electromagnetic field which is rectified to power the tag. The reader

transmits information to the tag by modulating the carrier wave. To transmit information back

to the reader, the tag backscatters the carrier wave by modifying its own impedance thereby

perturbing the field.

The CR14 chip contains six volatile registers of which three allow to configure the CR14 and

to transmit/receive frames to/from the tag (see Table 1):

●

Parameter register

●

Input/Output Frame register

●

Slot Marker register

datasheet.

Table 1.

CR14 control registers

Address

Description

Access

Purpose

00h

Parameter register

1 byte

W

Set parameter register

R

Read parameter register