AN3255 Datasheet, PDF (6/24 Pages) STMicroelectronics – Building an RFID short-range reader using the STM8S-DISCOVERY

Application description
AN3255
1.2
1.2.1
Short-range contactless communication principles
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 STM8S105C6T6 through the I²C bus.
The CR14 is organized as 4 functional blocks:
● The I²C bus controller
It handles the serial connection with the STM8S105C6 application host, and controls
the read/write accesses to all CR14 registers. It is compliant with the 400 KHz I²C bus
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
For details regarding registers description and CR14 I²C protocol, refer to the CR14
datasheet.
Table 1. CR14 control registers
Address
Description
00h
Parameter register
1 byte
Access
W
R
Purpose
Set parameter register
Read parameter register
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Doc ID 17784 Rev 1