AN3255
Application note
Building an RFID short-range reader
using the STM8S-DISCOVERY
Application overview
High frequency 13,56 MHz RFID solutions offer ideal close-proximity identification for
product authentication, parcel tracking, document management, library and ticketing
applications.
This application note describes how to build an RFID (radio frequency identification) shortrange reader using STMicroelectronics STM8S-DISCOVERY and ISO14443 type-B CR14
contactless coupler. The STM8S-DISCOVERY and the CR14 communicate through an I
bus.
The resulting RFID reader can exchange data with ISO 14443-2 type-B proximity PICCs
(proximity integrated coupling cards) also called tags. Communications are possible only
when the tags are present in the electromagnetic field generated by the reader built-in
antenna.
Once the STM8S-DISCOVERY is powered up through a USB cable connected to the host
PC, an electromagnetic field is generated by the RFID reader. A beep is emitted and the
LED LD1 briefly lights up when an ISO 14443-2 type-B proximity tag is detected by the
reader and its unique identifier (UID) successfully read.
2
C
The STM8S-DISCOVERY can be used to evaluate the main features of all STM8S MCUs,
even if it is built on an STM8S105C6T6.
Reference documents
■ STM8S-DISCOVERY evaluation board user manual (UM0817).
■ Developing and debugging your STM8S-DISCOVERY application code (UM0834).
■ User manual “CR14 and CRX14 reference design PCB Gerber files” (UM0672) and RFID
Gerber files.
■ CR14 datasheet: “Low cost ISO14443 type-B contactless coupler chip with anti-collision
and CRC management”
■ SRI2K datasheet: “13,56 MHz short-range contactless memory chip with 2048 bit
EEPROM and anti-collision functions”
■ Application note “Antenna (and associated components) matching-circuit calculation of
the CRX14 coupler” (AN1806)
■ Application note “How to design a 13.56 MHz customized antenna” (AN2866)
These documents can be downloaded from http://www.st.com.
October 2010 Doc ID 17784 Rev 1 1/24
www.st.com
Contents AN3255
Contents
1 Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Short-range contactless communication principles . . . . . . . . . . . . . . . . . . 6
1.2.1 CR14 contactless coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 I
1.2.3 Reader-tag protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.4 Commands and tag states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Hardware requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4 Application schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 STM8S-DISCOVERY configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1 Power supply configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2
C polling using Ack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Option byte configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1 STM8S peripherals used by the application . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Configuring the STM8S standard firmware library . . . . . . . . . . . . . . . . . . 15
3.3 Application principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A I2C memory addressing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix B I2C Read and Write functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
B.1 I2C Page Write flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.2 I
2
C post polling buffer read flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2/24 Doc ID 17784 Rev 1
AN3255 List of tables
List of tables
Table 1. CR14 control registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 2. List of PCB passive components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 3. List of PCB packaged components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 4. List of tag packaged components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 5. List of other passive components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 6. TAG commands used within this application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 7. Device Select code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 8. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Doc ID 17784 Rev 1 3/24
List of figures AN3255
List of figures
Figure 1. Short range RFID reader solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. Ack Polling flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 3. TAG state transition diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 4. Contactless RFID reader schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 5. Main application loop flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 6. I
Figure 7. I
2
C Page Write flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2
C post polling buffer read flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4/24 Doc ID 17784 Rev 1
AN3255 Application description
1 Application description
1.1 Overview
This application is built around a short range RFID reader PCB developed by
STMicroelectronics. The PCB Gerber files and the user manual "CR14 and CRX14
reference design PCB Gerber files" (UM0672) are available from http://www.st.com.
This board is designed to be connected to a digital host, in this case an STM8SDISCOVERY, which manages data transmission and reception through an I
(see Figure 1).
The tags supported by this application must be based on ST contactless memories
compliant with ISO 14443 part2 type-B standard for the radio-frequency power and signal
interface.
This application has been tested using a tag based on ST SRI2K short range contactless
EEPROM. Refer to application note AN2866 explaining how to design a 13.56 MHz tag
antenna.
Figure 1. Short range RFID reader solution
Resistors
TS1
GPIO's
LD1
Buzzer
USB
STLINK
SWIM
5V
3V3
STM8S105C6T6
SO16
footprint
2
C interface
RP1
RP2
I2C bus
J1
HE10
STMicroelectronics
CR14
STM8S-DISCOVERY
RFID reader PCB
Doc ID 17784 Rev 1 5/24
Application description AN3255
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 STM8S105C6T6 through the I
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 I
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) subcarrier decoder.
2
C bus.
2
C bus
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 Ta bl e 1 ):
● Parameter register
● Input/Output Frame register
● Slot Marker register
For details regarding registers description and CR14 I
datasheet.
Table 1. CR14 control registers
Address Description Access Purpose
00h Parameter register 1 byte
6/24 Doc ID 17784 Rev 1
2
C protocol, refer to the CR14
W Set parameter register
R Read parameter register
AN3255 Application description
Table 1. CR14 control registers (continued)
Address Description Access Purpose
Store and send request
01h
02h ST reserved N/A
03h Slot Marker register 1 byte
04h ST reserved N/A R and W
Input/Output Frame
register
36 bytes
W
R
W
R
W
R Return FFh
frame to the tag. Wait for
tag answer frame
Transfer tag answered
frame data to host
ST reserved, must not be
used
Launch the automated anticollision process from
Slot_0 to Slot_15
ST reserved, must not be
used
05h ST reserved N/A R and W
1.2.2 I2C polling using Ack
During radio frequency data exchange, the CR14 disconnects itself from the I2C bus. The
time needed to complete the exchange is not fixed as it depends on the tag command
format. To know when the exchange is complete before starting reading the data in the
Input/Output Frame register, the bus master uses an Ack polling sequence that performs the
following actions:
1. Initial condition: a radio frequency data exchange is in progress.
2. Step 1: the master issues a START condition followed by the first byte of the new
instruction (Device Select Code plus R/W bit = 1) (see Appendix A: I2C memory
addressing).
3. Step 2: if the CR14 is busy, no Ack is returned and the master goes back to Step 1. If
the CR14 has completed the radio frequency data exchange, it responds by sending
back an ACK, thus indicating that it is ready to receive the second part of the next
instruction (the first byte of this instruction has been sent during Step 1).
Figure 2 shows the detailed I²C Ack polling flowchart.
ST reserved, must not be
used
Doc ID 17784 Rev 1 7/24
Application description AN3255
2ADIO&REQUENCY
DATAEXCHANGE
INPROGRESS
3END
34!24CONDITION
3END#2
$EVICE3ELECT
#ODEWITH27
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RETURNED
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2EADDATABYTES
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DISCONNECTED
AI
Figure 2. Ack Polling flowchart
1.2.3 Reader-tag protocol
Standard tag commands such as Read and Write are generated by the CR14 using its
Input/Output Frame register. To send a command to the tag, the STM8S105C6 host first
generates internally the complete frame containing the command code followed by the
command parameters (if required). Only the 2 CRC bytes must not be generated as the
CR14 automatically adds them during the RF transmission.
Once the frame is ready, the host writes it into the Input/Output Frame register using an I
Write command.
If the tag answers, the characters received are demodulated, decoded and stored into the
input/output frame buffer. During the entire RF transmission, the CR14 disconnects itself
from the I
the I
2
C bus. On reception of the tag EOF, the CR14 checks the CRC and reconnects to
2
C bus. The host can then get the tag answer frame by issuing an Input/Output Frame
Register Read command on the I
Refer to the CR14 datasheet for details on Read and Write commands.
1.2.4 Commands and tag states
The tag can be switched into different states (see Figure 3). The tag only answers specific
commands depending on its current state. These states are specified by the ISO 15693
standard. For details concerning these states, refers to one of ST short range contactless
EEPROM datasheet (for example SRI2K) available from http://www.st.com.
2
C bus.
2
C
8/24 Doc ID 17784 Rev 1
AN3255 Application description
Power-off
Ready
Inventory
Selected
Deselected
Desactivated
Chip_ID
8bits = RND
Select(Chip_ID)
Read_block()
Write_block()
Get_UID()
Completion()
Select(Chip_ID)
Selct( != Chip_ID)
Select
(Chip_ID)
Reset_to_inventory()
Initiate() or Pcall16()
or Slot_marker(SN) or
Select(wrong Chip_ID)
Initiate()
On field
Out of
field
Out of
field
Out of
field
Out of
field
Out of
field
ai18400
Figure 3. TAG state transition diagram
1.3 Hardware requirements
The following STM8S-DISCOVERY on-board resources are used:
● LED LD1
Refer Ta bl e 2 , Tab le 3 , Tab le 4 , and Tab le 5 for the list of additional hardware required to
make this application software run on the STM8S-DISCOVERY.
Table 2. List of PCB passive components
Component description Value Comment
R1 N/A
R2 0 Ω
R3 N/A
R4 0 Ω
R5 N/A
R6 0 Ω
R7 0 Ω -
Doc ID 17784 Rev 1 9/24
Resistors
Hardwired addressing of the
CR14 (bits b3, b2, and b1 of the
7-bit I2C Device Select Code.
Allow up to 7 CR14 readers to
be addressed on the same I
2
C
bus
Application description AN3255
Table 2. List of PCB passive components (continued)
Component description Value Comment
Serial resistor allowing to fine
R8 0 Ω
Capacitor
C1, C2 7 pF crystal oscillator capacitors
C3 22 nF -
C4 22 μF-
C5 5pF (50V) -
C6 100 nF -
C7, C7’ 220 pF (50 V), 56 pF (50 V)
C8, C8’ 82 pF (50 V), 22 pF (50 V)
Diode
D1 (N4148) Optional
tune the quality factor of the
reader antenna
Capacitors allowing to fine tune
the RLC resonant frequency
impedance adaptation of the
RLC resonant circuit.
Ferrite
L3,L4,L5,L6, L7 multilayer SMD
ferrites
Table 3. List of PCB packaged components
Removal of parasites for tag
data reception and I
Part name Component name Description Package
Contactless coupler
CR14/CRX14
Short-range RFID
couplers
compliant with short-
range ISO14443 type-B
standard
13.56 MHz crystal Crystal
Table 4. List of tag packaged components
Generates a 13.56 MHz
carrier frequency
Part name Component name Description Package
13,56 MHZ short-range
SRI2K
contactless memory
chip
Table 5. List of other passive components
Short-range contactless
memory used to build a
tag
SO16
XTALCMS
SBN18
2
C
Component description Value Comment
Resistor
RP1, RP2 resistors 4,7 KΩ Pull-up for I2C open-drain
10/24 Doc ID 17784 Rev 1
AN3255 Application description
Table 5. List of other passive components (continued)
Component description Value Comment
Other
Piezo buzzer 5 V operating voltage Supports 4 KHz input frequency
1.4 Application schematics
Figure 4 shows the contactless reader implementation schematics. The reader is made of
two parts:
● STMicroelectronics RFID reader PCB based on the CR14 short range contactless
coupler (see Section 1.2.1).
● The STM8S-DISCOVERY which STM8S105C6T6 microcontroller controls the
bidirectional communications with the CR14 through the I
CR14 datasheet).
2
C serial bus (refer to the
Doc ID 17784 Rev 1 11/24
Application description AN3255
Vref1RF IN2E03E14E25GND6GND7GND
8
SDA
9
SCL
10
GND
11
OSC212OSC1
13
GND_RF
14
RF OUT
15
VCC
16
U1
CRX14
X1
C1C3
22nF50V
C8
C8'
C7 C7'
VCC
VCC
C6
+
C4
L4
WURTH 742-792-042
L5
WURTH 742-792-042L6WURTH 742-792-042
L7
WURTH 742-79 2-042
R7
R8
R1
OPT ANT1
ANT2
E0
E1
E2
SCL
SDA
123
4
J1
R2
R3
OPT
R4
R5
OPT
R6
VCC
D1
1N4148 (OPTIONA L)
C5
L3
WURTH 742-792-042
Vref1RF IN2E03E14E25GND6GND7GND
8
SDA
9
SCL
10
GND
11
OSC212OSC1
13
GND_RF
14
RF OUT
15
VCC
16
U1
X1
13.56 MHz
C1 7 pF 50 V
C2
7 pF 50 V
C3
22nF50V
C8
82 pF 50 V
C8'
22 pF 50 V
C7
220 pF 50 V
C7'
56 pF 50 V
VCCant
VCCant
C6
100 nF 50 V
+
C4
22 μF 10V
L4
WURTH 742-792-042
L5
WURTH 742-792-042L6WURTH 742-792-042
L7
WURTH 742-79 2-042
R7
R8
R1
OPT ANT1
ANT2
E0
E1
E2
SCL
SDA
123
4
R2
0 ΩR
R3
OPT
R4
R5
OPT
R6
VCCant
D1
1N4148 (OPTIONA L)
C5
10 pF 50 V
L3
WURTH 742-792-042
STM8S
I²C
PD0
VCCVCC
Buzzer
Piezo
PD4
PE1/SCL
PE2/SDA
VCCVCC
BEEP
Rp1
Rp2
GNDant
GNDant
GNDant
GNDant
GNDant
GNDant
GNDant
GNDant
LD1
(green)
0 ΩR
0 ΩR
0 ΩR
0 ΩR
AI
4,7Ω
4,7Ω
RFID reader PCB
Figure 4. Contactless RFID reader schematics
12/24 Doc ID 17784 Rev 1
AN3255 Application description
The efficiency of data transfers between the RFID reader and tag depends on the tuning of
their respective antenna. This is done by adjusting the following components:
● C
is the serial capacitance used to adapt the impedance of the reader antenna RLC
S
equivalent circuit:
C
= C8 + C8’
S
● C
is the parallel capacitance used to tune the resonant frequency of the reader RLC
P
equivalent circuit:
C
= C7 + C7’
P
● R
is the serial resistor used to fine tune the quality factor of the reader RLC resonant
S
equivalent circuit:
R
= R8
S
Refer to application note AN1806 for details on how to design a reader antenna and infer the
values of C
, CS and RS. This document is associated with a software tool using the Grover
P
method to calculate the inductance of rectangular planar antennas.
Warning: The values of CS, CP, and RS are dependant. Tuning one of
them impacts the 2 others. The best compromise must be
found to achieve a good tuning for the reader antenna (refer
to AN1806).
Doc ID 17784 Rev 1 13/24
STM8S-DISCOVERY configuration AN3255
2 STM8S-DISCOVERY configuration
2.1 Power supply configuration
The CR14 of the RFID reader PCB must be supplied from 5 V± 500 mV. This board being
powered from STM8S-DISCOVERY V
be set to V
= 5 V (see UM0817).
DD
2.2 Option byte configuration
The STM8S105C6T6 Beeper output is enabled through the alternate function remapping
option (AFR7) of the OPT2 option byte:
AFR7 = 0: port D4 alternate function is TIM2_CH1 (default)
AFR7 = 1: port D4 alternate function is BEEP (required)
For details on the option byte and alternate function remapping, refer to UM0834 and to the
STM8S105xx datasheet, respectively.
, the jumper JP1 of the STM8S-DISCOVERY must
DD
14/24 Doc ID 17784 Rev 1
AN3255 Software description
3 Software description
3.1 STM8S peripherals used by the application
The application software uses STM8S standard firmware library to control general purpose
functions. These peripheral functions are the following:
● Clock (CLK)
The clock control enables and delivers the correct clock frequency to the CPU and
peripherals. It configures the HSI prescaler division factor to 4. The I
frequency is 4 MHz to be able to generate correct timings compliant with Fast mode.
● GPIOs
They drive the MCU I/Os to interface with external hardware. They configure PD0 port
as output push-pull high to drive LD1 and switch it off at initialization. PD4 port is
configured through alternate function remapping to enable the Beeper output pin.
2
● I
C
This peripheral handles the serial connection with the CR14 contactless coupler of the
RFID reader board. It controls the read/write access to the CR14 registers.
● Auto wake-up (AWU)
This peripheral is used to provide an internal wake-up timebase that is used when the
MCU goes into Active-halt power saving mode. It is configured to wake up the MCU
after 512 ms, which is a good trade-off between the time during which the
microcontroller remains in Active-halt power saving mode and the time required by the
RFID reader to identify the tag.
● Beeper
This peripheral drives the Beeper output pin with a signal of 4 KHz for sound
generation.
● TIM3
The TIM3 timer is used to measure the LSI frequency with Input Capture 1 to reach
with a better accuracy the standard Beeper frequency outputs.
2
C input clock
3.2 Configuring the STM8S standard firmware library
The stm8s_conf.h file of the STM8S standard firmware library is used to configure the
library by enabling the peripherals used by the application.
The following define statements must be present:
#define _CLK 1 enables the clock control (CLK),
#define _GPIO 1 enables the GPIOs,
#define _I2C 1 enables the I
#define _AWU 1 enables the Auto wake-up,
#define _BEEP 1 enables the beeper,
#define _TIM3 1 enables timer 3.
2
C interface,
Doc ID 17784 Rev 1 15/24
Software description AN3255
3.3 Application principle
This application initiates communications with the tags present in the range of the
electromagnetic field generated by the reader antenna, and read the tag UIDs. If the
operation succeeds, the STM8S105C6T6 emits a beep and blinks LD1.
When running the code in debug mode from STVD development tool, the UID value can be
displayed in the Watch windows.
The application uses the I
board, and to transmit/receive frames to/from the tags (see Ta bl e 6 ). Communications with
the tags are managed by the state transition diagram shown in Figure 3.
A possible way to improve this application would be to use an LCD screen to display the tag
unique identifier (UID). Another improvement could be to implement the CR14 anti-collision
capability which allows the reader to select up to 16 tags (one at a time) during predefined
time slots.
Refer to Figure 5 for the flowchart of the application software main loop.
Table 6. TAG commands used within this application
Tag commands Description
Initiate() To detect if a tag in Ready state is present in the reader field range
Select(Chip_ID) Allow the tag to enter the Selected state
2
C protocol to configure the CR14 embedded on the RFID reader
Get_UID() On receiving this command, the tag returns its 8 UID bytes
16/24 Doc ID 17784 Rev 1
AN3255 Software description
34!24
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34-3ENTERS!CTIVEHALT
MODEFORMS
.EW#HIP?)$)#2EAD
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SUCCESSFULLY
DETECTED
./TAGNOTSELECTED
9%3
-AKE,$BLINK
%MITABEEP
)#7RITE@'ET5)$
5)$)#2EAD
-AKE,$BLINK
FORS
)#7RITE@)NITIATE
2EADTAG#HIP?)$
INITIALIZATIONOF
ALLPERIPHERALS
2EADNEW#HIP?)$OFTHETAG
AIB
Figure 5. Main application loop flowchart
During peripheral initialization, the application first configures the CR14 parameter register
to generate the 13,56 MHz RF field on the reader antenna.
Prior to issuing a Read/Write command (such as Get_UID()) to the memory tag, the tag
state machine must be put in the Selected state by sending a Select(Chip_ID) command. All
commands sent to the tag before this command is issued are ignored.
Once the peripheral initialization has completed, the application code enters a loop in which
it checks if a tag is present in the RF field by writing periodically the Initiate() command in
the CR14 Input/Output Frame register (see Appendix A: I2C memory addressing). If no tag
is present in the electromagnetic field, the STM8S105C6T6 enters the Active-halt power
saving mode for 512 ms and automatically wakes up to issue a new Initiate() command.
As soon as a tag is present in the electromagnetic field, it automatically enters the Ready
state in which its 8-bit random Chip_ID is initialized. When receiving the Initiate() command,
the tag then switches to the Inventory state in which its new 8-bit Chip_ID random value is
set and returned to the CR14. Since this application software does not implement the anti-
Doc ID 17784 Rev 1 17/24
Software description AN3255
collision mechanism, the Chip_ID is stored by the host and sent as parameter to the
Select(Chip_ID) command issued by the host to the tag. The tag then switches directly to
the Selected state.
The Get_UID() command is then sent to the tag that answers by returning its 8 UID bytes.
The host emits a beep and lights LD1 for about 1 s.
18/24 Doc ID 17784 Rev 1
AN3255 I2C memory addressing
Appendix A I2C memory addressing
To start communicating with the CR14, the bus master initiates a START condition and
sends 8 bits (with Most Significant Bit first) on the serial data line SDA. These bits contain
the Device Select Code (7 bits) and the RWbar bit.
According to the I
Code are the Device Type Identifier. For the CR14 these bits definition is given in Tab le 7 .
Table 7. Device Select code
CR14
select
The 8th bit is the Read/Write bit (RWbar). It is set to ‘1’ for I2C read and to ‘0’ for I2C write
operations.
If the data sent by the bus master matches the Device Select Code of the CR14, it returns
an acknowledgement on the bus during the 9th bit time. The CR14 device generates a
NACK if its Device Select Code does not correspond to the data sent. It deselects himself
from the bus and goes in standby mode.
Refer to the CR14 datasheet for details regarding the CR14 I
reference manual for a functional description on I
2
C bus definition, the seven Most Significant Bits of the Device Select
Device Code Chip Enable RWbar
b7 b6 b5 b4 b3 b2 b1 b0
1 0 1 0 E2 E1 E0 RWbar
2
2
C peripheral.
C protocol, and to the STM8S
Doc ID 17784 Rev 1 19/24
I2C Read and Write functions AN3255
Appendix B I2C Read and Write functions
This section gives an overview of the main functions controlling the read and write accesses
to the CR14.
20/24 Doc ID 17784 Rev 1
AN3255 I2C Read and Write functions
Start
I²C bus busy ?
YES
NO
Send
START condition
START condition
generated ?
YES
NO
Send
CR14 Device Select Code
with R/W=0
END of Address
transmission ?
Send register
write address
Data byte transfer
succeeded ?
Number of data
to be written > 0 ?
Send Data byte
Data byte transfer
succeeded ?
Generate
STOP condition
END
NO
YES
NO
YES
YES
NO
Number of data
to be written --
ai18402
B.1 I2C Page Write flowchart
The Page Write function performs write accesses to the CR14 registers. Its parameters are
the buffer containing the bytes to be written to the CR14, the CR14 write address and the
number of bytes to be written.
Figure 6 shows the detailed flowchart of the I2C_Page_Write function.
Figure 6. I
2
C Page Write flowchart
Doc ID 17784 Rev 1 21/24
I2C Read and Write functions AN3255
B.2 I2C post polling buffer read flowchart
This function has been implemented to be used together with the I2C polling function. Once
an ACK has been returned by I2C_CR14_EE_AckPolling, the
I2C_CR14_EE_PostPolling function reads one by one all the bytes that are available in
the Input/ Output Frame register of the CR14.
Figure 7 shows the detailed flowchart of the I2C_CR14_EE_PostPolling function.
Figure 7. I
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C post polling buffer read flowchart
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22/24 Doc ID 17784 Rev 1
AI
AN3255 Revision history
Revision history
Table 8. Document revision history
Date Revision Changes
12-Oct-2010 1 Document migrated from UM0927 rev 1.
Doc ID 17784 Rev 1 23/24
AN3255
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