AN2399

RFID library

19/22

8.3.2

CRX14 I/O frame register access

The Input/Output Frame Register is a 36-byte buffer that is accessed serially from Byte 0

through to Byte 35 (see Figure 11). It is located at the I²C address 01h. The Input/Output

Frame Register is the buffer in which the CRX14 stores the data Bytes of the request frame

to be sent to the PICC. It automatically stores the data Bytes of the answer frame received

from the PICC. The first Byte (Byte 0) of the Input/Output Frame Register is used to store

the frame length for both transmission and reception.

Figure 11.

CRX14 I/O frame register description

When accessed in I²C Write mode , the register stores the request frame Bytes that are to

be transmitted to the PICC. Byte 0 must be set with the request frame length (in Bytes) and

the frame is stored from Byte 1 onwards. At the end of the transmission, the 16-bit CRC is

automatically added. After the transmission, CRX14 waits for the PICC to send back an

answer frame. When correctly decoded, the PICC answer frame bytes are stored in the

Input/Output Frame Register from Byte 1 onwards. Byte 0 stores the number of bytes

received from the PICC.

Figure 12.

CRX14 I/O frame register: write operation

When accessed in I²C Read mode, the Input/Output Register sends back the last PICC

answer frame Bytes, if any, with Byte 0 transmitted first. The 16-bit CRC is not stored, and it

is not sent back on the I²C bus. The Input/Output Frame Register is set to all 00h between

transmission and reception. If there is no answer from the PICC, Byte 0 is set to 00h. In the

case of a CRC error, Byte 0 is set to FFh, and the data Bytes are discarded and not

appended in the register.

Figure 13.

CRX14 I/O frame register: read operation

Byte 0

Byte 1

Byte 2

Byte 3

...

Byte 34

Byte 35

Frame Length

1st data Byte

2nd data Byte

Last data Byte

<------------- Request and Answer Frame Bytes exchanged on the RF ------------->

00h No Byte transmitted

FFh CRC Error

xxh Number of transmitted Bytes

S

T

A

R

T

10 1 0 XX X

01h

N

S

T

O

P

ACK

ACK

ACK

Request Frame

Length N

Input/Output

Register

Address

Device

Select

Code

Bus

Master

CRX14

Write

Bus

Slave

R/W

Data 1

Data 2

PICC

Command

Parameter

PICC

Command

Code

Data N

PICC

Command

Parameter

PICC

Command

Parameter

ACK

ACK

ACK

ACK

S

T

A

R

T

10 10 XXX

01h

N

S

T

O

P

ACK

ACK

ACK

Received

Frame

Length

Input/Output

Register

Address

Device

Select

Code

Bus

Master

CRX14

Read

Bus

Slave

R/W

Data1

Data 2

Answer

Frame

Data

Answer

Frame

Data

Data N

Answer

Frame

Data

Answer

Frame

Data

NoACK

ACK

ACK

ACK

R

E

S

T

A

R

T

10 1 0 XXX

Device

Select

Code

R/W

ACK