Rainbow Electronics AT88RF1354 - Manual (Page 33)

13.56 MHz Type B RF Reader
33
8547A −RFID−10/08
Appendix C. The TWI Serial Interface
The TWI Interface mode is selected by shorting the ISEL pin to V
SS
. Four microcontroller pins are required to operate
AT88RF1354 in TWI mode. TWI ACK polling is not supported; the ISTAT signal is used for handshaking between the
microcontroller and RF reader.
C.1. TWI Interface
The AT88RF1354 2-wire serial interface (TWI) operates as a slave device. The TWI interface allows the device to
share a common 2-wire data bus with other compatible devices. The bus consists of a serial clock (SCK) and a serial
data (SDA / SDI) line. The serial clock is generated by the TWI bus master. Serial data bytes are transmitted bi-
directionally on the SDA / SDI line, most significant bit first, synchronized to the SCK. The ISTAT signal reports the
serial interface status to the microcontroller.
Figure C-1. Serial Interface Wiring to TWI Microcontroller
AT88RF1354
(TWI Slave)
ResetB
Reader IC
Microcontroller
(TWI Master)
N.C.
output
SDA
ISEL VSS
input
SCK
SDI
SDO
SSB
Istat
N.C.
SCK
A high level on the ISTAT pin signals the host microcontroller that a byte of data is ready to be read from the
AT88RF1354 serial interface. If another byte is immediately available on the serial port, ISTAT will go low for 150 uS,
then return high. ISTAT will remain high until the last bit of the byte is read, when it will return low.
Data on the SDA / SDI line is sampled by the receiving device when the SCK clock is high. Data is allowed to be
changed by the transmitting device only when the SCK clock is low. All data must be clocked out of the AT88RF1354
before it can receive a command.