U3280M_07 Datasheet, PDF (8/21 Pages) ATMEL Corporation – Transponder Interface for Microcontroller

4.3.3
Buffer Capacitor C_B
The buffer capacitor connected at V_DD is used to buffer the supply voltage for the microcontroller
and the EEPROM during field supply. It smoothes the rectified AC from the coil and buffers the
supply voltage during modulation and gaps in the field. The size of this capacitor depends on the
application. It must be of a dimension so that during modulation and gaps the ripple on the sup-
ply voltage is in the range of 100 mV to 300 mV. During gaps and damping the capacitor is used
to supply the device, which means the size of the capacitor depends on the length of the gaps
and damping cycles.
Table 4-1.
Example for a 350 µA Supply Current, 200 mV Ripple at V_DD
No Field Supply During
Necessary C_B
250 µs
470 nF
500 µs
1000 nF
4.4 Serial Interface
The transponder interface has a serial interface to the microcontroller for read and write access
to the EEPROM. In a special mode, the serial interface can also be used to control the
Bi-phase/Manchester modulator or the power management of the U3280M.
The serial interface of the U3280M device must be controlled by a master device (normally the
microcontroller) which generates the serial clock and controls the access via the SCL and SDA
lines. SCL is used to clock the data in and out of the device. SDA is a bi-directional line and used
to transfer data into and out of the device. The following protocol is used for the data transfers.
4.4.1
Serial Protocol
• Data states on the SDA line change only when SCL is low.
• Changes in the SDA line while SCL is high will be interpreted as a START or STOP condition.
• A STOP condition is defined as a high-to-low transition on the SDA line while the SCL line is
high.
• Each data transfer must be initialized with a START condition and terminated with a STOP
condition. The START condition awakens the device from standby mode, and the STOP
condition returns the device to standby mode.
• A receiving device generates an acknowledge (A) after the reception of each byte. For that
purpose the master device must generate an extra clock pulse. If the reception was
successful, the receiving master or slave device pulls down the SDA line during that clock
cycle. If an acknowledge has not been detected (N) by the interface in transmit mode, it will
terminate further data transmissions and switch to receive mode. A master device must finish
its read operation by a not acknowledge and then issue a STOP condition to switch the
device to a known state.
8 U3280M
4688D–RFID–03/07