U3280M Datasheet, PDF (6/18 Pages) ATMEL Corporation – TRANSPONDER INTERFACE FOR MICROCONTROLLER

Controlling Power
Management via the Serial
Interface
Buffer Capacitor C_B
Serial Interface
Serial Protocol
The automatic mode of the power management can be switched off and on by a com-
mand from the microcontroller. If the automatic mode is switched off, the IC is always
supplied by the battery up to the next power-on reset or to a switch-on command. The
power management’s on and off command must be transferred via the serial interface.
If the power management is switched off and the device is supplied from the battery, it
can communicate via the field without loading the field. This mode can be used to real-
ize applications with battery supply if the field is too weak to supply the IC with power.
The buffer capacitor connected at V_DD is used to buffer the supply voltage for the micro-
controller 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 mod-
ulation and gaps the ripple on the supply 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 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
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 con-
trol 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 (nor-
mally 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 pro-
tocol is used for the data transfers.
• 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.
6 U3280M
4688B–RFID–12/04