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4688D–RFID–03/07
U3280M
5.2.4 Special Modes
Data Transfer Sequence for Bi-phase and Manchester Modulation
By using special control bytes, the serial interface can control the modulator stage or the power
management. The EEPROM access and the serial interface are disabled in these modes until
the next STOP condition. If no START or STOP condition is generated, the SCL and SDA lines
can be used for the modulator stage. SCL is used for the modulator clock and SDA is used for
the data. In this mode, the same conditions for clock and data changing, as in normal mode, are
valid. The SCL and SDA lines can be used for continuous bit transfers, an acknowledge cycle
after 8 bits must not be generated.
Note: After a reset of the microcontroller it is not assured that the transponder interface has been reset
as well. It could still be in a receive or transmit cycle. To switch the device’s serial interface to a
known state, the microcontroller should read one byte from the device without acknowledge and
then generate a STOP condition.
5.2.5 Power-on Reset, NRST
The U3280M transponder front end starts working with the applied field. For the digital circuits
like the EEPROM serial interface and registers there is reset circuitry. A reset is generated by a
power-on condition at V
DD
, by switching back from field to battery supply and if a low signal is
applied at the NRST-pin.
The NRST-pin is a bi-directional pin and can also be used as a reset output to generate a reset
for the microcontroller if the circuit switches over from field to battery supply. This sets the micro-
controller in a well-defined state after the uncertain power supply condition during switching.
5.2.6 Antenna
For the transponder interface a coil must be used as an antenna. Air and ferrite cored coils can
be used. The achievable working distance (passive mode, not battery assisted) depends on the
minimum coupling factor of an application, the power consumption, and the size of the antennas
of the IC and the base station. With a power consumption of 150 µA, a minimum magnetic cou-
pling factor below 0.5% is within reach. For applications with a higher power consumption, the
coupling factor must be increased.
The Q-factor of the antenna coil should be in a range between 30 and 80 for read only applica-
tions and below 40 for bi-directional read-write applications.
Table 5-1. Control Byte Description
Control Byte Description
1100x111b Bi-phase modulation
1101x111b Manchester modulation
11xx0111b Switch power management off → disables switching from battery to field supply
11xx1111b
Switch power management on → enables automatic switching between battery
and field supply
xxxxx110b Reserved
START Control byte Ackn Bit 1 Bit 2 Bit 3 - - - - - - - - - - - Bit n STOP
