U3280M Datasheet, PDF (10/18 Pages) ATMEL Corporation – TRANSPONDER INTERFACE FOR MICROCONTROLLER

Initialization after a Reset
Condition
Special Modes
Power-on Reset, NRST
Antenna
The EEPROM with the serial interface has reset circuitry on-chip. In systems with micro-
controllers that have their own reset circuitry for power-on reset, watchdog reset or
brown-out reset, it may be necessary to bring the U3280M into a known state indepen-
dently of the internal reset. This is performed by reading one byte without
acknowledging and then generating a STOP condition.
Table 2. 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
Data Transfer Sequence for Bi-phase and Manchester Modulation
START Control byte Ackn Bit 1 Bit 2 Bit 3 - - - - - - - - - - -
Bit n
STOP
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 contin-
uous 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.
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 microcontroller in a well-defined state after the uncertain power supply condi-
tion during switching.
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 con-
sumption, and the size of the antennas of the IC and the base station. With a power
consumption of 150 µA, a minimum magnetic coupling 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
applications and below 40 for bi-directional read-write applications.
10 U3280M
4688B–RFID–12/04