Standard Read/Write ID Transponder with Anti ... - Atmel Corporation

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7. Measurement Assembly All parameters are measured in a Helmholtz arrangement, which generates a homogenous magnetic field (see Figure 7-1 and Figure 7-2). A function generator drives the field generating coils, so the magnetic field can be varied in frequency and field strength. Figure 7-1. Testing Application Sensing Coils (in Phase) Atmel ATA5551 Subtractor Output Voltage Reference Coil (in Phase) Field Generating Coils (in Phase) Reference Coil (in Phase) Amplifier 1:10 Function Generator Figure 7-2. Testing Geometry 30mm 15mm Atmel ATA5551 24mm 60mm Reference Coil 2mm Reference Coil Sensing Coil Sensing Coil 5mm Field Generating Coil Field Generating Coil 6 Atmel ATA5551 9211A–RFID–10/10
Atmel ATA5551 8. Writing Data into the Atmel ATA5551 The write sequence of the Atmel ® ATA5551 is shown below. Writing data into the transponder occurs by interrupting the RF field with short gaps. After the start gap the standard write OP code (10) is followed by the lock bit. The next 32 bits contain the actual data. The last three bits denote the destination block address. If the correct number of bits have been received, the actual data is programmed into the specified memory block. Figure 8-1. Write Protocol RF Field Standard OP-code Address bits (e.g. block 4) 32bit 1 0 0 1 0 0 > 64 clocks Start gap Lock bit Read mode Write mode 9. Write Data Decoding The time elapsing between two detected gaps is used to encode the information. As soon as a gap is detected, a counter starts counting the number of field clock cycles until the next gap is detected. Depending on how many field clocks elapse, the data is regarded as “0” or “1”. The required number of field clocks is shown in Figure 9-1. A valid “0” is assumed if the number of counted clock periods is between 16 and 32, for a valid “1” it is 48 or 64 respectively. Any other value being detected results in an error, and the device exits write mode and returns to read mode. Figure 9-1. Field clock cycles Write Data Decoding Scheme 1 16 32 48 64 Write data decoder fail 0 fail 1 writing done 10. Actual Behavior of the Device The Atmel ATA5551 detects a gap if the voltage across the coils decreases below the threshold value of an internal MOS transistor. Until then, the clock pulses are counted. The number given for a valid “0” or “1” (see Figure 9-1) refers to the actual clock pulses counted by the device. However, there are always more clock pulses being counted than were applied by the base station. The reason for this is the fact that an RF field cannot be switched off immediately. The coil voltage decreases exponentially. So although the RF field coming from the base station is switched off, it takes some time until the voltage across the coils reaches the threshold value of an internal MOS transistor and the device detects the gap. Referring to the following diagram (see Figure 10-1 on page 8), this means that the device uses the times t 0 internal and t 1 internal . The exact times for t 0 and t 1 are dependent on the application (e.g., field strength, etc.). 9211A–RFID–10/10 7
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Standard Read/Write ID Transponder with Anti ... - Atmel Corporation

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