SCBS881E January 2010 – October 2018 TMS3705
An SCI encoder performs the data transmission to the microcontroller. As the transmission rate of the transponder is lower than the SCI transmission rate, the serial bit flow received from the transponder is buffered after demodulation and before SCI encoding.
The SCI encoder uses an 8-bit shift register to send the received data byte-wise (least significant bit first) to the microcontroller with a transmission rate of 15.625 kbaud (±1.5 %), 1 start bit (high), 1 stop bit (low), and no parity bit (asynchronous mode indicated by the SYNC bit of the Mode Control register is permanently low). The data bits at the SCIO output are inverted with respect to the corresponding bits sent by the transponder.
The transmission starts after the reception of the start bit. The start byte detection is initialized with the first rising edge. Typical values for the start byte are 81_H or 01_H (at SCIO). The start byte is the first byte to be sent to the microcontroller. The transmission stops and the base station returns to the Idle state when TXCT becomes low or 20 ms after the beginning of the read phase. TXCT remains low for at least 128 µs to stop the read phase and less than 900 µs to avoid starting the next transmission cycle.
The SCI encoder also sends the diagnostic byte 2 ms after beginning of the charge phase. In case of a normal operation of the antenna, the diagnostic byte AF_H is sent. If no antenna oscillation can be measured or if at least one of the full-bridge drivers is switched off due to a detected short circuit, the diagnostic byte FF_H is sent to indicate the failure mode.
The SCI encoder can be switched into a synchronous data transmission mode by setting the mode control register bit SYNC to high. In this mode, the output SCIO indicates by a high state that a new byte is ready to be transmitted. The microcontroller can receive the 8 bits at SCIO when sending the eight clock signals (falling edge means active) for the synchronous data transmission through pin TXCT to the SCI encoder.