Honored Contributor
Honored ContributorHi,
the RS-232 interface is defined as +/-6V to +/-12V for logic "0" and "1". Thus these voltages (being negative and far more than the I/O voltages the FPGA can deal with) cannot be connected w/o RS-232 to logic level converters like the MAX232 or similar devices. It might be a "workaroung" (in case the sensor can send data w/o being configured, as I assume for you wrote "I only wish to receive data") that you connect the TxD line (transmit data) from the sensor via a serial resistor and a zener diode at the FPGA pin (referenced to FPGA GND) to ensure the level does not exceed the I/O voltage and that it does not fall below max. negative of the FPGA I/O voltage (for this a schottky diode in parallel to the zener diode would be recommended). In fact, after configuration turning the clamping diode in the I/O structure would eliminate the zenerdiode (not the serial resistor and the shottky to GND), but for that the sensor may not operate prior the FPGA has configured (unconfigured / during configuration the clamping diode is not active!) I would first check this resistor/diode network for matching level rather connecting it directly to the FPGA. If the transmission rate is too high, the capacitance of the zener diode may cause timing issues as the signal at the FPGA rises too slow - one more thing to check for. To be honest - comparing costs for resistance, zenerdiode and shottky by a simple RS-232 to logic converter... It might not be worth the effort. And - the converter IC allows you to send data as well (you don't want to - now...) If this is not a serial production design requiring optimization to the last "tenth of a cent", I would recommend using the converter... Best regards :-)
