Discussion in 'General Electronics Chat' started by mikebdawg, Feb 27, 2011.

  1. mikebdawg

    Thread Starter New Member

    Feb 27, 2011
    I have a Trimble Thunderbolt GPS device with a 1PPS output, and I am looking to connect it to a 3.3V LVCMOS input on an FPGA, and I'm wondering the best way to do this. The inputs on the FPGA can't take the 5V.

    I'm not sure what the following line means from the datasheet:

    "BNC Connector TTL levels into 50 Ω
    10 microseconds-wide pulse with the leading edge synchronized to UTC within 20 nanosec- onds (one sigma) in static, time only mode. The rising time is <20 nanoseconds and the pulse shape is affected by the distributed capacitance of the interface cable/circuit."

    What does "TTL levels into 50 Ω" mean? Do I need to be concerned with this? I am using a very short cable length.

    To go from 5V of this output to my 3.3V input, I was thinking of either using an NPN transistor to switch between a pull-up resistor to 3.3V and GND or using a resistor dividor to bring the 5V down to 3.3V. Any other suggestions for this?

    Are there any other concerns I need to think about?