Electronic – Oscilloscope displaying two voltage levels (on the same channel) for square wave

500kHz is a ways past the maximum specified frequency for square waves for this generator: 100kHz. It appears to have a single-pole RC of about 3.5µs, which would work great for a 0.35/(3.5µs)=100kHz square wave. The output may have a LPF for slew limiting. Also, it is a 50Ω source, so it should be terminated properly to avoid ringing. Try using the TTL and CMOS waveforms, too. B&K have put together this document: Function & Arbitrary Waveform Generator Guidebook .

The external CMOS inverter is not a 50Ω source -- it's source impedance is only a few ohms at most (for low currents) due to V_CC and ground impedances and FET R_ON equivalent resistances. Notice that the output duty cycle isn't 50%, and the edges are ringing.

There are a couple of issues with your code that can be easily fixed. First, you are calling wiringPiSetup which means you will need to refer to the pins by their WiringPi designation which can be found on this page:

https://projects.drogon.net/raspberry-pi/wiringpi/pins/

You are then setting pin 7 (wiringPi designation) to be an output which is actually GPIO 4 (physical pin 7) and not the CE1 pin as you mentioned. CE1 would actually be wiringPi pin 11.

The next problem is in the while loop. You are doing a digitalWrite on wiringPi pin 4 (not 7 as you configured) and are always setting it low so it would never toggle high even if configured correctly. The following code should be what you are looking for but is not tested. This assumes you want to use CE1 which is normally used for SPI but should work with WiringPi as an general I/O pin:

  if (wiringPiSetup () == -1)
    exit (1) ;

  pinMode(11, OUTPUT);

  while(1) {
    digitalWrite(11, 0);
    digitalWrite(11, 1);
  }