I'm building a simple motor shield for Arduino based on the L293D bridge driver. I'm doing some tests on a breadboard everything is fine as long as I drive the motor at full throttle with DC inputs (enable HIGH, inputs HIGH and LOW to spin CW/CCW).
I can regulate the speed applying a TTL PWM signal on the enable pin with a function generator. It works fine but using the oscilloscope I can see the TTL signal drop from 5 to 3,5/4 Volts as soon as I connect it to the L293D enable pin, even with no motor attached. Why is that happening? Is that supposed to be? I thought an enable pin should draw no current and cause no voltage drop.
I tried replacing the IC and the function generator but I still get that behavior. I get no voltage drop using a regular PS and 5V DC.