Hello EEVBlog,
First post here. I'm Cody, I love the videos and what is being done here, I just didn't want to clutter up your forum until I had a real question to ask. My background is in programming and 3D animation, but I've always wanted to learn electronics so when I found the channel I fell in love with it pretty much instantly.
OK my issue, I have two microcontrollers, the
VEX Cortex and a
Raspberry Pi which I am hoping to use together, so I developed this enclosure to mate the two:
So for the electronics bit, there is a little switch on the Cortex that turns it on (it's powered by a 7.2v battery), and I essentially want the Raspberry Pi to turn on when that switch is pulled as well. Now I cannot modify the Cortex, this is for competitive robotics and we aren't allowed to mess with the Cortex. But there are a host of 5v pins that go hot when this thing turns on, a bunch are part of PWM connectors but there are UART ports that have both ground and +5v pins as well, so for simplicity reasons I want to use them.
Now the Pi will be powered by another battery (a 9.6v battery), the reason for this is current related. The Cortex has a 1A breaker in it, and we need all of that 1A to power motors so the Pi with a 200-400 mA draw needs to be powered separately. Our competition lets us use either another 7.2v battery or a 9.6v battery to power sensors and co-processors so this worked out well.
The original idea was to use a relay, but I've had issues sourcing one that fits and plays nice so I was looking for alternatives when I found
this. It's a power regulator from Pololu with a shutdown pin. Now we already needed a regulator for the Pi so my thought was that this shutdown pin on this regular might just save the day, so I started reading the product page and I found this:
The SHDN pin can be driven low (under 0.4 V) to power down the regulator and put it in a low-power state. The quiescent current in this sleep mode is dominated by the current in the 100k pull-up resistor from SHDN to VIN. With SHDN held low, this resistor will draw 10 µA per volt on VIN (for example, the sleep current with a 5 V input will be 50 µA). The SHDN pin can be driven high (above 1.2 V) to enable the board, or it can be connected to VIN or left disconnected if you want to leave the board permanently enabled.
So I found myself confused on that part at the end there. Basically in a nutshell, I don't understand the difference between not connected at all, and being driven low. I talked to my dad about it (he has some old college background with electronics) and he tried to explain it, but my understanding is really fuzzy.
So EEVBlog, what is the difference between not connected and being driven low? Also, since the switch on the Cortex probably kills the connection to it's battery, will a +5v pin from the Cortex, connected to this shutdown pin on the regulator be considered "low" or not connected?
Thank you so much in advance! -Cody