Duane Benson 6/26/2012 4:51:49 PM User Rank Blogger
Re: Progress
Adam - I just double checked and mine is a rev-A board and that the wire goes between pin 4 (VCC) and pin 32 (AVCC). I don't know what other changes happened between rev A and Rev B but based on the note on the schematic title page, there shouldn't be any functional differences if the board has been modded.
Duane Benson 6/26/2012 12:21:18 PM User Rank Blogger
Re: Progress
Adam - I do feel like I'm making progress and I agree. It is a neat little device. The chip with the mod wires is, I beleive, an Atmel MCU that acts as the USB interface for the board. The schematic diagrame notes "Floating Avcc pin 32 was connected to +3V3_USB_A for Rev B." So, I think the red wire fixes an oopsie.
Duane Benson 6/25/2012 3:46:50 PM User Rank Blogger
Re: Why 680-ohm resistors?
Hi Brian - Thanks for the kind words. I'm glad this is helping.
As for the 680 ohms, it's somewhat of a personal choice. In reality I could likely have gotten away with no resister at all for this board. If I read their schematic right, they don't have any current limiting resistors on those 1.8 volt outputs. If the LED drops about that much and draws, say, 20 mA, then it should be okay without the resistor.
That being said, I'm never comfortable without some sort of a current limiting resistor. 680 ohms is large, but it will light up at 1.8 volts (at least with the LEDs I had on hand) and will also work fine in 5v, 9v and 12v applications. With the higher resistor, I have the flexibility of using them for test in other projects and I can use lots of them without ever worrying about jumping over the total current limit of an MCU or FPGA I'm fiddling with.
Minor question, but why did you choose 680-ohm resistor values for your current-limiting resistors?
We learned from your More About the UCF blog that the "LVCMOS18" identifier means 1.8V. So, using 680-ohms limits the current to ~2.65mA (nominal). That seems a little low (reads: a dim LED).
Duane has decided that the time is ripe to get his ZedBoard bolted onto his robot with a Linux distribution up and running. That was the ultimate plan anyway, so why wait?
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