Basys2 board's built-in Oscillator - Page 1
Forum section: electronics / general

author: Abstr7ct
author_rank: Regular Contributor
timestamp: October 19, 2013, 12:13:10 pm
content: I'm working with Basys2 FPGA board and I'm having trouble figuring out the frequency of the built-in oscillator at location B8.I checked the reference manual of the board and found the following details on the oscillators:Quotehttp://www.digilentinc.com/Data/Products/BASYS2/Basys2_rm.pdfWhat I understand is that I must physically solder in one of the three jumpers at JP4, 25 MHz, 50 MHz or 100 MHz and this is how it's user-settable. I managed to implement clocked sequential circuits by mapping the input clock to 'the global clock input' at location B8, but I don't know at which frequency it oscillates and this is what I need to know.Hint: I don't have any equipments, neither an oscilloscope nor a frequency meter.

author: andersm
author_rank: Super Contributor
timestamp: October 19, 2013, 01:14:45 pm
content: Figure 5 in the manual shows that when no jumper pins are bridged, the frequency is 50MHz.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: October 19, 2013, 03:11:58 pm
content: Oh thanks, I've just been shown how to figure it out!The gray-shaded jumpers are soldered. Wondering why the frequency is solder-controlled rather than software-controlled.

author: andersm
author_rank: Super Contributor
timestamp: October 19, 2013, 03:32:02 pm
content: Look up the oscillator part number. The frequency is set by a resistor.

author: c4757p
author_rank: Super Contributor
timestamp: October 19, 2013, 04:02:19 pm
content: Not quite. They're using the fixed frequency version of LTC6905 (datasheethere, separate from the main one), which does not have a set resistor. The jumper controls pin 4 ("div"), which divides the frequency by 1 (short to vcc), 2 (open), or 4 (short to ground). That could have been done easily by software control.My guess is this: the LTC6905 uses a pair of 15uA current sources to bias the pin to detect when it's open. Perhaps the high impedance tri-state mode of the FPGA outputs isn't high enough, and there is an asymmetric leakage to one rail or the other, causing the pin not to float properly. In that case, they'd have had to switch using transistors on the output - but an unpopulated jumper is free.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: October 19, 2013, 06:39:53 pm
content: However the frequency is set electronically, I was making fun of the 'user-settable' frequency feature of the board. It would appropriate and user-friendly to make it software-controlled rather than forcing the user to do some messy solder on those jumpers to complete a line connection.

author: Fantasma25
author_rank: Regular Contributor
timestamp: October 19, 2013, 06:52:19 pm
content: I also use the Basys2. The default frequency when no jumpers are connected is 50MHz.

author: wornwinter
author_rank: Newbie
timestamp: October 24, 2013, 10:14:10 am
content: It's 50 MHz by default, and also terribly jittery. If you're planning on driving any sort of VGA monitor with it, I'd recommend shelling out for a proper crystal oscillator (goes in the DIP-8 socket). I had a horrible time trying to get any sort of stable image to display with the onboard oscillator, but the second I plugged in a crystal and used it as my clock input, it worked a treat!Anyways, fantastic little board. I've wasted plenty of hours tinkering with it and had a lot of fun. Next, to work out some sort of project involving it and my raspberry pi