Re: Is 50 MHz doable for a complete newbie?
From: John Larkin (jjSNIPlarkin_at_highTHISlandPLEASEtechnology.XXX)
Date: 02/19/05
- Next message: CWatters: "Re: 1.5v to 3.3v LED circuit"
- Previous message: nanotech1: "name that wierd lc tank circuit ?"
- In reply to: Tobias Weingartner: "Is 50 MHz doable for a complete newbie?"
- Next in thread: Tobias Weingartner: "Re: Is 50 MHz doable for a complete newbie?"
- Reply: Tobias Weingartner: "Re: Is 50 MHz doable for a complete newbie?"
- Messages sorted by: [ date ] [ thread ]
Date: Sat, 19 Feb 2005 10:24:08 -0800
On Sat, 19 Feb 2005 05:25:21 +0000 (UTC), weingart@cs.ualberta.ca
(Tobias Weingartner) wrote:
>Hello all,
>
>I'm a newbie when it comes to actually designing circuits at much
>of anything above about 1MHz or so. And at that speed, you basically
>just slap them together, and they basically work.
>
>Now I'm looking at a circuit that will want to run at some 50MHz,
>a CPU, FPGA, some SDRAM... I will be doing a PCB, but are there
>other things I should be aware of? And good books/software, or even
>practices to make this as painless as the 1MHz stuff was? :-)
>
>Any help, any pointers? :-)
>
>
>PS: I've read the "black magic" book, and I understand the ideas that
>are presented within, but the actual application of most of those ideas
>are completely beyond me at the current time.
50 MHz with conventional CMOS logic parts isn't bad. But spend a few
bucks more for a 4-layer (or more, if you need it) board with a solid
ground plane and a single or split power layer. After that, just keep
all the traces short (don't trust an autorouter! They tend to give
your most critical signals the Grand Tour of the board.)
Sprinkle surface-mount bypass caps around, with vias to the ground
plane and the power plane/islands. Maybe four or so per chip per
supply voltage, and a few more here and there. Contrary to folklore,
you don't need one per power pin, nor do they have to be super-close
to the power pins. The best bypass cap is the capacitance between the
power and ground planes, so make that dielectric layer as thin as the
board house allows.
Plan your FPGA pinouts to minimize trace lengths and crossovers, and
contribute to the general beauty of the board.
Use series resistors (small surfmount arrays) in the lines to the
SDRAMS; try 33 ohms maybe, close to the CPU or FPGA driving the
memory.
Put test points (just pads with thru-holes big enough for a scope
probe tip to park in) on everything interesting: powers, clocks, chip
selects, ras/cas type things, one CPU port pin for timing subroutines.
No matter how many you put, you'll regret not having more. At speed, a
scope probe needs a good ground, so add a bunch of small (4-40 or
2-56) sized holes, grounded, and put in screws, pointing up, as places
to clip probe grounds.
Don't forget board mounting holes. Grounded, of course.
LEDs are always fun. We use SOT-23 led's to indicate successful FPGA
configuration, CPU heartbeat, power-on, stuff like that.
Which CPU and FPGA are you going to use? Bring out a few unused CPU
port pins and FPGA pins to test points.
John
- Next message: CWatters: "Re: 1.5v to 3.3v LED circuit"
- Previous message: nanotech1: "name that wierd lc tank circuit ?"
- In reply to: Tobias Weingartner: "Is 50 MHz doable for a complete newbie?"
- Next in thread: Tobias Weingartner: "Re: Is 50 MHz doable for a complete newbie?"
- Reply: Tobias Weingartner: "Re: Is 50 MHz doable for a complete newbie?"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
