Reply by Theo Markettos April 23, 20142014-04-23
Tim Wescott <tim@seemywebsite.really> wrote:
> On Mon, 21 Apr 2014 05:33:13 -0700, Kshitiz Bartariya wrote: > > E.x. I am unable to understand why a 22ohm resistor is present in the > > clock signal (SCK,SCK# pins - W2, Y1 ) from A10s to SDRAM > > (h5tq2g83cfr-h9c) (CK, CK# pins - F7, G7). It is not given in either of > > their datasheets. > > In your specific case, in general the reason you see series resistors at > logic outputs is because circuit board traces act like transmission > lines, and a series resistor at an output knocks down the reflections and > hence ringing.
In this specific case, the value was picked either: 1) As a rule of thumb, because the designer didn't know any better or because it isn't a critical part of the design. or 2) Because they simulated the PCB and that value was what the simulation showed best matched the PCB layout. To do this you use tools like HyperLynx with IBIS (for example) models of the pins of the CPU, DRAM, etc. If this is the clock to DDR3 then I would hope approach 2) was taken. Theo
Reply by April 21, 20142014-04-21
On Mon, 21 Apr 2014 16:01:24 -0500, Tim Wescott
<tim@seemywebsite.really> wrote:

>On Mon, 21 Apr 2014 05:33:13 -0700, Kshitiz Bartariya wrote: > >> I am new to schematic design and studying OLinuXino A10s schematics. >> A generic question : >> >> How do we decide the resistor & capacitor values required to interface >> multiple ICs on a pcb ? >> >> E.x. I am unable to understand why a 22ohm resistor is present in the >> clock signal (SCK,SCK# pins - W2, Y1 ) from A10s to SDRAM >> (h5tq2g83cfr-h9c) (CK, CK# pins - F7, G7). It is not given in either of >> their datasheets. > >I repeat what John Speth said about getting an engineering education -- >there's an enormous amount of detail to learn. > >In your specific case, in general the reason you see series resistors at >logic outputs is because circuit board traces act like transmission >lines, and a series resistor at an output knocks down the reflections and >hence ringing.
You might also search for some old (1970's) ECL design handbooks. In those days 100 MHz was considered as real high tech. While 100 MHz is common these days, the physics has not changed, so check the old literature. This literature was originally written for engineers of the time without RF experience. In the search term, use "ECL 10000" and you will get quite a lot of hits, find one concerning PCB design.
Reply by Tim Wescott April 21, 20142014-04-21
On Mon, 21 Apr 2014 05:33:13 -0700, Kshitiz Bartariya wrote:

> I am new to schematic design and studying OLinuXino A10s schematics. > A generic question : > > How do we decide the resistor & capacitor values required to interface > multiple ICs on a pcb ? > > E.x. I am unable to understand why a 22ohm resistor is present in the > clock signal (SCK,SCK# pins - W2, Y1 ) from A10s to SDRAM > (h5tq2g83cfr-h9c) (CK, CK# pins - F7, G7). It is not given in either of > their datasheets.
I repeat what John Speth said about getting an engineering education -- there's an enormous amount of detail to learn. In your specific case, in general the reason you see series resistors at logic outputs is because circuit board traces act like transmission lines, and a series resistor at an output knocks down the reflections and hence ringing. And if all that "transmission line" and "reflections" stuff makes no sense, circle back to what John Speth said... If you _do_ have that degree and you're _still_ at a loss, feel free to ask specific questions, like "what is this resistor for?" -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by John Speth April 21, 20142014-04-21
On 4/21/2014 5:33 AM, Kshitiz Bartariya wrote:
> I am new to schematic design and studying OLinuXino A10s schematics. > A generic question : > > How do we decide the resistor & capacitor values required to interface multiple ICs on a pcb ?
Answering your generic question is not possible in this forum because the complete answer is huge. I suggest you start your electrical engineering course of study at a good university and then practice circuit design as a student and professional. It will take years to learn it. In your example, I'll bet the 22 ohm resistor is for impedance matching. It should tame the clock edges for a reliable clock distribution system. JJS
Reply by Kshitiz Bartariya April 21, 20142014-04-21
I am new to schematic design and studying OLinuXino A10s schematics.
A generic question :

How do we decide the resistor & capacitor values required to interface multiple ICs on a pcb ?

E.x. I am unable to understand why a 22ohm resistor is present in the clock signal (SCK,SCK# pins - W2, Y1 ) from A10s to SDRAM (h5tq2g83cfr-h9c) (CK, CK# pins - F7, G7). It is not given in either of their datasheets.