Asynchronous communication 
Author Message
 Asynchronous communication

Hi all,

I need to recive external data asynchron to my design serial with a 25 Mhz
clock. My design runs up to 100 Mhz. Serial data reveived is vaild when
another line (load) is low. To convert serial data to paralel byte i did the
following:
- sampled data and load with external 25 Mhz clock
- sampled data, load and external 25 Mhz clock with my 100 Mhz clock
- detect rising edge of external clock (now is syncronized)
- strobe data and load to a shift register using infered edge

Is this safe? Is another way to do this?

Thanks in advance.

Florin



Sat, 06 Aug 2005 06:42:14 GMT  
 Asynchronous communication
Quote:
>I need to recive external data asynchron to my design serial with a 25 Mhz
>clock. My design runs up to 100 Mhz. Serial data reveived is vaild when
>another line (load) is low. To convert serial data to paralel byte i did the
>following:
>- sampled data and load with external 25 Mhz clock
>- sampled data, load and external 25 Mhz clock with my 100 Mhz clock
>- detect rising edge of external clock (now is syncronized)
>- strobe data and load to a shift register using infered edge

>Is this safe? Is another way to do this?

You'll need at least 2 FFs in your synchronization of the 25 MHz to your 100
MHz to handle metastability.  
Load should occur when data is stable. You may wnat to consider an edge that
insures that data is statble during the load signal.  Idially, you want LOAD
near the middle or 3/4 down of a data sample (i.e., falling edge of 25MHz, if
data is on rising edge).
Do some analysis.

----------------------------------------------------------------------------
Ben Cohen     Publisher, Trainer, Consultant    (310) 721-4830  

Author of following textbooks:
* Real Chip Design and Verification Using Verilog and VHDL, 2002 isbn
0-9705394-2-8
* Component Design by Example ",  2001 isbn  0-9705394-0-1
* VHDL Coding Styles and Methodologies, 2nd Edition, 1999 isbn 0-7923-8474-1
* VHDL Answers to Frequently Asked Questions, 2nd Edition, isbn 0-7923-8115
------------------------------------------------------------------------------



Sat, 06 Aug 2005 08:16:47 GMT  
 
 [ 2 post ] 

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