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10:47 2010-05-20

From: Ed Jastrzembski <***@jlab.org>                                                                 

Subject: Re: 150ns deadtime in FADC data triggers?                                                            

To: Brad Sawatzky <***@jlab.org>

CC: David Abbott <****@jlab.org>, Hai Dong <****@jlab.org>                                                
                                                                                                              
 I checked on the default values for the internal trigger register and                                         
 burst guard registers:                                                                               
 For every internal trigger:                                                                                   
  Trigger width = 48 ns                                                                                         
  Hold off after trigger = 40 ns.                                                                               
  This should give a dead time of 88 ns. However, there may be several 4                                        
  ns clock cycles used to synchronize the communication between my FPGA                                         
  and Hai's - this will add to the 88 ns.. I'll check on this.                                                  
                                                                                                              
 If the burst guard is enabled (default values may be loaded by writing                                        
 a '1' to bit 17 of the RESET register):                                                                       
 A total of 5 triggers are allowed to occur within any 2048 ns window.                                         
 If more triggers than this occurs, a dead time of 8192 ns is forced so                                        
 that the internal pipelines will not overflow.                                                                
                                                                                                              
On Thu, 20 May 2010, Ed Jastrzembski wrote:
                                                                                                             
 This dead time does not apply to the scalers. - Ed J.                                                      
 
On Thu, 20 May 2010, David Abbott wrote:                                                                  
                                                                                                                                                                                                                   
 There is a minimum time the FADC needs to "accept" and buffer one                                       
 trigger for processing. This is as I recall about 70-80ns. This is                                        
 true regardless of whether the trigger is external or internally
 generated.                                                                                               

 Now there are certain controls we have provided for the internal                                        
 trigger (widths and delays like you would have with NIM electronics)                                     
 so that you can have some control these triggers.                                                        
 There is one general register (itrig_cfg) which takes two 16 bit                                        
 parameters - width and holdoff (in clock ticks - 4ns/tick) that                                          
 define the width of the internal trigger pulse and then a deadtime                                       
 before the FADC will accept another. The sum of these two values                                         
 will define the trigger deadtime for the FADC.                                                           
 I believe this register comes programmed with default values. Ed                                        
 would know what they are. but 150 ns sounds like it would be about                                       
 right. In practice the sum of these two cannot be less than 80ns or                                      
 the FADC will choke in some strange ways...                                                              

*[http://hallaweb.jlab.org/equipment/moller//wiki/fadc/plots/TrigTime1371.png Trigger time distribution for run# 1371 ]
*[http://hallaweb.jlab.org/equipment/moller//wiki/fadc/plots/TrigTime1371_d.png  zoomed ]