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AR# 23346

8.1 AccelDSP Synthesis Tool - When Is a Persistent Variable Mapped to a Register?

Description

In AccelDSP, sometimes MATLAB persistent variables are latched on a clock edge and sometimes they are not. Why?

Solution


Whether or not a MATLAB variable is latched on a clock edge is dependent upon the data flow within the MATLAB statement list. Thus, if one has MATLAB of the form 
 
[c] = my_func(a); 
persistent b; 
if isempty(b) 
b = 0; 
end 
b = a; 
c = b; 
 
then "b" need not be mapped to a FF/Latch, even though "b" is a persistent variable. In this simple example, the data flow does not require any value to be "stored" in order to determine the output "c." 
 
However, if the last two lines are switched, we have 
 
[c] = my_func(a); 
persistent b; 
if isempty(b) 
b = 0; 
end 
c = b; 
b = a; 
 
In this MATLAB, the assignment to "c" requires a "previous" value of "b." Then "b" is overwritten after the assignment to "c." In this case, the persistent variable "b" will be latched on the clock edge so that we can retain its value for the "next" call to the function, "my_func." The key is the order of the MATLAB statements. In this case, "b" must be persistent in order to simulate in MathWorks.
AR# 23346
Date Created 09/04/2007
Last Updated 05/20/2014
Status Archive
Type General Article