Documentation

Implementation of RAM

You can write MATLAB^® code that maps to RAM during HDL code generation by using:

The following examples model the same line delay in MATLAB. However, one example uses a persistent array and the other uses an hdl.RAM System object to model the RAM behavior.

The line delay uses memory in a ring structure. Data is written to one location and read from another location in such a way that the data written is read after a delay of a specific number of cycles. The RAM read address is generated by a counter. The write address is generated by adding a constant value to the read address.

For a comparison of the ways you can write MATLAB code to map to RAM during HDL code generation, and for an overview of the tradeoffs, see RAM Mapping Comparison for MATLAB Code. For more information, see Map Persistent Arrays and dsp.Delay to RAM.

Implement RAM Using a Persistent Array or System object Properties

This example shows a line delay that implements the RAM behavior using a persistent array with the function mlhdlc_hdlram_persistent. Changing a specific value in the persistent array is equivalent to writing to the RAM. Accessing a specific value in the array is equivalent to reading from the RAM.

You can implement RAM by using user-defined System object private properties in the same way.

%#codegen
function data_out = mlhdlc_hdlram_persistent(data_in)

persistent hRam;
if isempty(hRam)
    hRam = zeros(128,1);
end

% read address counter
persistent rdAddrCtr;
if isempty(rdAddrCtr)
    rdAddrCtr = 1;
end

% ring counter length
ringCtrLength = 10;
ramWriteAddr = rdAddrCtr + ringCtrLength;

ramWriteData = data_in;
%ramWriteEnable = true;

ramReadAddr = rdAddrCtr;

% execute single step of RAM

hRam(ramWriteAddr)=ramWriteData;
ramRdDout=hRam(ramReadAddr);

rdAddrCtr = rdAddrCtr + 1;

data_out = ramRdDout;

Implement RAM Using hdl.RAM

This example shows a line delay that implements the RAM behavior using hdl.RAM with the function, mlhdlc_hdlram_sysobj. In this function, the step method of the hdl.RAM System object reads and writes to specific locations in hRam.

%#codegen
function data_out = mlhdlc_hdlram_sysobj(data_in)
persistent hRam;
if isempty(hRam)
    hRam = hdl.RAM('RAMType', 'Dual port');
end

% read address counter
persistent rdAddrCtr;
if isempty(rdAddrCtr)
    rdAddrCtr = 0;
end

% ring counter length
ringCtrLength = 10;
ramWriteAddr = rdAddrCtr + ringCtrLength;

ramWriteData = data_in;
ramWriteEnable = true;

ramReadAddr = rdAddrCtr;

% execute single step of RAM
[~,ramRdDout] = step(hRam,ramWriteData,ramWriteAddr, ...
                      ramWriteEnable,ramReadAddr);

rdAddrCtr = rdAddrCtr + 1;

data_out = ramRdDout;