Digital blocks typically communicate with each other using bus protocols, a few examples of which includes AMBA AXI, WishBone, OCP, etc. Bus masters that send out data adhering to a certain protocol provide control signals that tell the slave when the packet is valid, and whether it is a read or write, and how many bytes of data is sent. The master also sends out an address followed by the data to be stored at that address.
Let's see a quick example where the testbench acts as the master and constrains the bus packet class object with valid data.
// Burst [ 0 -> 1 byte, 1 -> 2 bytes, 2 -> 3 bytes, 3 -> 4 bytes]
// Length -> max 8 transactions per burst
// Protocol expects to send only first addr, and slave should calculate all
// other addresses from burst and length properties
class BusTransaction;
rand int m_addr;
rand bit [31:0] m_data;
rand bit [1:0] m_burst; // Size of a single transaction in bytes (4 bytes max)
rand bit [2:0] m_length; // Total number of transactions
constraint c_addr { m_addr % 4 == 0; } // Always aligned to 4-byte boundary
function void display(int idx = 0);
$display ("------ Transaction %0d------", idx);
$display (" Addr = 0x%0h", m_addr);
$display (" Data = 0x%0h", m_data);
$display (" Burst = %0d bytes/xfr", m_burst + 1);
$display (" Length = %0d", m_length + 1);
endfunction
endclass
module tb;
int slave_start;
int slave_end;
BusTransaction bt;
// Assume we are targeting a slave with addr range 0x200 to 0x800
initial begin
slave_start = 32'h200;
slave_end = 32'h800;
bt = new;
bt.randomize() with { m_addr >= slave_start;
m_addr < slave_end;
(m_burst + 1) * (m_length + 1) + m_addr < slave_end;
};
bt.display();
end
endmodule
Simulation Log
ncsim> run ------ Transaction 0------ Addr = 0x6e0 Data = 0xbbe5ea58 Burst = 4 bytes/xfr Length = 5 ncsim: *W,RNQUIE: Simulation is complete.
