APS/Labs/07. Datapath/board files

Testing processor_system on FPGA

After verifying the design through simulation, you need to test it on an FPGA prototype.

Instructions for implementing the prototype are described here.

Fig. 1 shows the structure of the FPGA prototype.

Figure 1. Block diagram of the nexys_processor_system module.

The prototype allows step-by-step execution of the program loaded into instruction memory. It also displays the operation currently being executed.

Note

The instance of the processor_core module inside the processor_system module must be named core. That is, the instantiation line must look as follows: processor_core core(...).

Description of the peripherals used

• Buttons

  • BTND — when pressed, generates a clock pulse delivered to the clk_i port of the design module. Note that instructions that access external memory require several clock cycles to complete.
  • CPU_RESET — connected to the rst_i input of the design module. Since processor_system uses synchronous reset (i.e., the reset signal is only sampled on the rising edge of the clock), resetting processor_system and its submodules requires holding the reset button while also pressing the clock button.

• Seven-segment displays

  The seven-segment displays are divided into 2 groups (see Fig. 1):

  • PC — displays the lower 16 bits of the program counter as a hexadecimal number, derived from the instr_addr_o output of the processor core module.
  • operation — displays the operation currently being executed by the processor.

Operations displayed by the prototype

The prototype determines the operation type from the lower 7 bits of the instruction.

If the operation type is legal within the processor implemented in the lab assignments, the corresponding opcode is displayed. Opcodes are defined in decoder_pkg.sv. If the operation type determined by the prototype is illegal, the seven-segment displays show ILL (from illegal).

The mapping between operations and their representation on the seven-segment displays is shown in Fig. 2:

Figure 2. Mapping between operations and their representation on the seven-segment displays.

Demo program

The recommended demo program is program.mem. A description of its operation can be found in the #task section.