Custom Logic Tutorial - Writing the Code

Our design requires two custom logic circuits:

• A clock pulse circuit – which takes as input a control signal from the Custom Instrument component. The circuit needs to generate a 1 clock cycle pulse (High) when this input control signal goes High.

• An accumulation circuit – which is enabled by the output of the clock pulse circuit being driven High. The accumulation circuit takes as input a new value specified on, and output by, the Custom Instrument component. It adds this new value to its internally-stored total and outputs the new accumulated total to the subsequent latch (D-type Flip-Flop).

Let's create the C source code for each of these required circuits, in two separate source files.

Code for the Clock Pulse Circuit

1. Right-click on the CHC_Accumulator.PrjFpg entry in the Projects panel and choose Add New to Project » C File. A new C source document is created (Source1.C) and opened as the active document in the main design window. Save this document with the name PulseGenerator.C, in the same location as the project file.
    

   All source C (and header) files containing functions to be exported must be added to the FPGA project. This allows the C-to-Hardware Compiler to find the files for subsequent HDL generation.

    
2. Type the following C code into the document (comments are optional but are always good to add for readability!):
    
   /*************************************************************************
|*
|* Function    : GeneratePulse
|*
|* Parameters  : GO_I, GO_PULSE_O
|*
|* Returns     : none
|*
|* Description : Generates a 1 clock cycle pulse when GO_I transitions
|*               from Low to High
*/
 
#include <stdbool.h>
 
static bool s_GoPrev = false;
 
void GeneratePulse(bool GO_I, bool* GO_PULSE_O)

     {

   
      *GO_PULSE_O = false;
   if (GO_I && (s_GoPrev == false))
      *GO_PULSE_O = true;
   s_GoPrev = GO_I;

     }

3. Save the document.

Code for the Accumulator Circuit

1. Right-click on the CHC_Accumulator.PrjFpg entry in the Projects panel and choose Add New to Project » C File. A new C source document is created (Source1.C) and opened as the active document in the main design window. Save this document with the name Accumulator.C, in the same location as the project file.
    
2. Type the following C code into the document:
    
   /********************************************************************
|*
|* Function    : Accumulate
|*
|* Parameters  : VALUE_I, TOTAL_O
|*
|* Returns     : none
|*
|* Description : Adds VALUE_I to the internal total. The total is
|*               placed in TOTAL_O.
*/
 
#include <stdint.h>
 
static uint32_t s_Total = 0;
 
void Accumulate(uint32_t VALUE_I, uint32_t* TOTAL_O)

     {

   
      s_Total += VALUE_I;
   *TOTAL_O = s_Total;

     }

3. Save the document.