Original document: Quadrature Encoder Receiver Module : An Implementation on FPGA (.pdf)
In the original document cited above, we discussed some designs of quadrature encoder receiver module where Verilog code listings were included. This supplementary article provides the same implementations using VHDL. The development flows remain the same for both design A and B, so we basically list the codes and show simulation results here without repeating the design details.
For Design A, the receiver module generates pulses at output pin U (up) or D (down), corresponding to whether A leads B, or A lags B, respectively (*). We showed the design process using STG (State Transition Graph) that yielded the results in equation (1) – (4) . The VHDL code for this implementation is in Listing 1 with simulation result shown in Figure 1. (The synthesis and behavioral simulation was performed using Xilinx Webpack 13.4 and Isim simulation, which could be downloaded from Xilinx website.
(*) It turns out that in the original document, the Karnaugh map for output U and D shuffles, so as the resulting equations (3) and (4). You could verify from the simulation in Figure 9. If this gives the wrong sense for your motor setup, simply switch equation (3) and (4). We do so in the VHDL code below.