Servo - August 2008

//*******************************************************
//* DETERMINE SERVO ROTOR DIRECTION
//*******************************************************
always @(posedge clk_1mhz)
 if(window_32ms == 0)
 case(btn_input)
 2’b01: servo_vector <= servo_vector + 1;
 2’b10: servo_vector <= servo_vector - 1;
 2’b11: servo_vector <= 500;
 endcase

endmodule

The only thing I really need to explain is the Verilog case structure. Otherwise, the Verilog case block works just like the C case block. All of the possible btn_input input logic levels are represented except 2’b00, whose omission brands it as a “don’t care” or “do nothing” logic level. The “2’b” in each case choice represents the number of binary bits in the case comparison argument. The stated number of binary bits follows. In our application, each bit in the case comparison arguments represents a particular input pin. The logic we have implemented stops the servo rotor when both btn_input pins are logically low. The servo rotor is centered when both btn_input pins are logically high.

If you’re wondering how we know which btn_input pin is associated with which bit, the ISE WebPACK generates a pin list report like the one you see in Screenshot 3 with

The CPLD Servo Driver

every successful CPLD or FPGA design implementation pass. The most significant bit of our register and wire bit range patterns is represented by the left-most bit in the range declaration ([most significant bit:least significant bit]). Now you can see how the order of the btn_input bit range declaration pattern ([1:0]) relates to the Verilog case comparison arguments (2’b01, 2’b10, etc.) and to the XC2C64A I/O pins.

Rotating Out

Driving hobby servos is only one of the tricks a CPLD can perform. In addition to turning servo rotors, you can use a CPLD to expand the I/O capability of your host microcontroller. You can also use a CPLD to replace a number of discreet logic ICs in your next robotic design.

If you see CPLD logic in your robotic future, I’ve provided all of the source code we generated in this discussion as a download package from the SERVO website ( www.servomag azine.com). You can get the hardware scoop on the XC2C64A development board as a download package from the Nuts & Volts website ( www.nutsvolts.com). CPLDs are easy to understand and easy to design into your projects. So, gather up a couple of hobby servos, build up the XC2C64A Development Board, and drive some servos with a CPLD yourself. SV

Fred Eady can be reached via email at fred@edtp.com.

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