VE270-Lab 6 Design of a Keypad Reader Solved

1. Objective
 

To design a Finite State Machine that reads the keys from a 4-by-4 keypad and displays the corresponding hexadecimal value on an SSD.

 

2. Background
 

Keypad readers are used to interpret data entered from keypads for digital devices such as phones, calculators, digital lock, etc. A keypad reader decodes a pressed key and outputs the corresponding binary code. Figure 1 shows the connection between a hexadecimal keypad and the keypad reader circuit.

 

When a button is pressed, it connects a row and a column at the location of the button. By providing a 1 to a column of the keypad, a 1 will be received from the connected row. In other words, the pressed key creates a loop for the signal 1 to travel from the keypad reader, through the loop created by the pressed key, and back to the keypad reader. 

 



 

Figure 1. Keypad Reader

 

The keypad reader must be able to fire a column line to detect the location of a pressed button. A keypad reader detects a pressed key in three steps: 

(1)   Detects whether a key is pressed; 

(2)   Identifies which key is pressed;

(3)   Generates the corresponding 4-bit hexadecimal code for the pressed key. 

 

Step (1) can be achieved by outputting “1111” on the col output, so that one of the row inputs will be asserted whenever there is a key pressed. An OR logic of all the row inputs (OR_row) will be used to indicate whether any row is high. If the keypad reader detects a high on the OR_row output, it performs step (2) in which the col outputs will be turned on sequentially. This will tell which row and column are connected. Thus, the pressed key can be located. Step (3) will be done by looking it up in Table 1. 

Table 1. Keypad code for the Hex keypad

Key
row (3 : 0)
col (3 : 0)
code (3 : 0)
0
0001
0001
0000
1
0001
0010
0001
2
0001
0100
0010
3
0001
1000
0011
4
0010
0001
0100
5
0010
0010
0101
6
0010
0100
0110
7
0010
1000
0111
8
0100
0001
1000
9
0100
0010
1001
A
0100
0100
1010
B
0100
1000
1011
C
1000
0001
1100
D
1000
0010
1101
E
1000
0100
1110
F
1000
1000
1111
 

This keypad reader can be designed as a Finite State Machine. The key detecting algorithm is captured in Figure 2. The machine begins in State0, with all of the col outputs asserted, until one of the row inputs is asserted. From State1 to State4, only one of the col outputs is asserted. If one of the row inputs is also asserted, the pressed key is located. An output code can be determined by the combination of col outputs and row inputs. The FSM then moves to State5 where it outputs the code with all the col outputs asserted until the OR_row is de-asserted which indicates the key is released. 



 

Figure 2. State diagram of the keypad reader

Note: use a 1 Hz clock to trigger the state machine, and use LEDs on the board to display the current state of the FSM.

 

3. Verilog Coding
 

Write a Verilog module for the Kaypad Reader as an FSM following the state diagram shown in Figure 2. The FSM should have an asynchronous reset. Connect the SSD Driver developed in Lab 3 to display the interpreted hexadecimal value.

 

4. Simulation, Synthesis, and FPGA Implementation
 

Simulate the top-level integrated circuit using HDL Bencher. Synthesize and implement your design on the Basys 3 FPGA board.