AVR Interfacing:Hex Kepad
    Revision as of 10:22, 24 March 2016 by Sandeep (Talk | contribs)

    Let's look at interfacing a Hex keypad, the one with 16 switches; arranged in 4 columns and 4 rows. The interesting challenge is to interface 16 switches on 8 pins. So how does this work? The simple answer is the MCU is faster than the your fingers.

    Basics

    Hook Up

    Code

    1. #include "keypad.h"
    2. #include "lcd.h"
    3. int main() 
    4. {
    5.     uint8_t key;
    6.  
    7.     /*Connect RS->PB0, RW->PB1, EN->PB2 and data bus PORTB.4 to PORTB.7*/
    8.     LCD_SetUp(PB_0,PB_1,PB_2,P_NC,P_NC,P_NC,P_NC,PB_4,PB_5,PB_6,PB_7);
    9.     LCD_Init(2,16);
    10.  
    11. 	/*Connect R1->PD0, R2->PD1, R3->PD2 R4->PD3, C1->PD4, C2->PD5 C3->PD6, C4->PD7 */
    12.     KEYPAD_Init(PD_0,PD_1,PD_2,PD_3,PD_4,PD_5,PD_6,PD_7);
    13.  
    14.     LCD_Printf("Key Pressed:");
    15.     while (1) 
    16.     {
    17.         key = KEYPAD_GetKey();
    18.         LCD_GoToLine(1);
    19.         LCD_DisplayChar(key);
    20.  
    21.     }
    22.  
    23.     return (0);
    24. }

    Let's look at the most important function in the code in a little more details 

    1.       key = KEYPAD_GetKey();

    As you can guess, the function return the ASCII value of the key being pressed. It follows the following sequences to decode the key pressed:

    1. Wait till the previous key is released.
    2. Wait for the new key press.
    3. Scan all the rows one at a time for the pressed key.
    4. Decodes the key pressed depending on ROW-COL combination and returns its ASCII value. 


    1. It is defined in keypad.c as below:
    1. uint8_t KEYPAD_GetKey(void)
    2. {
    3.     uint8_t i,j,v_KeyPressed_u8 = 0;
    4.  
    5.  
    6.     keypad_WaitForKeyRelease();
    7.     keypad_WaitForKeyPress();
    8.  
    9.     for (i=0;i<C_MaxRows_U8;i++)
    10.     {
    11.         GPIO_PinWrite(A_RowsPins_U8[i],HIGH); 
    12.     }
    13.  
    14.     for (i=0;(i<C_MaxRows_U8);i++)
    15.     {
    16.         GPIO_PinWrite(A_RowsPins_U8[i],LOW);
    17.  
    18.         for(j=0; (j<C_MaxCols_U8); j++)
    19.         {
    20.             if(GPIO_PinRead(A_ColsPins_U8[j]) == 0)
    21.             {
    22.                 v_KeyPressed_u8 = 1;
    23.                 break;
    24.             }
    25.         } 
    26.  
    27.         if(v_KeyPressed_u8 ==1)
    28.         {
    29.             break;
    30.         }
    31.  
    32.         GPIO_PinWrite(A_RowsPins_U8[i],HIGH);
    33.     }
    34.  
    35.     if(i<C_MaxRows_U8)
    36.         v_KeyPressed_u8 = A_KeyLookUptable_U8[i][j];
    37.     else
    38.         v_KeyPressed_u8 = C_DefaultKey_U8;
    39.  
    40.  
    41.     return v_KeyPressed_u8;
    42. }
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