ATMega644P SPI SN74HC595 and SN74HC165 4x8 Matrix KeyPad Driving

In previous post I use these two SPI chips, SN74HC595 and SN74HC165 to expand the inputs/outputs of an MCU. Here I use these two chip pair to make a matrix keypad. These two 8-bit I/O expansion chip can build up to a 8x8 matrix keypad that yield 64 different keys. But I make only a 4x8 matrix keypad since it's not important.

Simulating Program

The 8-bit output port of the SN74HC595 is a column scanner while the lower nibble input port of the SN74HC165 is a row detector (active high). Whenever any key press found the program assign a specific key in its keypad array.

Source Code "main.c":

1. /*
2. * 12-spi_sn74hc165_sn74hc595n_keypad.c
3. *
4. * Created: 2/18/2026 2:03:34 PM
5. * Author : Admin
6. */
7. 
   
8. #include <avr/io.h>
9. #include <util/delay.h>
10. #define F_CPU 16000000UL
11. 
    
12. #define DDR_SPI DDRB
13. #define PRT_SPI PORTB
14. 
    
15. #define DD_SH 3
16. #define DD_SS 4
17. #define DD_MOSI 5
18. #define DD_MISO 6
19. #define DD_SCK 7
20. 
    
21. 
    
22. void SPI_MasterInit(void)
23. {
24. /* Set MOSI and SCK output, all others input */
25. DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK)|(1<<DD_SS);
26. /* Enable SPI, Master, set clock rate fck/16 */
27. SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
28. }
29. 
    
30. void SPI_MasterTransmit(char cData)
31. {
32. /* Start transmission */
33. SPDR = cData;
34. /* Wait for transmission complete */
35. while(!(SPSR & (1<<SPIF)))
36. ;
37. }
38. 
    
39. void SPI_SlaveInit(void)
40. {
41. /* Set MISO output, all others input */
42. DDR_SPI = (1<<DD_MISO);
43. /* Enable SPI */
44. SPCR = (1<<SPE);
45. }
46. char SPI_SlaveReceive(void)
47. {
48. /* Wait for reception complete */
49. while(!(SPSR & (1<<SPIF)))
50. ;
51. /* Return Data Register */
52. return SPDR;
53. }
54. 
    
55. const char key_16[4][8]={'1','2','3','4','5','6','7','8',
56. '9','0','A','B','C','D','E','F',
57. 'G','H','I','J','K','L','M','N',
58. 'O','P','Q','R','S','T','U','V'};
59. 
    
60. char key_scan(void){
61. char data=0,temp,key;
62. for(char i=0;i<8;i++){
63. SPI_MasterTransmit(1<<i);
64. PRT_SPI&=~(1<<DD_SS);
65. _delay_us(10);
66. PRT_SPI|=(1<<DD_SS);
67. _delay_ms(5);
68. PRT_SPI&=~(1<<DD_SH);
69. PRT_SPI|=(1<<DD_SH);
70. SPI_MasterTransmit(0);
71. data=SPI_SlaveReceive();
73. if(data==0x01) {temp=key_16[0][i]; break;}
74. else if(data==0x02) {temp=key_16[1][i]; break;}
75. else if(data==0x04){temp=key_16[2][i]; break;}
76. else if(data==0x08){temp=key_16[3][i]; break;}
77. else temp=0;
78. _delay_ms(10);
79. }
80. return temp;
81. }
82. 
    
83. int main(void)
84. {
85. /* Replace with your application code */
86. SPI_MasterInit();
87. lcd_init();
88. DDR_SPI|=(1<<DD_SH);
89. char data=0;
90. char temp,charCount=0,newLine=0,line=1;
91. lcd_text("ATMega644P SPI");
92. lcd_xy(1,2);
93. lcd_text("MATRIX KEYPAD");
94. _delay_ms(5000);
95. lcd_clear();
96. while (1)
97. {
98. temp=key_scan();
99. if(temp!=0){
100. lcd_data(temp);
101. charCount++;
102. _delay_ms(500);
103. }
104. if(charCount>16){
105. newLine=1;
106. charCount=0;
107. line+=1;
108. }
109. if(newLine){
110. newLine=0;
111. if(line==2) lcd_xy(1,2);
112. else{
113. lcd_xy(1,1);
114. lcd_command(0x01);
115. _delay_ms(5);
116. line=1;
118. }
119. }
120. _delay_ms(100);
122. }
123. }
124. 
     
125. 
     
126. 
     
127. 
     
128. 
     

The character LCD uses the parallel port of the ATMega644P. But we can control this LCD by adding one more SN74HC595 and one additional Slave Select (SS) pin. 

Simulating Program

I separate these two chip using different Slave Select pins. The SN74HC165 doesn't need serial data input. It only need serial clock and Slave Select pins controlled by the MCU. However I think we can cascade this two chip using a shared SPI pins. If I have more time I will test it using this method.

 

For a full tutorial page of ATMega644P Programming Using C please visit this page.