The next step in my homebrew computer adventures is the Cosmac Elf. The Cosmac Elf was a DIY computer that appeared in Popular Electronics in 1976 and 1977 using the RCA 1802 microprocessor.
The schematics that I used are here. ELF Schematic 1 Elf Schematic 2
Other good resources can be found here:
I was able to find all of the parts except for the TIL311 displays. Well, I could find them, but the cheapest I saw was $30 per display, so I ended up using regular 7-segment displays with an arduino driving them. The arduino code is below.
// Seven Segment Display Connections
#define A_SEGMENT 2
#define B_SEGMENT 3
#define C_SEGMENT 4
#define D_SEGMENT 5
#define E_SEGMENT 6
#define F_SEGMENT 7
#define G_SEGMENT 8
#define DIGIT_1_GND 9
#define DIGIT_2_GND 10
// Data Bus Pins
#define bus0 11
#define bus1 12
#define bus2 13
#define bus3 A0
#define bus4 A1
#define bus5 A2
#define bus6 A3
#define bus7 A4
int COUNT = 0;
void setup() {
TIMSK2 = (TIMSK2 & B11111110) | 0x01;
TCCR2B = (TCCR2B & B11111000) | 0x05;
pinMode(A_SEGMENT, OUTPUT);
pinMode(B_SEGMENT, OUTPUT);
pinMode(C_SEGMENT, OUTPUT);
pinMode(D_SEGMENT, OUTPUT);
pinMode(E_SEGMENT, OUTPUT);
pinMode(F_SEGMENT, OUTPUT);
pinMode(G_SEGMENT, OUTPUT);
pinMode(DIGIT_1_GND, OUTPUT);
pinMode(DIGIT_2_GND, OUTPUT);
pinMode(bus0, INPUT);
pinMode(bus1, INPUT);
pinMode(bus2, INPUT);
pinMode(bus3, INPUT);
pinMode(bus4, INPUT);
pinMode(bus5, INPUT);
pinMode(bus6, INPUT);
pinMode(bus7, INPUT);
}
void loop() {}
void read_bus() {
byte value = 0;
bitWrite(value, 0, digitalRead(bus0));
bitWrite(value, 1, digitalRead(bus1));
bitWrite(value, 2, digitalRead(bus2));
bitWrite(value, 3, digitalRead(bus3));
bitWrite(value, 4, digitalRead(bus4));
bitWrite(value, 5, digitalRead(bus5));
bitWrite(value, 6, digitalRead(bus6));
bitWrite(value, 7, digitalRead(bus7));
String output_string = '0' + String(value, HEX);
output_string = output_string.substring(output_string.length() - 2, output_string.length());
// alternate writes to each display
if ((COUNT & 0x01) == 0) {
write_char(output_string.charAt(0), 2);
write_char(output_string.charAt(1), 1);
} else {
write_char(output_string.charAt(1), 1);
write_char(output_string.charAt(0), 2);
}
COUNT += 1;
}
ISR(TIMER2_OVF_vect) {
// time based interrupt routine
read_bus();
}
void write_char(char character, int digit) {
byte char_val;
switch (character) {
case '0':
char_val = 0b11111100; // 0
break;
case '1':
char_val = 0b01100000; // 1
break;
case '2':
char_val = 0b11011010; // 2
break;
case '3':
char_val = 0b11110010; // 3
break;
case '4':
char_val = 0b01100110; // 4
break;
case '5':
char_val = 0b10110110; // 5
break;
case '6':
char_val = 0b10111110; // 6
break;
case '7':
char_val = 0b11100000; // 7
break;
case '8':
char_val = 0b11111110; // 8
break;
case '9':
char_val = 0b11110110; // 9
break;
case 'a':
char_val = 0b11101110; // A
break;
case 'b':
char_val = 0b00111110; // B
break;
case 'c':
char_val = 0b10011100; // C
break;
case 'd':
char_val = 0b01111010; // D
break;
case 'e':
char_val = 0b10011110; // E
break;
case 'f':
char_val = 0b10001110; // F
break;
default:
break;
}
digitalWrite(A_SEGMENT, (char_val & 0b10000000) >> 7 );
digitalWrite(B_SEGMENT, (char_val & 0b01000000) >> 6 );
digitalWrite(C_SEGMENT, (char_val & 0b00100000) >> 5 );
digitalWrite(D_SEGMENT, (char_val & 0b00010000) >> 4 );
digitalWrite(E_SEGMENT, (char_val & 0b00001000) >> 3 );
digitalWrite(F_SEGMENT, (char_val & 0b00000100) >> 2 );
digitalWrite(G_SEGMENT, (char_val & 0b00000010) >> 1 );
if ( digit == 1 ) {
digitalWrite(DIGIT_1_GND, LOW);
digitalWrite(DIGIT_2_GND, HIGH);
} else if ( digit == 2 ) {
digitalWrite(DIGIT_1_GND, HIGH);
digitalWrite(DIGIT_2_GND, LOW);
}
delay(2);
}
The code runs a timer based interrupt and each pass reads the data from the bus, and outputs the correct character to the 7-segment display. That’s all it does really. There are probably better ways to do this, but this works and I didn’t spend too much time on it. You can see the arduino hiding in the hideous mess of wiring below.
Below is a quick video demo of a program that reads the input from the switches and outputs them on the 7-segment displays.
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