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.
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// 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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