Midi to control voltage converter

I've started working on this little circuit which will allow me to use a MIDI keyboard as control voltage for my VCO. Since MIDI is a serial protocol, I will need a microcontroller to interpret these signals. I will also need a DAC to get my control voltages. The DAC needs to be of the higher bit-type, since the VCO will need careful tuning to sound nice. I didn't find any simple microcontroller with more than 5 bits DAC, which is all little small. I did however find a nice cheap combo. A 12-bit I2C controlled DAC, MCP4725, and a 8 pin microcontroller with built in support for both UART and I2C, PIC12F1822. The DAC needs a voltage reference as both reference and supply, ZRC400F01 seems to work. All components are on the cheap and costs me less than 3€ for the lot. DAC and reference are small, surface mount components, so they are trickier to prototype with.

Midi 2 VCO converter, first steps. 

DAC

The DAC part can be seen to the right in the schematic. I've tested it out using an Aardvark I2C host adapter I borrowed from work, and it seems to work fine. When I choose the voltage reference, I thought that the 4.096V reference would fit well with a 12 bit DAC (0.001V/bit), but the DAC isn't perfectly rail to rail, so I effectively get a 0-4V DAC. I might just upgrade this with a 5V reference instead. Ideally it would be better with a higher voltage DAC since I intend to build a 1 octave/V VCO, and 0-4 V only gives me 3 octaves (4 if I upgrade to 5 V).

Microcontroller

Microcontroller  is seen to the left in the schematic. The symbol for the PIC doesn't have the correct pin descriptions, but is generally the same as the one I use, regarding supply, IO's and programming pins. I made a hello world program that blinks an LED using interrupts. This may seems simple, but it gives me a starting point where I know that IO's, clocks and interrupts are working. 

Next thing will be getting some UART communication working, and connecting it to the I2C bus.

Some Hello World!-action. Microcontroller to the right, DAC and reference to the left.

Microcontroller source code

1:  //Hello world with LED connected to RA0 on a PIC12F1822  
2:    
3:  #include <xc.h>  
4:    
5:  //Configuration bits  
6:  #pragma config WDTE=OFF, PWRTE = OFF, MCLRE=OFF, BOREN=OFF, FCMEN=OFF, CLKOUTEN = OFF, IESO=OFF, FOSC=INTOSC, CPD=OFF, LVP = ON, BORV = 0  
7:    
8:    
9:  void main()  
10:  {  
11:    //0 Internal oscillator, 3 <fosc> on, 6-4 31kHz  
12:    OSCCON = 0b00000000;  
13:    
14:    //Interrupt controller  
15:    //5 Timer0 overflow interrupt enabled  
16:    //7 Global interrupt enabled  
17:    INTCON = 0b10100000;  
18:    
19:    //Timer0  
20:    TMR0 = 0x00;  
21:    
22:    //Option Register  
23:    //2-0 Prescaler 1:4  
24:    //3 prescaler assigned to Timer0  
25:    //4 timer edge, high to low  
26:    OPTION_REG = 0b11010001;  
27:    
28:    //TRISA  
29:    //All A I/O's are set as outputs  
30:    TRISA = 0b0000000;  
31:    
32:    //PORTA  
33:    //RA0 is set  
34:    PORTA = 0b00000001;  
35:    
36:    //Loop forever, interrupts when Timer0 overflows  
37:    while(1)  
38:    {  
39:    }  
40:    
41:  }  
42:    
43:  //Interrupt routine  
44:  interrupt void isr (void)  
45:  {  
46:    //Ghost register of PORTA  
47:    int GPORTA;  
48:    
49:    //Read latch A into ghost register  
50:    GPORTA = LATA;  
51:    //Toggle bit 0  
52:    GPORTA = GPORTA^(1<<0);  
53:    //Write back into port register  
54:    PORTA = GPORTA;  
55:    
56:    //Clear interrupt flag  
57:    TMR0IF = 0;  
58:      
59:    return;  
60:  }  
61: