MIDI
Here is an example of code that will allow the MICrODEC to receive MIDI data over the serial port. Because of the way interrupts are handled, it is difficult to have a generic MIDI interface in the Main loop which communicates with the Function programs. It is possible to do this, but it would cut down on the processing time within the Function programs. Therefore, to accommodate MIDI, each Function must have its own implementation. This has the net effect of maximizing processing time, program by program.
== Initialize the USART
This first section of code is an initialization routine for talking MIDI, and must be placed at the beginning of the Function program. This sets up the USART for the 31.25kbps data transfer rate, and sets the start bits and stop bits.
1 ; initialize USART for midi transfer and new jump address
2 ldi r30,$0f ; set new jump address
3 ; this value will need to be changed if there is more
4 ; initialization code that follows the USART
5 ; initialization.
6 ldi r17,$00
7 sts ucsr0a,r17 ; set USART to single speed
8 sts ubrr0h,r17 ; set USART to 31.25kbps baud rate for midi
9 ldi r17,$27
10 sts ubrr0l,r17
11 ldi r17,$06
12 sts ucsr0c,r17 ; set USART to 8 bit mode
13 ldi r17,$10
14 sts ucsr0b,r17 ; turn on USART reciever
15
1 ; get delay settings
2 lds r17,ucsr0a ; get USART control register
3 sbrs r17,rxc0 ; check if byte has been recieved
4 rjmp shift_000023 ; skip buffer read
5 lds r17,udr0 ; get data byte
6 brts update_000023 ; skip if command byte already recieved
7 andi r17,$f0 ; mask off channel number - recieve all channels for now
8 cpi r17,$b0 ; check if controller
9 brne shift_000023 ; do nothing if not correct command byte
10 set ; set t register to indicate correct command byte recieved
11 rjmp shift_000023 ; finish off
12
13 update_000023: ; update delay time with pitch bend value
14
15 sbrc r17,$07 ; check if msb is set - command byte
16 rjmp resetusart_000023 ; dont process data if command byte
17 tst r14 ; check if first byte
18 brne secondbyte_000023 ; skip to second byte if not
19 cpi r17,$47 ; check if controller number 71
20 brne resetusart_000023
21 inc r14 ; set byte counter to first byte recieved
22 rjmp shift_000023 ; finish off
23
24 secondbyte_000023: ; get second byte
25
26 lsl r17 ; shift 14b number to 16b number
27 ;lsl r0
28 ;rol r17
29 mov r27,r17 ; move data to desired delay register
30 ;mov r26,r0
31 cpi r27,$03 ; check if desired delay is less than 9ms
32 brsh resetusart_000023 ; finish off if delay time is greater than 9ms
33 ldi r27,$03 ; set desired delay to 9ms
34 clr r26
35
36 resetusart_000023: ; reset usart
37
38 clt ; reset command byte indicator
39 clr r14 ; reset byte counter
40