Editing Use the VS1053b chip (section)

====== This chip is only present in the F256K2 or the F256Jr2. It isn't available on the earlier F256K nor the F256Jr. ======
Massive cleanup has to be done, code has to be tried.
Here's an info dump straight from discord

normally, let's assume we begin with a playback of a MP3, you read the file and you will need to dump like a 2K in before it tells you to stop. But, the FIFO inside the FPGA is also 2K, so in theory before things get ugly, you can upload in the MP3 playback system 2K+2K bytes, which ought to be enough to get it going. Now, if you monitor the FPGA FIFO info when empty you can put more in. Normally, after the VS1053 queue is full, it only need 32 bytes at the time and the FPGA takes care of that. Now, basically need to make sure the FPGA FIFO is topped off. Now, when come to end, you will need to fill extra bytes with 00 to empty the VS1053 Fifo and then the tune will end.

Now, the tricky parts is when one needs to stop the sound, there is also a process of filling the FIFO with 00 to cleanse the MP3 pipe. The specs talks about that.

This is as far as I went, now, I believe that if someone wants to play oggs and shit like that the controller need to be updated with new code since there are some issues in the latest tape out of the chip because remember people this is a DSP that actually can be programmed. There is actually an app note on how to use it to come out with Histogramic data (VUE bar app, with small LCD? anybody picking up on that? LOL) for one's pleasure

This is in a nutshell the process to playback stuff.
Here is the Register File Details for the VS1053 interface:

$D700..$D707

    case( CPU_A_i[2:0] )
        3'b000: begin CPU_D_o = {Busy, VS1053B_Registers[0][6:0]}; end 
        3'b001: begin CPU_D_o = VS1053B_Registers[1]; end 
        3'b010: begin CPU_D_o = VS1053B_Command_Read[7:0]; end 
        3'b011: begin CPU_D_o = VS1053B_Command_Read[15:8]; end 
        3'b100: begin CPU_D_o = Data_FIFO_Count[7:0]; end
        3'b101: begin CPU_D_o = {VS1053B_FIFO_Empty, VS1053B_FIFO_Full, 3'b000, Data_FIFO_Count[10:8]}; end
        3'b110: begin CPU_D_o = 8'h00; end 
        3'b111: begin CPU_D_o = 8'h00; end
    endcase

'''VS1053B_Registers[0] - Bit Fields: ($D700..$D703)'''

[0] 1: Start Transfer

[1] 1: Read Register, 0: Write Register

[7] 1: SPI Transfer in Progress [Busy]

'''VS1053B_Registers[1]''' 

[3:0]  Register to Address

'''VS1053B_Registers[2] Data Access Low'''

'''VS1053B_Registers[3] Data Access Hi'''

There are 2 Accessible Area for the VS1053
The Control/Register Section with 16bits wide Data and 16 Addresses (See specs)
The Other is the Data(Stream) Port, there is no Address and it is 8bits

When one wants to control the chip, one need to setup the address and data (for write) and address for read, one needs to setup the direction and when everything is ready, one needs to trigger the transaction.
When one needs to write file data for playback, one needs only to write data to FIFO port @ $D704 and monitor FIFO @ $D705/$D706 
Here is a piece of code, not the best, prolly not the most kosher either, but that ought to get you started.
I am simply playing back a very tiny sound effect (small enough to fit in the memory so not much to play back)

MP3_Playing: 

                lda #$42
                sta $d702   ; Set the Stream mode
                lda #$48
                sta $d703
                lda #$21    ; Start Transaction
                sta $d700
                lda #$20    ; Return to Zero
                sta $d700 

;                lda #$00
;                sta $d702
;                sta $d703
;                lda #$23    ; Go read the Command Register
;                sta $d700 
;                lda $d702 
;                lda $d703

                ldx #$00
VS_Pause:                
                inx 
                cpx #$80
                bne VS_Pause

                lda #$80
                sta $00     ; Let's enable the MMU Edit, keep the MMU 00 in play
                lda #$08    ; Bring about the first 8K 
                sta $0D     ; It starts @ $08 == page 0

                setaxl 

                ldx #$0000
                ldy #$0000
; Go Fill the FIFO
                setas
MP3_Fill_FIFO:  ;lda @l MP3_Music,x 
                lda $A000,x
                sta VS1053_STREAM_DATA      ; THere is 2K FIFO
                inx 
                iny 
                cpy #$0800                  ; Fill 2K
                bne MP3_Fill_FIFO

MP3_Fill_FIFO_Wait:
                ldy #$0000
                lda VS1053_FIFO_COUNT_HI    ; Load High Portion is monitor for 
                and #$80
                cmp #$80
                bne MP3_Fill_FIFO_Wait

                cpx #$2000
                bne MP3_Fill_FIFO
                ldx #$0000

                lda $0D
                inc A
                sta $0D 
                cmp #$0c 
                bne MP3_Fill_FIFO

                lda #$05
                sta $0D
MP3_We_are_Done:
                setaxs 
                rts