Saturday 2 March 2019

Towards Dual SD-Card Support

The MEGA65 was intended from the outset to support multiple SD Cards / storage devices, and there is considerable provision made for this in our boot ROM, if not actual current support.  This takes the form of allowing multiple drives, and for each drive to indicate which storage device it has come from.

Full active support is still a while off, but what we have done today is to add support for two SD cards, and to have the boot ROM work out which one has an SD Card inserted.  This will get used in the R2 PCBs that are being designed at the moment.  But the pressing reason for this was to get the R1 PCB that Falk is using to develop GEOS back working, as a couple of FPGA pins have died on it that were responsible for the SD card interface.  We have rerouted a couple of unused pins that connect to the HDMI controller to get a physical connection:

I have then also modified the SD controller so that it has a multiplexer to select which of two SD Card busses is active at any point in time.  This is controlled in software by writing $C0 (for Card 0) or $C1 (for Card 1) to $D680, the SD Card command register.  This turned out to be more of a pain than it should have been, because things kept going strange for no apparent reason.  I had to refactor the multiplexer a couple of times until it was working reliably, even though the problems, as far as I can see, were nothing to do with the SD card interface.

This kind of thing happens more often than I would like when working with VHDL. I am sure some of the problems are subtle (or not so subtle) things that I have done wrong, but others seem to defy explanation.  This was one of those: I added simply a multiplexer for the SD card busses, and suddenly the MEGA65 had keyboard problems.

Anyway, after some considerable effort, we managed to get it mostly working, but then suddenly the keyboard stopped working at 40MHz again -- a problem we have seen before. But then after doing some other unrelated fixes to the SID, suddenly the keyboard is again working at 40MHz.  Hardware is annoying, sometimes.

What was then left was to add support to the boot ROM to work out which SD card slot to use (as mentioned, support for using both at once will come later). Basically we try to reset the SD card in slot 0, and if that fails, then we try resetting the other one:

   ; Work out if we are using primary or secondard SD card

                ; First try resetting card 0
                lda #$c0
                sta $d680
                lda #$00
                sta $d680
                lda #$01
                sta $d680

                ldx #$0f
                jsr sdwaitawhile

                lda $d680
                and #$03
                bne trybus1


                ldx #<msg_usingcard0
                ldy #>msg_usingcard0
                jsr printmessage


                jmp tryreadmbr
                bne @morewaiting

                lda #$c1
                sta $d680
                ldx #<msg_tryingcard1
                ldy #>msg_tryingcard1
                jsr printmessage


Whichever we choose is then the SD card used by the system until next reboot.  The freeze menu and FDISK needed to be patched to handle the bit that indicates which SD card is being used, but other than that, it was pretty uneventful, and now when you boot, you get a message that indicates which SD card bus is being used, in this case, bus 0:

So now we can send Falk his board back, so that he can finish working on the GEOS port for the MEGA65, which we are all very much looking forward to.

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