R32V2020 SD Card Interface

Based on Neal Crook's improvements to Grant Searle's Multicomp project.

• Not all FPGA cards have built-in SD Card sockets
  • On other cards, should be able to wire up to IO pins (if there are any)
• SD Card Test Application dumps SD Card to the ANSI VGA screen
• Original VHDL code was based on the Multicomp design by Grant Searle
• Enhanced by Neal Crook to support SDHC cards
• SDHC can be upto 32GByte but this design can only access the low 8GByte
• Code for R32V2020 RISC CPU

; SD Card base address is x1000
; Register Addresses
;    0x1000    SDDATA        read/write data
;    0x1001    SDSTATUS      read
;    0x1001    SDCONTROL     write
;    0x1002    SDLBA0        write-only
;    0x1003    SDLBA1        write-only
;    0x1004    SDLBA2        write-only (only bits 6:0 are valid)
;
; To read a 512-byte block from the SDCARD:
; Wait until SDSTATUS=0x80 (ensures previous cmd has completed)
; Write SDLBA0, SDLBA1 SDLBA2 to select block index to read from
; Write 0 to SDCONTROL to issue read command
; Loop 512 times:
;     Wait until SDSTATUS=0xE0 (read byte ready, block busy)
;     Read byte from SDDATA
;
; To write a 512-byte block to the SDCARD:
; Wait until SDSTATUS=0x80 (ensures previous cmd has completed)
; Write SDLBA0, SDLBA1 SDLBA2 to select block index to write to
; Write 1 to SDCONTROL to issue write command
; Loop 512 times:
;     Wait until SDSTATUS=0xA0 (block busy)
;     Write byte to SDDATA
;