Atari XL/XE SD cartridge

Atari XL/XE SD cartridge is a device allowing access (read and write) to FAT32 formatted
SD cards by Atari 8-bit computers.

Atari-side interface

The cartridge occupies address space $8000..$BFFF. Read and write operations
are controlled through 8 registers mapped to $D5E8..$D5EF. Each read/write
operates on SD card sectors <SECTOR_NUM>..<SECTOR_NUM>+<SECTOR_COUNT>-1 and
RAM region $8000+512*<OFFSET>..$8000+512*(<OFFSET>+<SECTOR_COUNT>)-1.
Additionaly read operation may be performed using single register - read address
is automatically incremented after performing the operation.

$D5E8 :

• bit0=1 - read sectors from SD card into buffer RAM; reset after the operation is completed
• bit1=1 - write data from buffer RAM to SD card; reset after the operation is completed
• bit5..bit2 - unused
• bit6=1 - flag: last operation succeeded
• bit7=1 - flag: last operation failed

$D5E9 :

• bit5..bit0 - <OFFSET>; data offset from $8000; measured in 512-byte blocks
• bit7..bit4 - unused

$D5EA :

• bit5..bit0 - <SECTOR_COUNT>; number of 512-byte sectors to read/write
• bit7..bit6 - unused

$D5EB :

• bit7..bit0 - <SECTOR_NUM> bits 7..0

$D5EC :

• bit7..bit0 - <SECTOR_NUM> bits 15..8

$D5ED :

• bit7..bit0 - <SECTOR_NUM> bits 23..16

$D5EE :

• bit7..bit0 - <SECTOR_NUM> bits 31..24

$D5EF (write):

• bit4..bit0 - <READ_ADDRESS> - offset from $8000; measured in 512-byte blocks
• bit5 - unused
• bit6=1 - turn on cartridge memory $8000..$9FFF
• bit7=1 - turn on cartridge memory $A000..$BFFF

$D5EF (read):

• bit7..bit0 - data in cartridge memory at current <READ_ADDRESS>; the

           address is automatically incremented by 1 after read

Example usage

Read

 ; read 10 consecutive sectors from SD card starting from $00123456
 ; into buffer RAM starting at $8600 and wait for the
 ; operation to complete
 lda #$56
 sta $D5EB
 lda #$34
 sta $D5EC
 lda #$12
 sta $D5ED
 lda #$00
 sta $D5EE
 lda #3
 sta $D5E9
 lda #10
 sta $D5EA
 lda #1
 sta $D5E8
 lda #$C0
wait
 bit $D5E8
 beq wait
 bmi error
ok

Write

 ; fill 7 consecutive sectors on SD card starting from $00987654
 ; with data in buffer RAM starting at $a000 and wait for the
 ; operation to complete
 lda #$54
 sta $D5EB
 lda #$76
 sta $D5EC
 lda #$98
 sta $D5ED
 lda #$00
 sta $D5EE
 lda #16
 sta $D5E9
 lda #7
 sta $D5EA
 lda #2
 sta $D5E8
 lda #$C0
wait
 bit $D5E8
 beq wait
 bmi error
ok

Hardware

Main silicon components:

• buffer RAM - 32 kB of 55 ns SRAM (16 kB + 256 B of which are actually used)
• ARM Cortex-M0 microcontroller (NXP LPC1114)
• MMU controlling access to SRAM from microcontroller and Atari (Xilinx XC95144XL CPLD)

Connectors:

• SD card socket
• SWD connector for programming/debugging the microcontroller
• JTAG connector for programming the CPLD
• UART connector (5V) for communication PC-microcontroller; may also be used for microcontroller flash programming
• debug connector with two pins connected to the CPLD, and two pins connected to both the CPLD and the microcontroller
• SPI header (for debugging communication between the microcontroller and the SD card)

LED indicators:

• red : error occured (e.g. SD card not present, SD card read or write error)
• yellow : SD card present

Software (microcontroller)

Boot algorithm:

1. Write Atari bootstrap code to buffer RAM at $BF00
2. Initialize SD card
3. Find Atari executable file boot.obx on the SD card
4. Write boot.obx first sector number to buffer RAM at $D5EB..$D5ED
5. Execute main loop

Main loop:

1. Execute command received through UART (if any):

   • echo
     UART —> microcontroller 'e'
     UART <— microcontroller 'e'

   • read data from buffer RAM (A, N - 16-bit little endian numbers)
     UART —> microcontroller 'r'
     UART —> microcontroller A
     UART —> microcontroller N
     UART <— microcontroller buffer_ram[A]
     UART <— microcontroller buffer_ram[A+1]
     …
     UART <— microcontroller buffer_ram[A+N-1]

   • write data to buffer RAM (A, N - 16-bit little endian numbers)
     UART —> microcontroller 'w'
     UART —> microcontroller A
     UART —> microcontroller N
     UART —> microcontroller buffer_ram[A]
     UART —> microcontroller buffer_ram[A+1]
     …
     UART —> microcontroller buffer_ram[A+N-1]
     UART <— microcontroller 'w'

2. Execute operation read from register at $D5E8:

   • bit0=1 - read <SECTOR_COUNT> 512-byte sectors starting at sector <SECTOR_NUM>

          from SD card into buffer RAM at address `$8000+512*<OFFSET>`

   • bit1=1 - write <SECTOR_COUNT> 512-byte sectors from buffer RAM

          at `$8000+512*<OFFSET>` to SD card starting at sector `<SECTOR_NUM>`

     After operation is finished set success and error flags in register at $D5E8.

Sources

bootstrap/ - Atari bootstrap sources

cpld/ - MMU design files

pcb/ - PCB design files (gEDA) and gerbers

tools/ - programs to control the microcontroller through UART interface

uc/ - microcontroller firmware sources

video/ - video converter