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134 lines
3.1 KiB
NASM
134 lines
3.1 KiB
NASM
; sdc
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;
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; Manages the initialization of a SD card and implement a block device to read
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; and write from/to it, in SPI mode.
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;
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; Note that SPI can't really be used directly from the z80, so this part
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; assumes that you have a device that handles SPI communication on behalf of
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; the z80. This device is assumed to work in a particular way.
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;
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; That device has 3 ports. One write-only port to make CS high, one to make CS
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; low (data sent is irrelevant), and one read/write port to send and receive
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; bytes with the card through the SPI protocol. The device acts as a SPI master
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; and writing to that port initiates a byte exchange. Data from the slave is
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; then placed on a buffer that can be read by reading the same port.
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;
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; It's through that kind of device that this code below is supposed to work.
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; *** Defines ***
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; SDC_PORT_CSHIGH: Port number to make CS high
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; SDC_PORT_CSLOW: Port number to make CS low
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; SDC_PORT_SPI: Port number to send/receive SPI data
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; Wake the SD card up. After power up, a SD card has to receive at least 74
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; dummy clocks with CS and DI high. We send 80.
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sdcWakeUp:
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out (SDC_PORT_CSHIGH), a
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ld b, 10 ; 10 * 8 == 80
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ld a, 0xff
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.loop:
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out (SDC_PORT_SPI), a
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nop
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djnz .loop
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ret
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; Initiate SPI exchange with the SD card. A is the data to send. Received data
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; is placed in A.
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sdcSendRecv:
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out (SDC_PORT_SPI), a
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nop
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nop
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in a, (SDC_PORT_SPI)
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nop
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nop
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ret
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; sdcSendRecv 0xff until the response is something else than 0xff for a maximum
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; of 20 times. Returns 0xff if no response.
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sdcWaitResp:
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push bc
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ld b, 20
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.loop:
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ld a, 0xff
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call sdcSendRecv
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inc a ; if 0xff, it's going to become zero
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jr nz, .end ; not zero? good, that's our command
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djnz .loop
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.end:
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; whether we had a success or failure, we return the result.
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; But first, let's bring it back to its original value.
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dec a
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pop bc
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ret
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; Sends a command to the SD card, along with arguments and specified CRC fields.
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; (CRC is only needed in initial commands though).
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; A: Command to send
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; H: Arg 1 (MSB)
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; L: Arg 2
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; D: Arg 3
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; E: Arg 4 (LSB)
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; C: CRC
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;
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; Returns R1 response in A.
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;
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; This does *not* handle CS. You have to select/deselect the card outside this
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; routine.
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sdcCmd:
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; Wait until ready to receive commands
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push af
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call sdcWaitResp
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pop af
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call sdcSendRecv
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; Arguments
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ld a, h
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call sdcSendRecv
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ld a, l
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call sdcSendRecv
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ld a, d
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call sdcSendRecv
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ld a, e
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call sdcSendRecv
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; send CRC
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ld a, c
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call sdcSendRecv
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; And now we just have to wait for a valid response...
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call sdcWaitResp
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ret
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; Send a command that expects a R1 response, handling CS.
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sdcCmdR1:
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out (SDC_PORT_CSLOW), a
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call sdcCmd
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out (SDC_PORT_CSHIGH), a
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ret
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; Send a command that expects a R7 response, handling CS. A R7 is a R1 followed
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; by 4 bytes. Those 4 bytes are returned in HL/DE in the same order as in
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; sdcCmd.
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sdcCmdR7:
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out (SDC_PORT_CSLOW), a
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call sdcCmd
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; We have our R1 response in A. Let's try reading the next 4 bytes in
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; case we have a R3.
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push af
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ld a, 0xff
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call sdcSendRecv
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ld h, a
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ld a, 0xff
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call sdcSendRecv
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ld l, a
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ld a, 0xff
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call sdcSendRecv
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ld d, a
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ld a, 0xff
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call sdcSendRecv
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ld e, a
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pop af
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out (SDC_PORT_CSHIGH), a
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ret
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