diff --git a/sdcard.py b/sdcard.py
new file mode 100644
index 0000000..687a96e
--- /dev/null
+++ b/sdcard.py
@@ -0,0 +1,306 @@
+"""
+MicroPython driver for SD cards using SPI bus.
+
+Requires an SPI bus and a CS pin.  Provides readblocks and writeblocks
+methods so the device can be mounted as a filesystem.
+
+Example usage on pyboard:
+
+    import pyb, sdcard, os
+    sd = sdcard.SDCard(pyb.SPI(1), pyb.Pin.board.X5)
+    pyb.mount(sd, '/sd2')
+    os.listdir('/')
+
+Example usage on ESP8266:
+
+    import machine, sdcard, os
+    sd = sdcard.SDCard(machine.SPI(1), machine.Pin(15))
+    os.mount(sd, '/sd')
+    os.listdir('/')
+
+"""
+
+from micropython import const
+import time
+
+
+_CMD_TIMEOUT = const(100)
+
+_R1_IDLE_STATE = const(1 << 0)
+# R1_ERASE_RESET = const(1 << 1)
+_R1_ILLEGAL_COMMAND = const(1 << 2)
+# R1_COM_CRC_ERROR = const(1 << 3)
+# R1_ERASE_SEQUENCE_ERROR = const(1 << 4)
+# R1_ADDRESS_ERROR = const(1 << 5)
+# R1_PARAMETER_ERROR = const(1 << 6)
+_TOKEN_CMD25 = const(0xFC)
+_TOKEN_STOP_TRAN = const(0xFD)
+_TOKEN_DATA = const(0xFE)
+
+
+class SDCard:
+    def __init__(self, spi, cs, baudrate=1320000):
+        self.spi = spi
+        self.cs = cs
+
+        self.cmdbuf = bytearray(6)
+        self.dummybuf = bytearray(512)
+        self.tokenbuf = bytearray(1)
+        for i in range(512):
+            self.dummybuf[i] = 0xFF
+        self.dummybuf_memoryview = memoryview(self.dummybuf)
+
+        # initialise the card
+        self.init_card(baudrate)
+
+    def init_spi(self, baudrate):
+        try:
+            master = self.spi.MASTER
+        except AttributeError:
+            # on ESP8266
+            self.spi.init(baudrate=baudrate, phase=0, polarity=0)
+        else:
+            # on pyboard
+            self.spi.init(master, baudrate=baudrate, phase=0, polarity=0)
+
+    def init_card(self, baudrate):
+        # init CS pin
+        self.cs.init(self.cs.OUT, value=1)
+
+        # init SPI bus; use low data rate for initialisation
+        self.init_spi(100000)
+
+        # clock card at least 100 cycles with cs high
+        for i in range(16):
+            self.spi.write(b"\xff")
+
+        # CMD0: init card; should return _R1_IDLE_STATE (allow 5 attempts)
+        for _ in range(5):
+            if self.cmd(0, 0, 0x95) == _R1_IDLE_STATE:
+                break
+        else:
+            raise OSError("no SD card")
+
+        # CMD8: determine card version
+        r = self.cmd(8, 0x01AA, 0x87, 4)
+        if r == _R1_IDLE_STATE:
+            self.init_card_v2()
+        elif r == (_R1_IDLE_STATE | _R1_ILLEGAL_COMMAND):
+            self.init_card_v1()
+        else:
+            raise OSError("couldn't determine SD card version")
+
+        # get the number of sectors
+        # CMD9: response R2 (R1 byte + 16-byte block read)
+        if self.cmd(9, 0, 0, 0, False) != 0:
+            raise OSError("no response from SD card")
+        csd = bytearray(16)
+        self.readinto(csd)
+        if csd[0] & 0xC0 == 0x40:  # CSD version 2.0
+            self.sectors = ((csd[8] << 8 | csd[9]) + 1) * 1024
+        elif csd[0] & 0xC0 == 0x00:  # CSD version 1.0 (old, <=2GB)
+            c_size = (csd[6] & 0b11) << 10 | csd[7] << 2 | csd[8] >> 6
+            c_size_mult = (csd[9] & 0b11) << 1 | csd[10] >> 7
+            read_bl_len = csd[5] & 0b1111
+            capacity = (c_size + 1) * (2 ** (c_size_mult + 2)) * (2**read_bl_len)
+            self.sectors = capacity // 512
+        else:
+            raise OSError("SD card CSD format not supported")
+        # print('sectors', self.sectors)
+
+        # CMD16: set block length to 512 bytes
+        if self.cmd(16, 512, 0) != 0:
+            raise OSError("can't set 512 block size")
+
+        # set to high data rate now that it's initialised
+        self.init_spi(baudrate)
+
+    def init_card_v1(self):
+        for i in range(_CMD_TIMEOUT):
+            time.sleep_ms(50)
+            self.cmd(55, 0, 0)
+            if self.cmd(41, 0, 0) == 0:
+                # SDSC card, uses byte addressing in read/write/erase commands
+                self.cdv = 512
+                # print("[SDCard] v1 card")
+                return
+        raise OSError("timeout waiting for v1 card")
+
+    def init_card_v2(self):
+        for i in range(_CMD_TIMEOUT):
+            time.sleep_ms(50)
+            self.cmd(58, 0, 0, 4)
+            self.cmd(55, 0, 0)
+            if self.cmd(41, 0x40000000, 0) == 0:
+                self.cmd(58, 0, 0, -4)  # 4-byte response, negative means keep the first byte
+                ocr = self.tokenbuf[0]  # get first byte of response, which is OCR
+                if not ocr & 0x40:
+                    # SDSC card, uses byte addressing in read/write/erase commands
+                    self.cdv = 512
+                else:
+                    # SDHC/SDXC card, uses block addressing in read/write/erase commands
+                    self.cdv = 1
+                # print("[SDCard] v2 card")
+                return
+        raise OSError("timeout waiting for v2 card")
+
+    def cmd(self, cmd, arg, crc, final=0, release=True, skip1=False):
+        self.cs(0)
+
+        # create and send the command
+        buf = self.cmdbuf
+        buf[0] = 0x40 | cmd
+        buf[1] = arg >> 24
+        buf[2] = arg >> 16
+        buf[3] = arg >> 8
+        buf[4] = arg
+        buf[5] = crc
+        self.spi.write(buf)
+
+        if skip1:
+            self.spi.readinto(self.tokenbuf, 0xFF)
+
+        # wait for the response (response[7] == 0)
+        for i in range(_CMD_TIMEOUT):
+            self.spi.readinto(self.tokenbuf, 0xFF)
+            response = self.tokenbuf[0]
+            if not (response & 0x80):
+                # this could be a big-endian integer that we are getting here
+                # if final<0 then store the first byte to tokenbuf and discard the rest
+                if final < 0:
+                    self.spi.readinto(self.tokenbuf, 0xFF)
+                    final = -1 - final
+                for j in range(final):
+                    self.spi.write(b"\xff")
+                if release:
+                    self.cs(1)
+                    self.spi.write(b"\xff")
+                return response
+
+        # timeout
+        self.cs(1)
+        self.spi.write(b"\xff")
+        return -1
+
+    def readinto(self, buf):
+        self.cs(0)
+
+        # read until start byte (0xff)
+        for i in range(_CMD_TIMEOUT):
+            self.spi.readinto(self.tokenbuf, 0xFF)
+            if self.tokenbuf[0] == _TOKEN_DATA:
+                break
+            time.sleep_ms(1)
+        else:
+            self.cs(1)
+            raise OSError("timeout waiting for response")
+
+        # read data
+        mv = self.dummybuf_memoryview
+        if len(buf) != len(mv):
+            mv = mv[: len(buf)]
+        self.spi.write_readinto(mv, buf)
+
+        # read checksum
+        self.spi.write(b"\xff")
+        self.spi.write(b"\xff")
+
+        self.cs(1)
+        self.spi.write(b"\xff")
+
+    def write(self, token, buf):
+        self.cs(0)
+
+        # send: start of block, data, checksum
+        self.spi.read(1, token)
+        self.spi.write(buf)
+        self.spi.write(b"\xff")
+        self.spi.write(b"\xff")
+
+        # check the response
+        if (self.spi.read(1, 0xFF)[0] & 0x1F) != 0x05:
+            self.cs(1)
+            self.spi.write(b"\xff")
+            return
+
+        # wait for write to finish
+        while self.spi.read(1, 0xFF)[0] == 0:
+            pass
+
+        self.cs(1)
+        self.spi.write(b"\xff")
+
+    def write_token(self, token):
+        self.cs(0)
+        self.spi.read(1, token)
+        self.spi.write(b"\xff")
+        # wait for write to finish
+        while self.spi.read(1, 0xFF)[0] == 0x00:
+            pass
+
+        self.cs(1)
+        self.spi.write(b"\xff")
+
+    def readblocks(self, block_num, buf):
+        # workaround for shared bus, required for (at least) some Kingston
+        # devices, ensure MOSI is high before starting transaction
+        self.spi.write(b"\xff")
+
+        nblocks = len(buf) // 512
+        assert nblocks and not len(buf) % 512, "Buffer length is invalid"
+        if nblocks == 1:
+            # CMD17: set read address for single block
+            if self.cmd(17, block_num * self.cdv, 0, release=False) != 0:
+                # release the card
+                self.cs(1)
+                raise OSError(5)  # EIO
+            # receive the data and release card
+            self.readinto(buf)
+        else:
+            # CMD18: set read address for multiple blocks
+            if self.cmd(18, block_num * self.cdv, 0, release=False) != 0:
+                # release the card
+                self.cs(1)
+                raise OSError(5)  # EIO
+            offset = 0
+            mv = memoryview(buf)
+            while nblocks:
+                # receive the data and release card
+                self.readinto(mv[offset : offset + 512])
+                offset += 512
+                nblocks -= 1
+            if self.cmd(12, 0, 0xFF, skip1=True):
+                raise OSError(5)  # EIO
+
+    def writeblocks(self, block_num, buf):
+        # workaround for shared bus, required for (at least) some Kingston
+        # devices, ensure MOSI is high before starting transaction
+        self.spi.write(b"\xff")
+
+        nblocks, err = divmod(len(buf), 512)
+        assert nblocks and not err, "Buffer length is invalid"
+        if nblocks == 1:
+            # CMD24: set write address for single block
+            if self.cmd(24, block_num * self.cdv, 0) != 0:
+                raise OSError(5)  # EIO
+
+            # send the data
+            self.write(_TOKEN_DATA, buf)
+        else:
+            # CMD25: set write address for first block
+            if self.cmd(25, block_num * self.cdv, 0) != 0:
+                raise OSError(5)  # EIO
+            # send the data
+            offset = 0
+            mv = memoryview(buf)
+            while nblocks:
+                self.write(_TOKEN_CMD25, mv[offset : offset + 512])
+                offset += 512
+                nblocks -= 1
+            self.write_token(_TOKEN_STOP_TRAN)
+
+    def ioctl(self, op, arg):
+        if op == 4:  # get number of blocks
+            return self.sectors
+        if op == 5:  # get block size in bytes
+            return 512
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