sdcard: Compute CRC7 for all SPI commands.

Some SD cards (e.g. SDXC) have CRC checking permanently enabled in SPI
mode. Sending commands with CRC=0x00 causes them to return ILLEGAL_COMMAND,
making initialisation fail. Fix this by computing a valid CRC7 for every
command using a viper function.

Signed-off-by: Kwabena W. Agyeman <kwagyeman@live.com>

Author: kwagyeman

micropython/drivers/storage/sdcard/sdcard.py

@@ -38,6 +38,18 @@
 _TOKEN_DATA = const(0xFE)
 
 
+@micropython.viper
+def _crc7(buf, n: int) -> int:
+    """CRC7 with end bit for SD card SPI command frames (polynomial x^7+x^3+1)."""
+    d = ptr8(buf)
+    crc = 0
+    for i in range(n):
+        crc ^= d[i]
+        for j in range(8):
+            crc = ((crc << 1) ^ (0x12 * (crc >> 7))) & 0xFF
+    return crc | 1
+
+
 class SDCard:
     def __init__(self, spi, cs, baudrate=1320000):
         self.spi = spi
@@ -76,13 +88,13 @@ def init_card(self, baudrate):
 
         # CMD0: init card; should return _R1_IDLE_STATE (allow 5 attempts)
         for _ in range(5):
-            if self.cmd(0, 0, 0x95) == _R1_IDLE_STATE:
+            if self.cmd(0, 0) == _R1_IDLE_STATE:
                 break
         else:
             raise OSError("no SD card")
 
         # CMD8: determine card version
-        r = self.cmd(8, 0x01AA, 0x87, 4)
+        r = self.cmd(8, 0x01AA, 4)
         if r == _R1_IDLE_STATE:
             self.init_card_v2()
         elif r == (_R1_IDLE_STATE | _R1_ILLEGAL_COMMAND):
@@ -92,7 +104,7 @@ def init_card(self, baudrate):
 
         # get the number of sectors
         # CMD9: response R2 (R1 byte + 16-byte block read)
-        if self.cmd(9, 0, 0, 0, False) != 0:
+        if self.cmd(9, 0, 0, False) != 0:
             raise OSError("no response from SD card")
         csd = bytearray(16)
         self.readinto(csd)
@@ -109,7 +121,7 @@ def init_card(self, baudrate):
         # print('sectors', self.sectors)
 
         # CMD16: set block length to 512 bytes
-        if self.cmd(16, 512, 0) != 0:
+        if self.cmd(16, 512) != 0:
             raise OSError("can't set 512 block size")
 
         # set to high data rate now that it's initialised
@@ -118,8 +130,8 @@ def init_card(self, 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:
+            self.cmd(55, 0)
+            if self.cmd(41, 0) == 0:
                 # SDSC card, uses byte addressing in read/write/erase commands
                 self.cdv = 512
                 # print("[SDCard] v1 card")
@@ -129,10 +141,10 @@ def init_card_v1(self):
     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
+            self.cmd(58, 0, 4)
+            self.cmd(55, 0)
+            if self.cmd(41, 0x40000000) == 0:
+                self.cmd(58, 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
@@ -144,7 +156,7 @@ def init_card_v2(self):
                 return
         raise OSError("timeout waiting for v2 card")
 
-    def cmd(self, cmd, arg, crc, final=0, release=True, skip1=False):
+    def cmd(self, cmd, arg, final=0, release=True, skip1=False):
         self.cs(0)
 
         # create and send the command
@@ -154,7 +166,7 @@ def cmd(self, cmd, arg, crc, final=0, release=True, skip1=False):
         buf[2] = arg >> 16
         buf[3] = arg >> 8
         buf[4] = arg
-        buf[5] = crc
+        buf[5] = _crc7(buf, 5)
         self.spi.write(buf)
 
         if skip1:
@@ -250,15 +262,15 @@ def readblocks(self, block_num, buf):
         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:
+            if self.cmd(17, block_num * self.cdv, 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:
+            if self.cmd(18, block_num * self.cdv, release=False) != 0:
                 # release the card
                 self.cs(1)
                 raise OSError(5)  # EIO
@@ -269,7 +281,7 @@ def readblocks(self, block_num, buf):
                 self.readinto(mv[offset : offset + 512])
                 offset += 512
                 nblocks -= 1
-            if self.cmd(12, 0, 0xFF, skip1=True):
+            if self.cmd(12, 0, skip1=True):
                 raise OSError(5)  # EIO
 
     def writeblocks(self, block_num, buf):
@@ -281,14 +293,14 @@ def writeblocks(self, block_num, buf):
         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:
+            if self.cmd(24, block_num * self.cdv) != 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:
+            if self.cmd(25, block_num * self.cdv) != 0:
                 raise OSError(5)  # EIO
             # send the data
             offset = 0