fix: use correct byte order for multi-byte read/write

Author: feeph

feeph/i2c/burst_handler.py

@@ -70,7 +70,7 @@ def read_register(self, register: int, byte_count: int = 1, max_tries: int = 5)
                 self._i2c_bus.writeto_then_readfrom(address=self._i2c_adr, buffer_out=buf_r, buffer_in=buf_w)
                 value = 0
                 for i in range(byte_count):
-                    value += buf_w[i] << i*8
+                    value += buf_w[byte_count - 1 - i] << i*8
                 return value
             except OSError as e:
                 # [Errno 121] Remote I/O error
@@ -94,11 +94,8 @@ def write_register(self, register: int, value: int, byte_count: int = 1, max_tri
             try:
                 buf = bytearray(1 + byte_count)  # buf[0], buf[1], buf[2]
                 buf[0] = register
-                # need to populate the bytes in reverse order:
-                #   0x##.. => buf[2]
-                #   0x..## => buf[1]
-                for i in range(byte_count, 0, -1):
-                    buf[i] = value & 0xFF
+                for i in range(byte_count):
+                    buf[1+i] = value & 0xFF
                     value = value >> 8
                 self._i2c_bus.writeto(address=self._i2c_adr, buffer=buf)
                 return

feeph/i2c/emulation.py

@@ -80,8 +80,7 @@ def writeto(self, address: int, buffer: bytearray, *, start=0, end=None):
             #  0x##.. -> buf[1]  -> i = 1, offset = 1
             # (buf[0] contains the register address)
             for i in range(len(buffer) - 1):
-                offset = len(buffer) - 1 - i
-                value += buffer[offset] << i*8
+                value += buffer[1 + i] << i*8
         self._state[i2c_device_address][i2c_device_register] = value
 
     def writeto_then_readfrom(self, address: int, buffer_out: bytearray, buffer_in: bytearray, *, out_start=0, out_end=None, in_start=0, in_end=None, stop=False):
@@ -93,6 +92,7 @@ def writeto_then_readfrom(self, address: int, buffer_out: bytearray, buffer_in:
         if i2c_device_register < 0:
             raise ValueError("device register can't be negative")
         value = self._state[i2c_device_address][i2c_device_register]
-        for i in range(len(buffer_in)):
-            buffer_in[i] = value & 0xff
+        byte_count = len(buffer_in)
+        for i in range(byte_count):
+            buffer_in[byte_count - 1 - i] = value & 0xff
             value = value >> 8