diff --git a/os/io/pal.h b/os/io/pal.h
index b75582ee1..2a1cbaf63 100644
--- a/os/io/pal.h
+++ b/os/io/pal.h
@@ -141,12 +141,12 @@
  *       not belonging to the bus.
  */
 typedef struct {
-  /** Port identifier. */
+  /** Port identifier.*/
   ioportid_t            bus_portid;
   /** Bus mask aligned to port bit 0. The bus mask implicitly define the bus
-   *  width. */
+      width. A logical AND is performed on the bus data.*/
   ioportmask_t          bus_mask;
-  /** Offset, within the port, of the least significant bit of the bus. */
+  /** Offset, within the port, of the least significant bit of the bus.*/
   uint_fast8_t          bus_offset;
 } IOBus;
 
@@ -273,7 +273,8 @@ typedef struct {
  * @brief Reads a group of bits.
  *
  * @param[in] port the port identifier
- * @param[in] mask the group mask
+ * @param[in] mask the group mask, a logical AND is performed on the input
+ *            data
  * @param[in] offset the group bit offset within the port
  * @return The group logical states.
  */
@@ -288,7 +289,8 @@ typedef struct {
  * @brief Writes a group of bits.
  *
  * @param[in] port the port identifier
- * @param[in] mask the group mask
+ * @param[in] mask the group mask, a logical AND is performed on the output
+ *            data
  * @param[in] offset the group bit offset within the port
  * @param[in] bits the bits to be written. Values exceeding the group width
  *            are masked.
diff --git a/os/io/platforms/LPC214x/pal_lld.h b/os/io/platforms/LPC214x/pal_lld.h
index 4547abc61..565b3333a 100644
--- a/os/io/platforms/LPC214x/pal_lld.h
+++ b/os/io/platforms/LPC214x/pal_lld.h
@@ -183,7 +183,8 @@ typedef FIO * ioportid_t;
  *          accesses.
  *
  * @param[in] port the port identifier
- * @param[in] mask the group mask
+ * @param[in] mask the group mask, a logical AND is performed on the output
+ *            data
  * @param[in] offset the group bit offset within the port
  * @param[in] bits the bits to be written. Values exceeding the group width
  *            are masked.
@@ -192,7 +193,7 @@ typedef FIO * ioportid_t;
  *       code.
  */
 #define pal_lld_writegroup(port, mask, offset, bits) {                  \
-  (port)->FIO_MASK = (mask) << (offset);                                \
+  (port)->FIO_MASK = ~((mask) << (offset));                             \
   (port)->FIO_PIN = (bits) << (offset);                                 \
   (port)->FIO_MASK = 0;                                                 \
 }
diff --git a/readme.txt b/readme.txt
index 6f9b05139..73fb81919 100644
--- a/readme.txt
+++ b/readme.txt
@@ -3,6 +3,8 @@
 *****************************************************************************
 
 *** 1.3.3 ***
+- FIX: Fixed bug in the LPC2148 PAL driver (bug 2881380).
+- FIX: Fixed bug in the AT91SAM7X PAL driver (bug 2879933).
 - NEW: New MAC and MII driver models and implementations for the AT91SAM7X.
   Removed the old EMAC driver, updated the uIP WEB demo to use the new
   driver model.