diff --git a/src/machine/machine_atsamd21.go b/src/machine/machine_atsamd21.go
index e0c5f2cc29..2e5fe604aa 100644
--- a/src/machine/machine_atsamd21.go
+++ b/src/machine/machine_atsamd21.go
@@ -1904,7 +1904,7 @@ func (f flashBlockDevice) ReadAt(p []byte, off int64) (n int, err error) {
 }
 
 // WriteAt writes the given number of bytes to the block device.
-// Only word (32 bits) length data can be programmed.
+// Data is written to the page buffer in 4-byte chunks, then saved to flash memory.
 // See Atmel-42181G–SAM-D21_Datasheet–09/2015 page 359.
 // If the length of p is not long enough it will be padded with 0xFF bytes.
 // This method assumes that the destination is already erased.
@@ -1921,8 +1921,10 @@ func (f flashBlockDevice) WriteAt(p []byte, off int64) (n int, err error) {
 	waitWhileFlashBusy()
 
 	for j := 0; j < len(padded); j += int(f.WriteBlockSize()) {
-		// write word
-		*(*uint32)(unsafe.Pointer(address)) = binary.LittleEndian.Uint32(padded[j : j+int(f.WriteBlockSize())])
+		// page buffer is 64 bytes long, but only 4 bytes can be written at once
+		for k := 0; k < int(f.WriteBlockSize()); k += 4 {
+			*(*uint32)(unsafe.Pointer(address + uintptr(k))) = binary.LittleEndian.Uint32(padded[j+k : j+k+4])
+		}
 
 		sam.NVMCTRL.SetADDR(uint32(address >> 1))
 		sam.NVMCTRL.CTRLA.Set(sam.NVMCTRL_CTRLA_CMD_WP | (sam.NVMCTRL_CTRLA_CMDEX_KEY << sam.NVMCTRL_CTRLA_CMDEX_Pos))
@@ -1944,7 +1946,7 @@ func (f flashBlockDevice) Size() int64 {
 	return int64(FlashDataEnd() - FlashDataStart())
 }
 
-const writeBlockSize = 4
+const writeBlockSize = 64
 
 // WriteBlockSize returns the block size in which data can be written to
 // memory. It can be used by a client to optimize writes, non-aligned writes