Fixed the problem that caused no chip present to verify OK after a write

The problem was that the data bus would "remember" the last data I wrote
to it if a chip was not driving the data bus on a read, so it would
think that the chip had returned the correct data. I fixed it by writing
the opposite data to the data bus before a verification read.

external_mem.c

@@ -346,6 +346,15 @@ uint8_t ExternalMem_Write(uint32_t startAddress, uint32_t *buf, uint32_t len, ui
 		ExternalMem_WriteByteToChips(startAddress, *buf, chipsMask);
 		if (doVerify)
 		{
+			// Write the opposite data to the data bus before attempting to read
+			// the chips for verification. At this point, the OE and WE lines
+			// will be deasserted (due to WriteByteToChips()) so writing a value
+			// to the data bus won't do anything. But if there's no chip on
+			// a data line, reading back seems to "remember" the last value
+			// written. So writing the opposite of what we expect will ensure
+			// that the readback will fail if a chip isn't responding.
+			ExternalMem_SetData(~(*buf));
+
 			#define VERIFY_EXTRA_READ_TRIES	2
 
 			// Read back the word we just wrote to make sure it's OK...