Henk,
On Montag, 9. März 2009, Henk Stegeman wrote:
> I don't understand how this would work,
>
> Now I do one byte-swap, which works.
> -I byteswap in software, for 16-bit cycles by byte swapping and for 8
> bit cycles by adding an offset of 1.
> (The byte swapping on the chipselect is off)
>
> Your advice includes two byteswaps, one by re-routing the data bus and
> one by enabling the byte swap on the chip-select.
My experience is the chip select byte swap feature only works correctly if you
connect a little endian device like I showed you.
> Or does one of them not really swap bytes?
Let me show you how it works. You must ensure you can write/read data in any
data width, but at the side of the little endian device it always must be in
the correct endianess. This example uses a 32 bit data width, but it works
for 16 bit, too.
- LE shows how a real litte endian CPU would write data
- MPC1 shows how MPC5200 will do it, without any byte swap and DO at the
MPC5200 side is also D0 at the little endian device
- MPC2 shows how MPC5200 will do it, with D0 at the MPC5200 side is D24 at the
little endian device
- MPC3 shows how MPC5200 will do it, connected like MPC2 but also the chip
select byte swap feature enabled
- LE DEV shows how the little endian device expects the data
You want to write this data at the given offset into the little endian device:
Bytes: 0:0x34, 1:0x12, 2:0x78, 3:0x56
Worte: 0:0x1234 2:0x5678
LONG: 0:0x56781234
Writing as bytes:
Bytes: 0:0x34, 1:0x12, 2:0x78, 3:0x56
Offset LE MPC1 MPC2 MPC3 LE DEV
0 0x34 0x56 0x34 0x34 0x34
1 0x12 0x78 0x12 0x12 0x12
2 0x78 0x12 0x78 0x78 0x78
3 0x56 0x34 0x56 0x56 0x56
^^^^--------^^^^--^^^^--^^^^--> these are correct
^^^^--------------------> this is wrong
Writing as words:
Words: 0:0x1234 2:0x5678
Offset LE MPC1 MPC2 MPC3 LE DEV
0 0x34 0x78 0x12 0x34 0x34
(1) 0x12 0x56 0x34 0x12 0x12
2 0x78 0x34 0x56 0x78 0x78
(3) 0x56 0x12 0x78 0x56 0x56
^^^^---------------^^^^--^^^^--> these are correct
^^^^--^^^^--------------> these are wrong
Writing as longs:
LONG: 0:0x56781234
Offset LE MPC1 MPC2 MPC3 LE DEV
0 0x34 0x34 0x56 0x34 0x34
(1) 0x12 0x12 0x78 0x12 0x12
(2) 0x78 0x78 0x12 0x78 0x78
(3) 0x56 0x56 0x34 0x56 0x56
^^^^---^^^^ -------^^^^--^^^^--> these are correct
^^^^--------------> this is wrong
So, the MPC3 example always writes correct data.
Hope it helps,
Juergen
--
Pengutronix e.K. | Juergen Beisert |
Linux Solutions for Science and Industry | Phone: +49-8766-939 228 |
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