USB-MODEVM Protocol
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If the command is sent with no problem, the returning byte [0] is the same as the sent one logically ORed
with 0x20 - in the preceding first example, the returning packet is:
[0]
0x31
[1]
0xA0
[2]
0x02
[3]
0x05
[4]
0xAA
[5]
0x55
If for some reason the interface fails (for example, the I
2
C device does not acknowledge), it comes back
as:
[0]
0x51
→
interface | INTF_ERROR
[1]
0xA0
[2]
0x02
[3]
0x05
[4]
0xAA
[5]
0x55
If the request is malformed, that is, the interface byte (byte [0]) takes on a value which is not as previously
described, the return packet is:
[0]
0x93
→
the user sent 0x13, which is not valid, so 0x93 returned
[1]
0xA0
[2]
0x02
[3]
0x05
[4]
0xAA
[5]
0x55
The preceding examples used writes. Reading is similar:
Read two bytes from device starting at register 5 of an I
2
C device with address A0:
[0]
0x01
[1]
0xA0
[2]
0x02
[3]
0x05
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USB-MODEVM Protocol
SLAU285 – July 2009
