2 reading and writing registers – Sundance SMT321 User Manual

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the appropriate settings be implemented. The first four bits indicate the operation which
must be preformed. There are four operations:

•

Comport Reset (bit sequence: 1111)

•

Loop back Mode (bit sequence: 0000)

•

Comport Register Write Command (bit sequence: 0001)

•

Comport Register Read Command (bit sequence: 0010)

The next 12 bits specifies the register to read or write. In the first two commands (Reset
and Loop back) the rest of the packet is NO CARES.

The read command packet only needs the first 2 bytes to read a specific register the last
two bytes are NO CARES.

Write commands use the whole packet, with the last 2 bytes being the data to be written
to the specific register.

This structure is illustrated in the following figure:

Byte Content

Byte

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Command

Address

Address 7

Address 6

Address 5

Address 4

Address 3

Address 2

Address 1

Address 0

Data 15

Data 14

Data 13

Data 12

Data 11

Data 10

Data 9

Data 8

Data 7

Data 6

Data 5

Data 4

Data 3

Data 2

Data 1

Data 0

Figure 11. Setup Packet Structure.

4.2 Reading and Writing Registers

Control packets are sent to the SMT321 over Comport 3. This is a uni-directional
interface and data can only be sent to the SMT321 over Comport 3. Comport 0 is used
to read control information back from the SMT321. Comport 0 is thus also a uni-
directional interface going from the SMT321 to the system host. Data is read by issuing
a ‘Read Request’ control packet containing the address to be read over Comport 3. The
SMT321 will collect the required data and send a ‘Read Packet’ out over Comport 0
containing the requested data. The format of a ‘Read Packet’ is the same as that of a
write packet.