PNI CommBoard - Manual (Page 21)

PNI Sensor Corporation DOC#1018122 r02
PNI CommBoard User Manual Page 19
6.1 I
2
C Commands
{ or [ – Start Read or Write Packet
The open brackets are used to start an I
2
C Read or Write packet. It does not matter
which is used, as the end of the packet defines the type of transaction.
} or R or r – Finish Read Packet
An end curly bracket, an „R‟ character, or an „r‟ character finishes construction of an
I
2
C packet to read data from the slave device and sends the packet to the slave device.
The syntax of the packet is:
{SLA REG NUM}
Where:
All numbers must be in 2-digit hexadecimal format. For example, the value
zero must be represented as “00” and the value one as “01”. As a result, no
delimiter is necessary.
SLA is the 7-bit slave address in the 7 MSBits, plus the r/w bit in the LSB.
However, the value of the r/w bit does not matter, as the CommBoard ensures
the r/w bit is properly set. For example, the SpacePoint Scout‟s 7-bit slave
address is 0x0C, and shifting it into the 7 MSBits makes SLA = 0x18. Even if
the LSB, or r/w bit, is set to “0” indicating a write operating, the CommBoard
will transmit SLA = 0x19, effectively changing the LSB to reflect a read
operation.
REG either can be the first register number to start reading from or a
command number depending on client hardware protocol.
NUM is the number of data bytes to read.
Example #1:
{183108} requests and returns 8 bytes of data starting at register 0x31. For the
SpacePoint Scout, the data will be quaternion data in 4x 2 bytes.
Example #2:
{193314} requests and returns 20 bytes (0x14 bytes) of data starting at register
0x33. While the r/w bit is populated with a “1”, such that the SLA byte = 0x19,
the CommBoard uses the closing curly bracket, “}”, to recognize this as a read
operation and ignores the r/w bit. For the SpacePoint Scout, the cursor position,
Hpos and Vpos, will be returned in the final 2x 4 bytes.