Building A Packet

computer guy

I am waiting for a CC-Debugger to program a 8051 MCU, to get a Bluetooth Module working.

In the meantime as I am impatient, I have proceeded to try and write a driver object for the module based on the documentation and wanted to see if I am doing it right. (As I am new to byte manipulation).

'Packet Structure

'  +------------+---------+--------+--------+--------+----------+------------+
'  |      0     |   0000  |   000  |00000000|00000000| 00000000 |00000000 ...|
'  +------------+---------+--------+--------+--------+----------+------------+
'  |Message Type|Tech Type|Len High|Len Low |Class ID|Command ID|  Payload   |
'  +------------+---------+--------+--------+--------+----------+------------+
'  |                            Header                          |  Payload   |
'  +------------------------------------------------------------+------------+

 ' BUILD HEADER
  txHeader[0] := messageType
  txHeader[0] |= techType
  txHeader[0] |= len << 8
  txHeader[1] := len
  txHeader[2] := class
  txHeader[3] := cmd

The following code is supposed to build the header section of the packet.

Where message type is one of either:

messageTypeCommand := %00000000
messageTypeEvent   := %10000000

techType is:

techTypeBT4S := %00000000

and len is the length of the payload.

It's just the first byte im not sure about. The ORing of the values together and shifting of the length.

Does it look right?

ericball

txHeader[0] |= len >> 8
txHeader[1] := len & 255

You could also have

txHeader[0] := messageType << 7
txHeader[1] := techType << 3

in which case messageType is 0 or 1 and techType is 0 to 15.

Duane Degn

If "txHeader" is an array of bytes, I think you could simplify:

txHeader[1] := len & 255

to:

txHeader[1] := len

Since only the least significant byte would be used from "len" anyway.

(Not that it matters much.)

Edit: I see this is the way Computer Guy originally had it.