AN201
Rev 1.5 | 17/91
www.cmostek.com
2.7.2 Hardware Packet Format
The CMT2189B supports hardware packet structure internally with data frame structure as follows.
Preamble
Head/
Sync
ID/ADDR
Key Value LBD
Status
Stop
Bit
1
3
2
4
5
6
Pause/Interval
7
Figure 7. Packet Structure
As shown in the packet structure in the above figure, it contains 7 parts.
1.
Preamble: optional, the value can be selected as 0x55 or 0xAA
[1]
, ranging from 0 to 256 bytes (select any value within the
range).
2.
Head/Sync: synchronous word, optional, ranging from 0 to 32 bit
s[2]
(select any value within the range).
3.
ID/ADDR: sequence number, mandatory, ranging from 1 to 32 logic bits
[3]
(select any value within the range).
4.
Key value: optional, ranging from 0 to 8 logic bits
[3]
(select any value within the range).
5.
LBD status: low battery detection status bit, optional, occupying 1 logic bit
[3]
;
6.
Stop bit: optional, ranging from 0 ~ 16 bits
[2]
(select any value within the range).
7.
Packet interval, fixed to send
0
, ranging from 0 ~ 256 bit
s[2]
;
Notes:
[1].
Preamble adopts NRZ format without encoding, e.g. for 0x55, the data flow is 0b01010101 at the set rate (0 represents low
level and 1 represents high level).
[2].
Head/Sync, stop bit and pause/interval will not be encoded. They are output at the set rate just like the preamble code.
[3].
Logic bits, representing the encoded bits, which are described in detail below. Item 3 ~ 5 in the message structure as shown
in the above figure all supports being encoded by the internal encoder.
For example, the coding rules choose at least 1 symbol as 1 logical bit, that is, 0b0 is logic 0, and 0b1 is logic 1 (NRZ enc oding)
with following settings.
Preamble: set to open. The value is 0xAA with a length of 5 bytes.
Head/Sync: set to open. The value is 0x2DD4 with a length is 2 bytes.
ID/ADDR: the value is 0x12345678 with a length of 4 bytes.
Key Value: the value is 0x9A.
LBD Status: set to close.
Stop bit: set to open. The value is 0xBCDE with a length of 16 bits (2 bytes).
Pause/interval: pause/interval: set to open. The length is 32 bits (4 bytes).
Then the data flow is as follows.
0xAA AA AA AA AA 2D D4 12 34 56 78 9A BC DE 00 00 00 00 AA AA AA AA
AA 2D D4 12 34 56 78 9A BC DE 00…
