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UART
Help > Basics > Interfaces > UART

UART is configured with 8 data bits, 1 stop bit, and no parity bit. UART baud rate is specified at TR Configuration. The size of both RX and TX buffers is 64 bytes.

 

HDLC byte stuffing protocol is used to frame, protect, and encode DPA messages. Every data frame (DPA message) starts and ends with byte 0x7e (Flag Sequence). When the actual data byte (applies to 8-bit CRC value too) equals 0x7e (Flag Sequence) or 0x7d (Control Escape) then it is replaced by two bytes: the 1st byte is 0x7d (Control Escape) and the 2nd byte equals the original byte value XORed by 0x20 (Escape Bit).

 

An 8-bit CRC is used to protect data. The CRC value is appended after all data bytes and it is coded by the same HDLC byte stuffing algorithm. CRC is compatible with 1-Wire CRC with an initial value 0xFF, the polynomial is x8+x5+x4+1. See CRC Calculation for the implementations of the CRC algorithm. There is also an online calculator available.

 


 

Example

 

The example shows encoded DPA Request “write bytes 0x7E, 0x7D at the RAM address 0 at [N] with address 0x2F”:

 

NADR=0x002F(Node address), PNUM=0x05(RAM peripheral), PCMD=0x01(RAM write), HWPID=0xFFFF, PData={0x00}(address), {0x7E,0x7D}(bytes to write)

 

CRC from bytes {0x2f, 0x00, 0x05, 0x01, 0xff, 0xff, 0x00, 0x7e, 0x7d} = 0x7e

 

Data in index

 

0

1

2

3

4

5

6

7

8

CRC

 

Data in

0x2f

0x00

0x05

0x01

0xff

0xff

0x00

0x7e

0x7d

0x7e

Data out index

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Data out

0x7e

0x2f

0x00

0x05

0x01

0xff

0xff

0x00

0x7d

0x5e

0x7d

0x5d

0x7d

0x5e

0x7e

Note

Flag Sequence

original byte

original byte

original byte

original byte

original byte

original byte

original byte

Control Escape

0x7e XOR 0x20

Control Escape

0x7d XOR 0x20

Control Escape

0x7e XOR 0x20

Flag Sequence