Header Lrc; Packet Id; Packet Length; Crc - Advanced Navigation Spatial Dual Reference Manual

Field Bytes
# Offset
1 0
2 1
3 2
4 3
Table 25: ANPP header format
13.2.1

Header LRC

The header LRC (Longitudinal Redundancy Check) provides error checking on the
packet header. It also allows the decoder to find the start of a packet by scanning for a
valid LRC. The LRC can be found using the following:
LRC = ((packet_id + packet_length + crc[0] + crc[1])^0xFF) + 1
13.2.2

Packet ID

The packet ID is used to distinguish the contents of the packet. Packet IDs range from
0 to 255.
Within this range there are three different sub-ranges, these are system packets, state
packets and configuration packets.
System packets have packet IDs in the range 0 to 19. These packets are implemented
the same by every device using ANPP.
State packets are packets that contain data that changes with time, i.e. temperature.
State packets can be set to output at a certain rate. State packets are packet IDs in
the range 20 to 179.
Configuration packets are used for reading and writing device configuration.
Configuration packets are packet IDs in the range 180 to 255.
13.2.3

Packet Length

The packet length denotes the length of the packet data, i.e. from byte index 5
onwards inclusive. Packet length has a range of 0 – 255.
13.2.4

CRC

The CRC is a CRC16-CCITT. The starting value is 0xFFFF. The CRC covers only the
packet data.
13.3

Packet Requests

Any of the state and configuration packets can be requested at any time using the
ANPP Header Format
Data Size
Type
u8 1
u8 1
u8 1
u16 2
Spatial Dual Reference Manual
Page 89 of 137
Description
Header LRC, see section 13.2.1
Packet ID, see section 13.2.2
Packet Length, see section 13.2.3
CRC16, see section 13.2.4
Version 2.3
12/04/2016