# Logging

For more information on how to use this and how this is implemented have a look here.

## Communication protocol

The log port is separated in 3 channels:

Port Channel Function
5 0 Table of content access: Used for reading out the TOC
5 1 Log control: Used for adding/removing/starting/pausing log blocks
5 2 Log data: Used to send log data from the Crazyflie to the client

### Table of content access

The first byte of each messages correspond a command. All communication on this port are initated by the client and all answer from the copter will contain the same command byte.

TOC command byte Command Operation
0 GET_ITEM Get an item from the TOC
1 GET_INFO Get information about the TOC and the LOG subsystem implementation

### Get TOC item

The GET_ITEM TOC command permits to retrieved the log variables name, group and types from the copter. This command is intended to be requested from all the ID from 0 to LOG_LEN (see GET_INFO).

Request (PC to Copter):
+--------------+----+
| GET_ITEM (0) | ID |
+--------------+----+
Length         1         1

+--------------+----+
| GET_ITEM (0) | ID |                                        If index out of range
+--------------+----+------+------------+--------------+
| GET_ITEM (0) | ID | Type |   Group    |     Name     |     If returning Item
+--------------+----+------+------------+--------------+
Length        1          1     1    < Null terminated strings >

Byte Request fields Content
0 GET_ITEM At 0 for GET_ITEM operation
1 ID ID of the item to be retrieved. The variables are numbered from 0 to LOG_LEN (see GET_INFO command)
0 GET_ITEM 0 for GET_ITEM operation
1 ID ID of the item returned
2 Type Variable type of the element. See variable types list
3.. Group Null-terminated string containing variable group
.. Name Null-terminated string containing the variable name

Type, group and name are not sent if the required ID is higher than TOC_LEN-1.

### Get Info

The get info command is intended to be requested first when connecting to the copter. This permits to know the number of variable, the limitations of the log implementation and the fingerprint of the log variables.

Request (PC to Copter):
+--------------+
| GET_INFO (1) |
+--------------+
Length         1

+--------------+---------+---------+-----------------+-------------+
| GET_INFO (1) | LOG_LEN | LOG_CRC |  LOG_MAX_PACKET | LOG_MAX_OPS |
+--------------+---------+---------+-----------------+-------------+
Length        1             1         4             1               1

Byte Request fields Content
0 GET_INFO At 1 for GET_INFO operation
0 GET_INFO 1 for GET_INFO operation
1 LOG_LEN Number of log items contained in the log table of content
2 LOG_CRC CRC values of the log TOC memory content. This is a fingerprint of the copter build that can be used to cache the TOC
6 LOG_MAX_PACKET Maximum number of log packets that can be programmed in the copter
7 LOG_MAX_OPS Maximum number of operation programmable in the copter. An operation is one log variable retrieval programming

### Log control

The log control channel permits to setup, activate, deactivate and remove log packets. Like the TOC access channel the first data byte represents a command.

Control command byte Command Operation
0 CREATE_BLOCK Create a new log block
1 APPEND_BLOCK Append variables to an existing block
2 DELETE_BLOCK Delete log block
3 START_BLOCK Enable log block transmission
4 STOP_BLOCK Disable log block transmission
5 RESET Delete all log blocks

### Log data

The log data channel is used by the copter to send the log blocks at the programmed rate. The packet format is

Answer (Copter to PC):
+----------+------------+---------//----------+
| BLOCK_ID | TIME_STAMP | LOG VARIABLE VALUES |
+----------+------------+---------//----------+
Length        1          3           0 to 26