32 bytes CRTP packets not supported

[ataffanel]

The Crazyflie radio supports 32 bytes packets however this line limits the max packet size to 31: https://github.com/bitcraze/crazyflie-firmware/blob/crazyflie2/modules/interface/crtp.h#L33

When the CRTP data size is set to 31 (to send 32 bytes packet), 32bytes packet are dropped somewhere. This needs to be fixed.

[AlexisTM]

Isn't it simply because CRTP_MAX_DATA_SIZE is the data which is prepended by a byte of the size of the message? It is using CRTP_MAX_DATA_SIZE+1 for the full message.

crazyflie-firmware/src/modules/interface/crtp.h

Lines 52 to 75 in 467742c

	typedef struct _CRTPPacket
	{
	uint8_t size; //< Size of data
	union {
	struct {
	union {
	uint8_t header; //< Header selecting channel and port
	struct {
	#ifndef CRTP_HEADER_COMPAT
	uint8_t channel : 2; //< Selected channel within port
	uint8_t reserved : 2;
	uint8_t port : 4; //< Selected port
	#else
	uint8_t channel : 2;
	uint8_t port : 4;
	uint8_t reserved : 2;
	#endif
	};
	};
	uint8_t data[CRTP_MAX_DATA_SIZE]; //< Data
	};
	uint8_t raw[CRTP_MAX_DATA_SIZE+1]; //< The full packet "raw"
	};
	} __attribute__((packed)) CRTPPacket;

[ataffanel]

The source of this problem is that CRTP_MAX_DATA_SIZE is set to 30 and not to 31 as it should (which would make 32 bytes max packet including the 1 byte header).

Though last time I tried to fix it (in 2015...), setting CRTP_MAX_DATA_SIZE to 31 was causing 32bytes packet to be dropped somewhere and I never put/had the time to look where the packet was dropped (or if this was still a problem).

[AlexisTM]

Could it be that the radio adds a CRC for the last byte?

[ataffanel]

In theory no, the nRF24 radio handles 32 bytes payload independently of the CRC (the CRC is added outside of the payload). Though, for Crazyflie 2.0 we implement the radio ourselves in the nRF51 so it is possible that there is a bug there.

[knmcguire]

#29 increased the packet size to 60, so that fixes this issue. However we still might need to address the issue that this cannot be even bigger than it already is. But that is not part of this issue!