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Bluetooth 4.2 compatibility with 4.0?

I read about compatibility between BT 4.0 and 4.2. I found here and in another forum people saying that it is fully compatible. In addition, in accordance with what I read, the main differences will be mainly the security, speed and IoT material. Therefore, if I start a simple BT connection (without IP) and do not require a maximum transfer speed, can I rely on 4.0 and 4.2 to be compatible?

My MCU library stack is based on 4.2, but I want to make the product 4.0 compatible. Is there anything I should know about? Any other functions / functions that I should not use?

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4.2 functions are discussed between the central and peripheral devices. If host 4.2 tries to reconcile with 4.0, reconciliation will fail because the messages are unknown. The error should be accepted as a soft failure function, which is not supported for host 4.2. For instance:

  • Length extension

  • in 6.B.5.1.9:

    If the communication level of the master or slave sends the LL_LENGTH_REQ PDU to a device that does not understand this PDU, then the device should expect an LL_UNKNOWN_RSP PDU in response. If the Link Layer receives an LL_UNKNOWN_RSP PDU with the UnknownType field set to LL_LENGTH_REQ, then it should not transfer the device to another LL_LENGTH_REQ PDU.

  • Secure connection in 3.H.3.5.2:

    The SC field is a 1-bit flag that is set to one for the LE Secure Connection request, otherwise it should be set to 0 based on the supported features of the initiator and responder, possible resulting pairing mechanisms: if both devices support LE Secure Connections, use LE Secure Connections; otherwise, use the previous LE pairing.

In principle, yes, they are fully compatible.

Side note. Even in specification 4.2, all “new” functions are optional (extension of length, secure connections, etc.). So, as long as the host answers the negotiations with messages meaning “I know about them, but I don’t support them”, it may require compatibility with 4.2 and actually do not implement anything unusual! Most SoC developers who have released a software update to 4.2 for their current hardware have actually done so.

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