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I am reading some general information about Bluetooth 5.0 improvements here:

https://www.itechtics.com/bluetooth-4-vs-bluetooth-5-feature-comparison/

It says clearly that Bluetooth 5.0 is not backwards compatible and needs new hardware

If we look at the firmware release notes for Qualcomm ADK 4.0.1 for the CSR8675 bluetooth chip, it says that Bluetooth v4.2 is supported

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However, if we look at the firmware release notes for Qualcomm ADK 4.2 (a newer version of their ADK, Audio Development Kit) for the CSR8675 bluetooth chip, it says it supports Bluetooth v5.0

enter image description here

The ADK is just software you install on the computer (IDE, library files, source code) and you can use the same CSR8675 development board and PC-to-board communication modules to download your programs, whether written in ADK 4.0.1 or ADK 4.2

If Bluetooth v5.0 requires new hardware, how is it that the same CSR8675 chip supports both v4.2(ADK 4.0.1) and v5.0(ADK 4.2)? Both ADK 4.0.1 and 4.2 come with their respective firmwares for the CSR8675 chip, and it's the firmware release note documents that state the bluetooth compatibility, but if I understand correctly this firmware is just software and the underlying CSR8675 hardware is still the same

How does the CSR chip do this if Bluetooth v5.0 and v4.2 are hardware incompatible?

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Bluetooth 5 is backward compatible with Bluetooth 4 until you use any of the new Bluetooth 5 features. If, for example, you tried to use the 2MBPS PHY with a Bluetooth 4 device, it would not work. If your Bluetooth 5 device uses the 1 MBPS PHY, however, it is able to communicate with a Bluetooth 4 device.

Because in Bluetooth 5's protocol stack, at a bottom layer which is a PHY Layer or Physical Layer, totally three variants have been added in which two variants are new and these two are for longer range and double the speed of Bluetooth 4 as mentioned above.

Name of those variants are

  1. LE 1M (1MBPS PHY)

LE 1M is the PHY used in Bluetooth 4. It uses Gaussian Frequency Shift Keying and has a symbol rate of 1 mega symbol per second (Ms/s). It continues to be available for use in Bluetooth 5 and its support is mandatory.

  1. LE 2M Double the Speed (2MBPS PHY)

The new LE 2M PHY allows the physical layer to operate at 2 Ms/s and thus enables higher data rates than LE 1M and Bluetooth 4.

  1. The LE Coded PHY

The LE Coded PHY allows the range to be quadrupled (approximately) compared to Bluetooth 4, and this has been accomplished without increasing the transmission power required.

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