ANT Forum

according to the document "Interfacing_with_ANT_General_Purpose_Chipsets_and_Modules_Rev_2.1" , which explain that in asynchronous mode ,the sync byte is always "0xA4" ,but in Synchronous mode ,the sync byte is "0xA5" when host-> ANT, and "0xA4" when ANT -> host.

I'm confused,Is that true?      

Yes, in SPI mode the sync byte differs depending on the intended direction of the message because ANT is the serial master in this case. Thus it uses 0xA5 to indicate when the host can send a message.