i'm trying to implement an application using a Bluetooth 4.0 module, and I wonder if using this application with an Iphone 3GS containing a simple Bluetooth 2.1 EDR module is going to enable me take advantage of the the low energy in my 4.0 module. Thank you for your answers
Bluetooth 4.0 is the standard, which includes both classical Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR) and Bluetooth Low Energy (BLE, or Bluetooth Smart, which is a marketing name).
Compatibility depends on the the device and Bluetooth chip e.g. is it a BR/EDR only chip (Bluetooth 3.0), a BLE only chip or a dual-mode chip capable of both BR/EDR and BLE. A dual-mode chip can (until 4.1, at least) only operates in either BR/EDR or BLE mode for any given connection.
A dual mode chip, certified as Bluetooth 4.0, can and should be able to connect to a Bluetooth 2.1 chip, in BR/EDR mode.
A Bluetooth BLE (Bluetooth Smart) chip will not be able to.
First, you must clearly determine if your Bluetooth 4.0 module is only capable of BLE.
According to Bluetooth 4.0 (ie BLE), there are two kinds of devices 1) A dual mode device , which support BR/EDR , ie Bluetooth 2.1 or 3.0 and same time it support Bluetooth 4.0 2) A BLE only device, which supports only Bluetooth 4.0
In general, sensors which supposed to be work in coin cell battery will be designed with BLE chipset and mobile devices will be designed with Dual mode chipset. So the mobile device can have interoperability with other BR/EDR device as well as it can connect and fetch data from sensors.
More over a BLE device can not communicate with a classic Bluetooth device[ BR/EDR or Bluetooth 2.1 device]
No of coures not, how can a Bluetooth 2.1 magically turn into a bluetooth 4 module.
If you apps requires Bluetooth 4.0 than add it to the Required device capabilities in the info.plist. Add bluetooth-le to the Required device capabilities.
Bluetooth 4.0 in low energy mode is actually quite different from what people consider classical Bluetooth. While Bluetooth 4.0 in the iPhone is backwards compatible with Bluetooth 2.1, Bluetooth low energy in general (or Bluetooth Smart as it is also known) is not backwards compatible.
The corebluetooth framework specifies functionality with Bluetooth low energy (4.0) and is only available with newer iPads and iPhone 4S.
You cannot by any means work with Bluetooth 4.0 devices using an older iOS device.
Bluetooth Low energy only chip cannot communicate with Classic BR/EDR Chips.
4.0 Bluetooth specification is an umbrella specification that contains both BLE and BR/EDR.
A Dual Mode chip contains BLE as well as BR/EDR capability. Of course, Host also should be capable of dual mode strictly speaking. Dual mode chips, obviously can communicate with both kinds of chips.
来源：https://stackoverflow.com/questions/12004171/bluetooth-4-0-with-older-bluetooth