5V Tolerant CSR Bluetooth Module

Here’s a 5V tolerant version of the Bluetooth Module covered in the post about the generic 3.3v Bluetooth Module. The module is known by a few names such as “the eBay bluetooth module”, “the dealextreme bluetooth module”, but it’s official name is the Cambridge Silicon Radio EGBT-04 module. This module is very affordable and seems robust. This circuit follows the recommended application note in the datasheet as follows:

  • The connection from the MCU’s TX line to the Bluetooth’s RX line uses a cheap 1N914 diode and 10K pull-up instead of 3.3K. The weaker pull-up has worked better with the Arduino configurations tested so far.
  • The connection from the Bluetooth’s TX line to the MCU’s RX line is straight-thru. Most MCUs will tolerate this. I haven’t memorized the module’s output current specification but it’s low and the datasheet is posted on this site.
  • The MCU is an Arduino MEGA.
  • The blue LED is connected to the Bluetooth module PIO for connection status. This is an active-high PIO.
  • The purple LED is wired to the module’s RX line so this this fixture can monitor the signal from the MCU. It blinks off when transmission occurs. At 115.2kbps the blink is too fast to see with short byte sequences but at 57.6k they’re noticeable. The firmware on this module has no PIOs for RX/TX (yet).
  • With Arduino IDE 1.0+ SoftwareSerial library the module can talk to Arduino at speeds up to 57.6k on a 16MHz Arduino (Pro Mini, Uno, etc). With the Arduino MEGA using a library for built-in USART the module can talk to Arduino at speeds up to 115.2k – perhaps higher but untested.

    Robustness:
    I’m not sure if the jury is out yet. I have no quantitative way to measure connection strength in the field yet but it seems acceptable. OS X has worked smoothly with this module so far. Android has been quirky with issues both at the SDK level and when fatal APK crashes cause disconnection. Google’s Bluetooth Chat won’t connect but Amarino and Amarino-Embed work. When an app crashes while connected to the module it has to be reset. Whitebox testing on Android hasn’t turned up any other surprises so far. I’d have to say the other modules I’ve worked with have their own quirks with Android. Eventually I’ll be able to make more quantitative statements about this topic.

    The module has performed well in my field-tests involving some real-world scenarios such as connection distance and robustness through walls. The firmware lacks features found in modules from Roving Networks and BlueGiga. In general, I’d say the firmware running on this module doesn’t offer anywhere near the amount of features offered in firmware running on Roving Networks and BlueGiga modules. But I rarely use the full feature set on those modules and there’s a well-featured firmware available and programmable for this one. Another thing is: even the pricey modules provide what’s called “single-mode” Bluetooth profile support. That means, if the firmware is written to support the SPP or HID profile, then that’s all it supports. To support a different profile it has to be re-programmed analogous to this module. All-in-all, I’d say this module measures well. I haven’t found multi-mode support for an affordable price yet, but that’s likely to change.

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