I'm choosing a Bluetooth module for my project. Reading datasheets like this:

I can see sentences like:

11 kbps (BR/EDR)@ UART baud Rate is 115200 bps • 
 8 Kbps (LE) UART baud Rate is 115200 bps 

Does "UART Baud rate" mean that the module can transfer only 1Kb/sec or 14Kb/sec? Is it not too slow for a standard Bluetooth 3.0 which promises at least 1 Mbit/sec?


UART Baud rate is the TX/RX rate between the host controller and the tranceiver. (Not the BT physical link speed). The maximum UART baud rate of this BT UART is just under 1 Mbps. This is the maximum throughput from the MCU (Host device) to the UART.

Bluetooth 3.0 uses an 802.11 (wifi) physical link for data transfer, in the Bluetooth 4.0 spec this is refered to as the HS (high speed) mode. With the other two modes being Classic and Low Energy.

This tranceiver supports Classic and LE mode , and does not offer the high throughput of the Bluetooth 3.0 (HS). The Description of the module is Bluetooth 4.0 Dual mode Module further indicating that HS support is not intended.


Data rate / throughput
Speaking of Bluetooth, there is a big difference between "Data Rate" (1MBit/s) and actual "Throughput" (which can be only 1/10 of that). Data transfer only happens during periodic intervals, not continuously. The BT physical layer is not comparable to a cable connection.
Furthermore there is some protocol overhead involved and some amount of post/pre-processing time for each packet.

Regarding the specs from the data sheet:

  • 11k bps (BR/EDR)@ UART baud Rate is 115200 bps
  • 8 Kbps (LE) UART baud Rate is 115200 bps

This is slightly irritating regarding the units. I am pretty sure (it also corresponds with the values from data sheet TABLE 10‐2) that those mean 11kByte/s and 8kByte/s for Bluetooth EDR (enhanced data rate) and Bluetooth LE (low energy) respectively.

Why the UART baud rate indication?
The most common UART framing (8N1) uses 2 bit per data byte overhead (start- and stop-bit).
So using 115200 baud, the actual throughput is only 92160 bps or 11520 Byte/s. This corresponds to the above mentioned 11kB/s. So when using Bluetooth EDR @115200 UART, the UART is the limiting factor!

Looking at the table "TABLE 10‐2: APP mode Throughput report" from the data sheet, the throughput is higher when using a higher UART baud rate. At higher baud rates, the Bluetooth connection is the limiting factor.

There is virtually no improvement (I can't explain why there even is one) when using a higher baud rate for Bluetooth LE, because it is already operating on its limit with 115200 baud UART.


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