In general, what is the serial port baud rate variation range of Bluetooth?

[Nancy Wang]

When using AT command to send data, the baud rate is set to 9600, and the two modules are connected at different temperatures, and the data is sent through the serial debugging assistant, and the other serial debugging assistant window is observed to confirm whether the communication is normal or not, and the logic analyzer is used to collect the baud rate of TX at the same time. Our questions are：

1. In general, what is the  serial port baud rate variation range of Bluetooth?

2.What is the maximum range of variation?

3.What would be the factors when there is a change in baud rate? (such as temperature, etc.?)

Here are our test data：

[Vidar Berg]

Hello,

Based on the variation you are observing I assume you are using HFINT as your HF clock source. This oscillator has a fairly low frequency tolerance to begin with. It will also have some variation across temperature and from chip to chip. The crystal oscillator is more accurate and stable (HFXO).

You can make the baudrate generator use the HFXO by starting it at the beginning of main() like this:

void main(void)
{
	NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
	NRF_CLOCK->TASKS_HFCLKSTART = 1;
	while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
	...

Best regards,

Vidar

[Nancy Wang]

If optimized as suggested, the responses to the following 3 questions are：

1. In general, what is the  serial port baud rate variation range of Bluetooth?

2.What is the maximum range of variation?

3.What would be the factors when there is a change in baud rate? (such as temperature, etc.?)

[Vidar Berg]

The variation should be negligible when using the crystal oscillator. Exactly how stable it will be depends on what 32MHz crystal you have chosen in your design. For Bluetooth, the crystal needs to have a frequency tolerance of 40 ppm or better (over the operating temperature range). 

1./2. This depends frequency tolerance of the HF clock source you have selected.

3. You may refer to the datasheet of your 32 MHz to find out how stable it is over temperature.