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Osc. frequency selection

Posted by Chandra V on May 8, 2009

I am using NEC v850 architecture freeRTOS porting code with 2MHz oscillator frequency for my project. One of the task is to implement data logger feature. Field data is received at 9600KBaud rate and needs to transmit those data's to PC application at 38400KBaud rate. Max. received data length is 32 bytes and it may be received approx. every 30ms.

My Question:

Most of RTOS based application uses higher osc. frequency like 40 to 100MHz. Should i consider higher frequencies for my application? Do you feel 2MHz osc. clock frequency is not at all sufficient to use freeRTOS API?


RE: Osc. frequency selection

Posted by Samuel Tardieu on May 8, 2009
FreeRTOS couldn't care less about how low your oscillator frequency is. It only needs to know it so that it can generate ticks at the right interval, but that's about it.

RE: Osc. frequency selection

Posted by Chandra V on May 8, 2009

I agree for your comment. But, i feel osc. frequency plays a crucial role. For example, if i configure tick rate as 10ms and my tick functionalities(In freeRTOS, function calls like Tick increment, Scheduler) takes around 6ms due to lower osc. frequency, then software is in trouble if i want respond to an external event within certain ms time frame using Queue or other kernel API services. Isn't?

So, my view is that osc. frequency must be fast enough(Higher osc. frequency) to execute kernel services to meet timing requirements of any embedded product by considering power consumption factor also.


RE: Osc. frequency selection

Posted by Samuel Tardieu on May 8, 2009
Having timing requirements (such as reacting to an event in a given time) does not necessarily imply that you need to use timer functionalities. Priority based systems may offer the proper guarantees. And in this case, having an unfrequent timer tick is perfectly fine.

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