AN2682 - Application note

STR711 implementation example

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3.2.1

CPU usage

In order to estimate the efficiency of this solution, for integration in different applications, we

can calculate the CPU load relative to the I2S communication. This value determines the

ratio of time dedicated to the I2S task. This value is calculated as follows:

T is time between the transmission of two words, so:

is the FIFO size.

Tc is the CPU usage time, obtained (considering a mean instruction length of 3 CPU

cycles), by adding:

●

●

The time needed to save and reload the context (16 + 26 CPU cycles).

●

The interrupt code execution time (47 x 3 CPU cycles).

●

Other durations need to be added such as the latency between the APB and the AHB

buses, the time related to the library operations, the optimization type used...

Determining the exact duration of all these terms is outside the scope of this document

and requires specific and detailed explanations for each case. The intention here is to

give a simplified overview of the performance.

For a 32 MHz MCLK frequency, 16-bit format and 10-word FIFO length, the CPU usage is

therefore:

C = 3.9%

C

Tc

T

-------

=

T

1

SCLK

----------------

16

DW

T

F

×

×

×

=