SPI communication speed calculation

AN3031

18/28

Doc ID 16144 Rev 1

4

SPI communication speed calculation

The chips are connected in cascade (daisy chain) decoupled from the control unit by high-

speed low power optocouplers. SPI signals are filtered by RC cells (Rf = 220, Cf = 100 pF)

placed close to each SCLT3-8BT8 and microcontroller SPI inputs. The signal filtering

implementation reduces the communication bandwidth of the SPI bus.

Using the following simple equations, we can estimate the upper limit of the communication

frequency:

Equation 1

then

Equation 2

and

Equation 3

where:

(for ST 32-bit microcontrollers like the STM32x, it is 5 ns)

SPI - CLK and MISO lines and write out propagation time (influenced by the

galvanic isolator, RC filtering cells and SCLT3-8BT8 chip)

The total propagation delay measured on the application is visible in the figure below; its

(data setup time of an SPI master) as 5 ns, is then:

Equation 4

Equation 5

T

CLKM IN

2

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

t

su

–

t

ptot

=

T

CLKMIN

2t

ptot

t

su

+

()

×

=

f

CL KM AX

1

T

CLKMIN

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

=

T

CL KM IN a pplicatio n

–

2t

pto t

t

su

+

()

×

2

160ns

5ns

+

()

×

330ns

==

=

f

CLKM AX ap plica tion

–

1

T

CL KM IN

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

1

330ns

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

3MHz

≅

==