AN3031 Datasheet, PDF (18/28 Pages) STMicroelectronics – SCLT3-8 input termination demonstration board

SPI communication speed calculation
4
SPI communication speed calculation
AN3031
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
T_{----C----L---K---M-----I-N--}
2
–
t_su
=
t_ptot
Equation 2
T_CLKMIN = 2 × (t_ptot + t_su)
and
Equation 3
f_CLKMAX
=
----------¹------------
T_CLKMIN
where: t_su is the data setup time of the microcontroller
(for ST 32-bit microcontrollers like the STM32x, it is 5 ns)
t_ptot is the total propagation delay which is a sum of the propagation delays on the
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
value is approximately 160 ns. The maximum communication frequency, considering t_su
(data setup time of an SPI master) as 5 ns, is then:
Equation 4
T_{CLKMIN–application} = 2 × ( t_ptot + t_su) = 2 × ( 160ns + 5ns) = 330ns
Equation 5
f_{CLKMAX–application}
=
-----------¹-----------
T_CLKMIN
=
-------¹---------
330ns
≅
3MHz
18/28
Doc ID 16144 Rev 1