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I am designing a circuit where MCU connects to LCD driver IC (FT813) using Quad SPI. Since there is a risk of Both MCU and FT813 pins becoming output at the same time with different values (i.e. one Gnd and the other VCC) and creating a short circuit, I plan on placing 220ohm resistors on the SPI lines for protection.

However, I am worried that these resistors will corrupt the signals. The distance from MCU to Ft813 is about 10cm and I'm aiming for an SPI clock rate of about 3 to 6 Mhz.

Am I taking the right approach? And where is the best position to place the resistors (middle or near closer to one of the ICs)?

thanks

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  • \$\begingroup\$ a.) datasheets, application notes? b.) 10cm is already a huge distance with 3-6MHz for SPI c.) you maybe need! series resistors for correct line impedance so signals are not corrupted (application note?) \$\endgroup\$
    – schnedan
    Commented Oct 11, 2020 at 12:18
  • \$\begingroup\$ @schnedan There was nothing in the applications notes. \$\endgroup\$
    – hadez
    Commented Oct 11, 2020 at 12:25
  • \$\begingroup\$ well, all I can say, I worked with a 25MHz SPI several years ago, altera demanded series resistors for data lines but not for clock. with a >1GHz osciloscope + active probes we found over/undershots resulting in additional clock edges... a series resistor healed the problem. so it might be a good idea just to prepare your PCB to place resistors if needed \$\endgroup\$
    – schnedan
    Commented Oct 11, 2020 at 12:47
  • \$\begingroup\$ @schnedan Those are termination resistors that are usually in the order of 30-50 ohms. They must be placed close to the driver. However QUAD SPI lines are bidirectional and that is my second problem. I don't know where to place these (in the middle of the lines?). I need to place protection resistors that are much higher than these. \$\endgroup\$
    – hadez
    Commented Oct 11, 2020 at 12:54
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    \$\begingroup\$ You're trying to 'fix' a problem which doesn't exist. First, write your code in a way that you don't write to the display when you've told the display you're going to read. Secondly, even if your code is badly written and both micro and display try to drive the QuadSPI data lines at the same time, neither of them will be driving 'hard' enough or long enough to cause any damage. Neither the micro nor the display connect the lines to Vcc or Gnd directly while driving them. At 3-6MHz you're not even likely to need series termination resistors, but you should probably plan for some anyway in case. \$\endgroup\$
    – brhans
    Commented Oct 11, 2020 at 14:54

1 Answer 1

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3.6V Family Logic were typically 22 +/-50% typ Ohm driver impedance so 33R series parts are used for 50 Ohm bus, given tolerances on source.

The FT813 datasheet indicates on p49 of 63, for VccIO=3.3, Rsw = 6 Ω typ, 10 Ω max, thus twice this for differential.

220R is not needed for short cct protection. Rather you choose according to cable type. Std. twisted pair is often 110 or 220 depending on insulation thickness and twists/ft or /m. On PCB Zo depends on track gap/width ratio to gnd plane or trk//trk, so if you don't know assume it is 100 Ohms. (get Saturn PCB ...exe free, don't guess)

12MHz is pretty low clock rate for 12 cm unless your 12 cm is a big ugly jumper wire pair. ( consider 10x BW for margin)

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  • \$\begingroup\$ Then how do I fix the protection for short circuit. QUAD SPI lines are bidirectional. Both ends might breifly be in driving mode and opposite polarities causing a short circuit. \$\endgroup\$
    – hadez
    Commented Oct 11, 2020 at 13:52
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    \$\begingroup\$ They add Rs to match transmission line impedance at each end. I suggested 33R for 50 Ohm, thus 100 Ohm for 120 twisted pair etc. if the load and line at matched the source impedance only attenuates, thus current limiting. But 3.3^2/50 Ohms is only 218mW and 10 ohm source with 40 in series means the driver dissipates 218/5 mW and if both are driving a bit more so no sweat \$\endgroup\$ Commented Oct 11, 2020 at 18:20
  • \$\begingroup\$ 33 R or 40R makes only a small difference. Too much and you lose voltage margin from noise immunity \$\endgroup\$ Commented Oct 11, 2020 at 18:27

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