GPIO output impedanace and series resistance

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1

Hi,

I am using the PRU to emulate an SPI bus and provide deterministic general-purpose digital lines. I am using:

P8:

GPIO1_30
GPIO1_31
GPIO2_22

GPIO2_23

GPIO2_25

GPIO2_24

GPIO2_6

GPIO2_7

GPIO2_8

GPIO2_9

GPIO2_10

GPIO2_11

GPIO2_12

GPIO2_13

I cannot find info concerning the output impedance of these lines and if there has been an attempt at the level of the BBB to match this output impedance with the impedance of the transmission lines by adding a series resistance. Does anyone know if:

  1. There are already series resistances on the expansion outputs of the board indented for impedance matching?
  2. The output impedance of the GPIOs from the AM335x
  3. If I should attempt impedance matching at the level at the cape level since there may be a fair bit of unmatched copper between my resistor and the GPIO pin on the AM335x.

Thanks.

  • Jon
2

Have you looked at the schematic? There are generally no resistors add, but some have them, like say the control lines for the LCD. You can look at thy schematic and figure which ones those are.

Impedance matching is not generally needed expect on say the LCD pins, but I suggest you use a buffer for that on a cape. Nothing else really runs that fast. You can add them to your cape if you like. You can always make the 0 ohms if they are not needed.

Gerald