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In Chapter 3 in this document there is a schematic showing a connection to VGA.

Why is there a 47 ohm resistor connected to VSYNC (GPIO17) and HSYNC (GPIO16)?

I managed to understand the other resistors but I couldn't find out why we need those.

large_schematic

vga_r

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2 Answers 2

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It's called source termination.

Since that is a long cable that goes into a VGA input, the HS and VS wires have a lot of capacitance, and the wires are also a transmission line with some characteristic impedance.

The resistors provide a current limiting to drive the wire capacitance so instantaneous currents are smaller.

It also forms an RC filter to reduce high frequencies so lower bandwidth signal goes in the wire.

The signals are also likely not terminated with the characteristic impedance at the monitor, so the signal gets reflected back. The resistor damps the reflected signal.

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  • \$\begingroup\$ This is what i was afraid of, I can only understand half of this. So... I think i'm going to accept this and in time i will try to learn... what the other half means. It's really irresponsible to give out cheap, ready-to-use microcontrollers like this. Thanks guys anyway \$\endgroup\$
    – user
    Jun 27, 2022 at 18:31
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    \$\begingroup\$ @user In case unaware of what is 'termination' ... practicalee.com/termination Go thru this .... \$\endgroup\$
    – Mitu Raj
    Jun 27, 2022 at 18:49
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For the VGA DAC ...

The result of this is that if all the bits are high (3.3V), corresponding with the maximum digital value, we have all five resistors in parallel to 3.3V. Basic circuit theory tells us that this is 1/Rparallel = 1/499 + 1/1000 + 1/2000 + 1/4020 + 1/8060 = 0.00388, so Rparallel is 258Ω.
If we have a monitor connected to this signal, then we will have a 75Ω resistor to ground inside the monitor (this isn’t shown on this schematic). This creates a potential divider, with 3.3V connected to 258Ω, which in turn is then connected to 75Ω to ground in the monitor. This means we have a full-scale voltage of 3.3 * 75 / (258 + 75) = 0.74V, which is close enough to the target of 0.7V.

From this ...

Why is there a 47 Ohm resistor connected to VSYNC (GPIO17) and HSYNC (GPIO16)?

Generally, when we add such resistors, it is because we will drive "transmission lines" of (50 Ohm - 75 Ohm - 120 Ohm ...).
So, we are "sure" that no "reflection" will occur in the fast pulses which will be transmitted correctly.

And here (internal impedance) 25 Ohm (see link strength = 10) + 50 Ohm = 75 Ohm (impedance cable used in monitors or TV).

enter image description here

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    \$\begingroup\$ Generally, the sync signals are just wired with random wires inside VGA cable so they rarely have a specific characteristic impedance. They are simple TTL interface signals. Sometimes a coaxial may be used for HS, but generally that's only in analog DVI. VGA to BNC adapters have coax on both HS and VS too. But the monitor defnitely terminate the HS and VS with 75 or 50 ohms. More like 2k2. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 8:53
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    \$\begingroup\$ Hum ... Cables, whatever there are, have "always" a "characteristic" impedance. So, do always if there were "transmission lines" ... \$\endgroup\$
    – Antonio51
    Jun 27, 2022 at 9:35
  • \$\begingroup\$ You are quite right. But what is the characteristic impedance of sync pins in a random VGA cable? Your guess is as good as mine, and the monitor still won't terminate it with characteristic impedance. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 11:04
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    \$\begingroup\$ Where did you get the value 25 ohms of driver impedance from? \$\endgroup\$
    – Mitu Raj
    Jun 27, 2022 at 18:37
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    \$\begingroup\$ @MituRaj here is the link, I took the value (for convenience) of Strength=10. thebox.myzen.co.uk/Raspberry/Understanding_Outputs.html \$\endgroup\$
    – Antonio51
    Jun 27, 2022 at 19:10

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