V9958 Video for 6502 SBC

[maded2]

thanks for all the feedback. I've actually tried using external 5V power supply and the voltage drop is the same. I've now finished a 4 layer PCB design with separate 5V & GND layer as well as an option for a separate USB power. Will see if this improves the output RGB signal more.

[Chromatix]

It strikes me that you have small bypass capacitors in appropriate places, but there is no big bulk power-supply stabilising capacitor next to the power input, where I'd expect to find it. Whenever you have a supply with significant impedance and a load with short-term transients, you'll need that to avoid dropouts.

[maded2]

thanks, I will add a 470uF capacitor next to the transistors.

Chromatix wrote:

It strikes me that you have small bypass capacitors in appropriate places, but there is no big bulk power-supply stabilising capacitor next to the power input, where I'd expect to find it. Whenever you have a supply with significant impedance and a load with short-term transients, you'll need that to avoid dropouts.

[drogon]

floobydust wrote:

On a different topic... USB cables. I've received some real cheap ones where the wiring is so thin a gauge, that it barely handles a switch transient without the voltage taking a dive. So, as another possible culprit, poorly made USB cables can also result in poor power to an end-user device.

Oh yes, this.

We learned early on in the Raspberry Pi days that it was a race to the bottom on both "PSUs" and USB cables )-:

-Gordon

[Chromatix]

To say nothing of outright violations of the spec. A Google engineer engaged in testing cables found out the hard way that one particular USB A-C cable had the power lines reversed (which blew the power input circuitry on his laptop, which used USB-C for charging), and the high-speed data lines were missing entirely. He'd bought the cable on Amazon.

[BigDumbDinosaur]

Chromatix wrote:

It strikes me that you have small bypass capacitors in appropriate places, but there is no big bulk power-supply stabilising capacitor next to the power input, where I'd expect to find it. Whenever you have a supply with significant impedance and a load with short-term transients, you'll need that to avoid dropouts.

Ditto on that.

While on the subject of bypass capacitors, radial-leaded parts usually introduce less series inductance into the circuit than axial-leaded ones. Series inductance is your enemy when it comes to effective bypassing. I use X7R MLCCs at all active devices, plus an X7R MLCC across the power input jack in parallel with a large, low-ESR electrolytic. The purpose of the MLCC at the power input is to suppress any switching transients that don't get suppressed by devices' MLCCs.

Ideally, each device's Vcc pin should be wired to the device's MLCC and the MLCC bonded to the inner power plane. The device's Vcc pin should not be directly connected to the power plane. The purpose of doing so is to force all switching transients back to the MLCC rather than allow transients to enter the power plane. This is a design technique I learned decades ago and has been advocated by industry gurus such as Dr. Howard Johnson.

[maded2]

BigDumbDinosaur wrote:

Chromatix wrote:

It strikes me that you have small bypass capacitors in

Ideally, each device's Vcc pin should be wired to the device's MLCC and the MLCC bonded to the inner power plane. The device's Vcc pin should not be directly connected to the power plane. The purpose of doing so is to force all switching transients back to the MLCC rather than allow transients to enter the power plane. This is a design technique I learned decades ago and has been advocated by industry gurus such as Dr. Howard Johnson.

thanks, I will implement this recommendation.