I see the same thing, very difficult to see any difference with the scope for the GPIO_SLEW_RATE option.I mean on oscilloscope ... GPIO_DRIVE_STRENGTH is clearly seen, but with GPIO_SLEW_RATE cannot tell the difference, loaded or not, even at 400 MHz output drive, maybe a hint, maybe too subtle.
Surely, some internal IC nodes must be switching with higher slew rate, externally, with normal loads (even with only the scope), it seems there is almost no difference.
Maybe some RPI expert can tell us more details.
The resistor DAC is a classic R2R network, with values chosen to meet the requirements of a video output:Do you use a resistive DAC ?
I'd like to try.
- 4 bits.
- Maximum output: 0.7V with a 75 ohm load.
- Output impedance: close to 75ohm.
- Limit current to acceptable levels for GPIO.
This is done to compensate for the output impedance of GPIO pins.
The 75 ohm resistor is not part of the network, it simulates the input of the target video device.
The resistor divider formed by 4.7 and 147 resistors can be modified to fine-tune the output voltage or impedance.
With the values shown (the real ones I used in the PCB), output impedance is 77.7 ohm, and output is a little bit over 0.7V.
Maximum current for GPIO is between 7 and 9mA (depending on the bit).
Theoretically, with 4.7 and 137 ohm, you get 74.8 ohm impedance and 0.701V output.
Of course, for RGB, I have 3 DACs like this in the same PCB.
Statistics: Posted by visenri — Sat Nov 30, 2024 4:06 pm