rebechs

Member Since: January 18, 2010

Country: United States

• So at the “3.3V Device” CS input in my
opinion lie 3.0V instead of 3.3
Sorry! I meant:
So at the “3.3V Device” CS input in my opinion lie 0.30V instead of 0.33V.

• Regarding the second schematic “Inline Resistor”:

1. The 5V devise asserts CS, current is limited through R5, and the
3.3V device should see 3.3-3.8V on the CS pin
2. The 5V device sets the CS line to low then the 3.3V device will see 0.33V on the CS line. (See voltage divider).
To #2: Don’t we also have a voltage divider?
3.3v —- 10K (R4) —- CS-Pin —- 1K (R5) —- 5V (powered on by 5V Device)
Voltage difference between 3.3 and 5.0 of 1.7 Volt:
Voltage over both contacts of the 10K resistor is
1.7V10K/(10K+1K) = 1.55
1K resistor:
1.7V
1K/(10K+1K) = 0.15
Now stack this additional Voltage at the 10K resistor (1.55V) on top of the 3.3V:
3.3V + 1.55V = 4.85V
This 4.85V are at “3.3V Device” CS input in my opinion.
To #3:
3.3v —- 10K —- 1K —- 0V
Voltage over both contacts of the 1K resistor is
3.3V*1K/(10K+1K) = 0.30V
So at the “3.3V Device” CS input in my opinion lie 3.0V instead of 3.3
I could be terribly wrong with all that, please let me know.

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