SC16IS752 IRQ Pull-Up Resistor value

[timbeyer]

In the data sheet for SC16IS752 (and even SC16IS750) the IRQ Pull-Up Resistor value is recommended 1 kOhm for 3.3 V supply. What is the cause ? What will occur with a pullup as high as 33 kOhm ? There is no value given for the leakage current (for GPIO it's 1 µA, for SPI it's 10 µA).

We design a low power circuit with a quite long interrupt response time and would not spend 3.3 mA over several seconds when the rest of the circuit runs on 2 mA.

[JozefKozon · Accepted solution]

Dear Tim,

Why 1 kΩ is recommended?

1. Fast rise time for IRQ signal
   The IRQ pin is an open-drain output, so the pull-up resistor determines how quickly the line goes high after the device releases it.

   • With 1 kΩ at 3.3 V, the current is about 3.3 mA when the pin is low, which ensures a strong pull-up and a fast transition when released.
   • This is important because IRQ signals often need to meet timing specs for SPI/I²C or interrupt controllers.

2. Noise immunity
   A strong pull-up reduces susceptibility to noise and ensures the voltage reaches a valid logic high quickly, even with some parasitic capacitance on the line.

What happens with 33 kΩ?

• Rise time increases significantly because the RC time constant grows: $\tau =R\times C$ If the line has, say, 50 pF of capacitance:

  • With 1 kΩ: $\tau =1,000\times 50\text{pF}=50\text{ns}$
  • With 33 kΩ: $\tau =33,000\times 50\text{pF}=1.65\text{µs}$ That’s still fast for many systems, but if your interrupt latency is long and the MCU samples asynchronously, it might not matter much.

• Logic level stability: With higher resistance, leakage currents (up to 10 µA for SPI pins) could cause noticeable voltage drop. The IRQ might only reach ~3.0 V instead of 3.3 V, which is usually okay but could be marginal in noisy environments.

• If your interrupt response time is in milliseconds and you can tolerate slower rise times, 33 kΩ is likely fine.
• Just ensure:
  • Total bus capacitance isn’t huge.
  • Leakage currents don’t pull the line below VIH (usually >2 V for 3.3 V logic).

With Best Regards,

Jozef