SC16IS752 IRQ Pull-Up Resistor value

timbeyer · ‎10-06-2025

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 · ‎10-06-2025

Dear Tim,

Why 1 kΩ is recommended?

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.
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: $τ=R×C\tau = R \times C$ If the line has, say, 50 pF of capacitance:
- With 1 kΩ: $τ=1,000×50pF=50ns\tau = 1{,}000 \times 50 \text{pF} = 50 \text{ns}$
- With 33 kΩ: $τ=33,000×50pF=1.65µs\tau = 33{,}000 \times 50 \text{pF} = 1.65 \text{µs}$ 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

View solution in original post

JozefKozon · ‎10-06-2025

Dear Tim,

Why 1 kΩ is recommended?

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.
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: $τ=R×C\tau = R \times C$ If the line has, say, 50 pF of capacitance:
- With 1 kΩ: $τ=1,000×50pF=50ns\tau = 1{,}000 \times 50 \text{pF} = 50 \text{ns}$
- With 33 kΩ: $τ=33,000×50pF=1.65µs\tau = 33{,}000 \times 50 \text{pF} = 1.65 \text{µs}$ 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