Push-Pull and Open-Drain Outputs

A push-pull output has two transistors: one connects the pin to VDD, the other to ground, and while it drives, exactly one is on (apart from a brief overlap during each transition). It drives both levels hard, so edges are fast and nothing else is needed on the wire.

An open-drain output (open-collector with a bipolar transistor) has only the transistor to ground. Writing 0 turns it on and pulls the wire low; writing 1 turns it off and lets go. Something else, a pull-up resistor, has to bring the wire high.

↑ This step uses the figure at the top of the page.

When two push-pull outputs disagree, one high-side and one low-side transistor are both on, in series across the supply. The only thing limiting the current is their on-resistance. With an illustrative 50 Ω each:

limited only by the transistors, well above the few milliamps a pin is characterised for (unit 1, lesson 5), and the wire sits at an indeterminate level. This is also what happens when a pin is accidentally configured as an output while another device drives it, a common bring-up bug.

With open-drain outputs nobody can fight. The wire is low if any device pulls it low and high only if all let go: a wired-AND. That is why I²C, interrupt lines shared by several chips and reset lines use open-drain drivers: any device can assert the signal, and a device can also see that someone else is holding it (I²C clock stretching and arbitration rely on this).

Because the pull-up sets the high level, an open-drain output can also drive a line pulled up to a different voltage, within the pin’s voltage rating: that is one simple way to signal between chips on different supplies.

The STM32F4 has an open-drain output type in OTYPER. The RP2040’s pads do not (its pad bit OD is output disable). Open-drain is emulated by keeping the output value at 0 and switching the output enable: enabled means “pull low”, disabled means “let go”. The pin must be under software control first (gpio_init()), and the pad’s reset pull-down stays on unless disabled, slightly loading the external pull-up.

gpio_put(pin, 0);                 /* the value is always 0 */
gpio_set_dir(pin, GPIO_OUT);      /* assert: drive low */
gpio_set_dir(pin, GPIO_IN);       /* release: the pull-up takes the line high */

Never emulate open-drain by writing 1 with the output enabled: that is push-pull high, and it will fight any device that pulls the line low.

A Fast-mode I²C bus has 150 pF of capacitance and runs at 3.3 V.

Upper limit from rise time: $R_{max} = \dfrac{300\ \text{ns}}{0.8473 \times 150\ \text{pF}} \approx 2.36\ \text{k}\Omega$.

Lower limit from drive strength (V_OL ≤ 0.4 V at 3 mA): $R_{min} = \dfrac{3.3 - 0.4}{3\ \text{mA}} \approx 967\ \Omega$.

Any value between about 1 kΩ and 2.2 kΩ works; 2.2 kΩ gives a 30–70 % rise time of 0.8473 × 2.2 kΩ × 150 pF ≈ 280 ns and sinks 1.5 mA when low. A 10 kΩ pull-up, fine for a slow interrupt line, would give about 1.3 µs here and break Fast-mode timing.