7 Programmable Open-Drain and Push-Pull Outputs

TI’s TCA and TCAL family of devices allow the GPIO pins to be configured for open-drain or push-pull outputs. TCA devices can configure open-drain I/O’s by writing to the configuration and output port registers in a specific sequence.

TCAL devices have a built-in output port configuration register from register address 0x4F. The output port configuration register selects port-wise, push-pull, or open-drain I/O stage. A logic 0 configures the I/O as push-pull. A logic 1 configures the I/O as open-drain. ODEN0 configures the Port 0x and ODEN1 configures Port 1x. This information is defined in Table 7-1.

The affects of using push-pull or open-drain GPIO configurations can be observed by the scope captures taken in Figure 7-1 and Figure 7-2.

The open-drain configuration requires a pull-up resistor on the p-port pin since the drain of the internal FET on the p-port is left floating. This configuration results in a slower rising edge due to the RC time constant caused by the combination of parasitic capacitance and pull-up resistance on the p-port.

The push-pull configuration differs from the open-drain as the push-pull configuration does not require an external pull-up resistor. Internal to the device exists two driving FETS, one to VCC and the other to GND in the push-pull architecture. This configuration results in very fast rise and fall times with steep slew rates, but also introduces larger overshoots and undershoots.