+ /* all these i2c/smbus pins should have external pullups for
+ * open-drain sharing among all I2C devices. SDA and SCL do;
+ * PB28/EXTINT3 doesn't; it should be SMBALERT# (for PMBus),
+ * but it's not available off-board.
+ */
+ at32_select_periph(GPIO_PIN_PB(28), 0, AT32_GPIOF_PULLUP);