Connect pull-down resistors to I/O port, why can not read low level when there is no external signal?
SONIX MCU general I/O port built-in pull-up resistors, the equivalent resistance is
about 100 Kohm @ 5V, 200 Kohm @ 3V. If client enables I/O pull-up resistor in program
while connects external I/O pull-down resistors, in the case of no signal input, that
is equivalent to internal pull-up resistor and external pull-down resistor endure
voltage pressure in series, which causes I/O port can not read low. For example, in
the case of VDD = 5V, internal pull-up resistor is equivalent to 100K, if there is
an 100K external pull-down resistor connected at this time, in the absence of an external
input signal, the I/O input level is about 0.5VDD, the level has exceeded SONIX MCU
VIL/VIH range, definitely result in an reading error.
Therefore, if user need I/O drop-down state in design, he can connected I/O external pull-down resistor, but please remember to disable I/O internal pull-up function.
Therefore, if user need I/O drop-down state in design, he can connected I/O external pull-down resistor, but please remember to disable I/O internal pull-up function.