hi

 if i want to use posc to output a a signal whose votage linear increase by time, how should i do?

thanks for answering.

Use a VDAC, simply increment a counter 8 bit variable and writes its value to VDAC

in a timed loop using a timer to set rate.

Or use a 256 entry 8 bit table, and fill the table with the arbitrary waveform you want

to  produce. Timer incs a pointer into table, that location value written to VDAC.

Or use an equation based on some f(t)......

Regards, Dana.

Note you can also can use a PWM changing its duty cycle,

followed by a RC LPF on the output (acting as an integrator)

to convert pulse width to DC.

Regards, Dana.

 if it's going to drive a significant load, it would also e a good idea to buffer it with an opamp betwing the DAC and the pin

Small addition to Zeta's comments -