The timekeeping gets an unplanned second part, for a fifth variant. Fifth time is lucky, right ? :-)
http://downloads.qi-hardware.com/people/werner/anelok/tmp/time-backup-20160328.pdf All the circuits presented so far have the issue that a voltage above Vcc is present at the MCU's GPIO(s) when Vc1 > Vcc. This is outside the chip's specification and may or may not cause trouble. Joerg suggested another variant, on page 5: the diode is now at the MCU and thus clamps Vadc to Vcc. A standard diode with Vf around 0.3 V at 1 uA (e.g., Panasonic DA2J10100L) should work nicely for this. One issue with the circuit is that the leakage current of the ADC pin flows through the 3.3 MOhm resistor, which may cause a voltage offset that is as large as the whole measurement range. The magnitude of the leakage should depend largely on the chip temperature. Joerg suggested an algorithm that may help there, too. To take a sample of the unknown Vc1, we would do something like this: static void pulse(float duty) { output(1); sleep(INTERVAL * duty); output(0); sleep(INTERVAL * (1 - duty)); output(Z); } float measure_vc1(void) { unsigned s0; // initial ADC reading, in ADC counts unsigned st; // further ADC readings float duty; // duty cycle, 0 <= duty <= 1 unsigned i; // change detection timeout s0 = adc(); // sample Vadc = Vc1 + Ileak(t0) * R1, // in ADC counts duty = s0 / ADC_RANGE; // fraction of Vcc while (1) { // PWM at "duty" * 100% i = SAMPLES; do { pulse(duty); // emit pulse st = adc(); if (!i--) // Vc1 is stable return duty * 3.3; } while (abs(st - s0) <= NOISE); // correct Vc1 if (st > s0) { // we were too high - pull down do pulse(0); while (adc() > s0); duty -= INCREMENT; } else { // we were too low - pull up do pulse(1); while (adc() < s0); duty += INCREMENT; } } } This basically implements a PWM with variable duty cycle. We start with a first guess based on the ADC value we obtain. Then the PWM pulls Vc1 towards 3.3 V * duty. This voltage is not affected by GPIO leakage. During all this, we read back Vc1 with the ADC (which is affected by leakage). If the values read back stay long enough within the expected noise bounds, our duty cycle matches the correct Vc1 voltage. If our measurements indicate that Vc1 drifts away, we emit 0% or 100% pulses until it goes back to the original value, and we adjust the duty cycle up or down, accordingly. For faster convergence, INCREMENT should be a function of the time it took to pull Vc1 back to the original value. This seems to look pretty decent, and is easy on the hardware. Thanks a lot, Joerg ! - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): discussion@lists.en.qi-hardware.com Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
