Circuit description

UM1559

10/13

Doc ID 023542 Rev 1

The voltage sensing is not needed for the open loop or closed loop speed control with

tachometer feedback.

Potentiometer

PCB mounted potentiometer R8 is connected between 5 V and ground with the wiper

providing an input signal to STM8 ADC input Ain4. This can be used as a general analog

input signal. In the example software, this signal is used as either a closed loop speed

command or an open loop gating phase angle command.

Tachometer interface circuit

For closed loop speed control, a tachometer signal can be connected to connector J3. The

circuit supports a pure magnet pickup coil type, a hall sensor type pulse tachometer or an

opto interrupter or reflector type of pick-up.

Figure 5.

Tachometer interface

For active devices like a hall sensor or opto pick-up, the 5 V power supply and ground is

available on pins 1 (GND) and 4 (5v) and the device output should be connected to pin 3. A

pull-up resistor, R28, is provided for open collector devices, but totem pole outputs can be

accommodated as well.

For AC pick-up coil type devices, the output wires should be connected at pins 2 and 3. The

differential signal is biased to approximately 1/2 of the supply voltage and diodes D2 and D3

clamp the signals to safe levels for the comparator inputs. A small hysteresis in the

comparator should still provide sufficient sensitivity for low speed operation. The sensitivity

can be improved by removing the pull-up resistor R28, which is not actually needed for the

AC tach.

The example software requires a setting of the number of pulses (rising and falling edges)

per rotation to scale the speed calculation.

User and debug interface

Connector J1 is the standard 4-pin SWIM connector for programming and debug

connections.

Up to three pushbuttons can be installed to provide simple command inputs. The standard

board configuration populates only PB1, which is used by the example software as a

start/stop command. The other two pushbuttons may be installed and used by user