1.3 3-Lead Contact Socket Interface
The basic circuitry needed to use a device in a 3-lead Contact socket includes:
- Socket U1 has pogo pins to connect
the 3-lead Contact device to the socket and the board.
- Pins 1 and 2 (PAD1) provide the SIO signal connection.
- Pins 3 and 4 (PAD2) provide the connection to GND.
- Pins 5 and 6 (PAD3) provide the connection to VCC_DVC.
- U3 provides a footprint for 3-lead Contact devices and is not populated. The footprint is beneath the socket. If so desired, the socket can be removed and a device can be soldered directly to the board.
- Jumper J3 connects the SIO signal to either RX/TX (SWI mode) or SIO_2 (SWI-PWM mode) header pins on header J2. R2 and R5 are pull-up resistors connected to the respective signal lines for each mode of operation. These resistors are required for correct operation.
- JPP1 is a parasitic power jumper that
either connects the power directly to the socket or enables the Parasitic Power
mode.
- JPP closed: VCC power is applied directly to the socket.
- JPP open: The device is powered parasitically through the SIO signal and diode D2.
- Diode D2 is used to allow charging of the C3 capacitor when in Parasitic Power mode. The C3 capacitor provides power to the device in Parasitic Power mode. When the SIO voltage is higher than the VCC_DVC voltage by the forward bias voltage of the diode, the C3 capacitor will charge. D2 also acts to prevent SIO from discharging VCC_DVC when the signal is driven LOW. Note that some devices have an internal diode that will be in parallel with diode D2. In Direct Power mode, diode D2 is always reverse biased and will have minimal impact on performance.