Attaching additional peripherals

The gateways has a number of headers that allow connecting LEDs, a reset button, or a temperature sensor. This page provides some details on how to connect these components.

LEDs

The LEDS header has 5 pins. Pin 1 is connected to VCC (5V), the other 4 pins can each drive a LED. Each LED should be connected via a current-limiting series resistor between VCC and one of the other 4 pins. The anode of the LED should be on the VCC side and the cathode toward the output for the specific LED. The resistor may be placed on either side of the LED, whatever is easiest in the design.

An additional 2 LEDs may be connected to pin 2 and 3 of the GPIO header. The GPIO header doesn't provide VCC, so the anode side of these LEDs must also be connected to pin 1 of the LEDS header. A configuration with the maximum number of LEDs would then be hooked up like this:

You can choose which color LED to use depending on the function you are planning to assign to the LED. I'm using the default assignments (that's how they came to be the defaults), and chose orange for LED A (Flame), green for LED B (Transmit) and LED C (Override), and red for LED D (Maintenance).

Different color LEDs have different forward voltage drops and will require different resistors to get the same current to pass through them. On the other hand, some LEDs may appear brighter than others when using the same current. I'm happy with 330 Ω for my orange LED, 470 Ω for the green ones, and 680 Ω for the red one. You may want to get a bunch of different resistor values (between 220 Ω and 1 kΩ should normally suffice) and experiment with them on the work bench to find the best fit for your LEDs.

Reset button

It can be useful at times to be able to reset the gateway. This is accomplished by pulling the MCLR pin of the PIC low for a short while. A momentary switch can be hooked up to the RST header for this purpose. The RST header has 2 pins: pin 1 (near the R) is connected to ground and pin 2 (near the T) is connected to the MCLR pin of the PIC. Simply connect one of the two wires from the switch to one pin of the RST header, and the other wire to the other pin. Which one way around is not important. Just make sure you have a momentary switch, i.e. one that only makes a connection as long as you push it. A switch that can stay in the "on" position may continuously reset the PIC, preventing it from running its program.

Temperature sensor

A DS18S20 or DS18B20 temperature sensor can be connected to the gateway to measure the outside temperature. The sensor has 3 leads: GND, DQ, and VDD. GND is available at pin 1 of the GPIO header, DQ should be connected to pin 2 of the GPIO header. VDD should be connected to 5V (VCC), which is available at pin 1 of the LEDS header. However, putting both VCC and GND in a long cable that even goes outside feels a bit risky to me. A short-circuit between those two would completely disable the gateway, and with it any communication between the thermostat and the boiler. So I suggest to use pin 3 of the GPIO header to provide the 5V. For that scenario, you should connect the sensor as shown in this picture:
The picture shows a DS18S20, but a DS18B20 is connected in exactly the same way. The resistor may be placed anywhere along the wires. Close to the gateway is probably most convenient. But it may also be next to the temperature sensor if that works out better. Configure the GPIO pins as follows:
  • GPIO A - Vcc (GA=2)
  • GPIO B - Temp sensor (GB=7)
In this configuration, a short-circuit in the cable to the temperature sensor will only pull the I/O port low. The current is limited to 25mA and the gateway can continue to do its normal job. Only the outside temperature reading will obviously no longer be working. When you become aware of the situation, you can even remotely change the GPIO A setting to None to stop maxing out the I/O port until you have time to investigate the problem.