I have to connect a remote device to an existing board. The interface includes:
- a 4-wire SPI port (SCK, MISO, MOSI, SS), where the board is master and the device is slave; the SPI maximum rate is 2 MHz: at the moment I am using it at lower rates (500 kHz), but it could be good to have room for higher speeds
- a “loopback” SPI SCK signal, carried back from the device in order to take into account all the delays
- a 1 MHz clock signal (generated on the board and used by the device)
- an interrupt line generated by the device
- power and ground lines.
The main problems are:
- I have only a 2.5V source on the board
- I have to reach a distance up to 100 meters
- I have to transfer power, so I need the lowest power solution available in order to keep the voltage drop across cables as small as possible.
The device contains only a digital accelerometer which draws < 1 mA and can be powered up to 3.6V. Moreover, the device should be as small as possible.
I was thinking about using LVDS, but I don’t know if it is a good idea for such a distance. Using SN65LVDS047 and SN65LVDT348 (4 drivers and 4 receivers), which I found to be the lowest power ones, on the device side, I have almost 50 mA at 3.3V. With an AWG26 cable I have 13.4ohm x 50 mA x2 = 1.34 V drop, which would prevent all devices to work (provided that a 3.3V was generated in a way from the 2.5V source). Sure, I could use a lower gauge wire, but that would raise the cable dimension and weight…
Moreover, should I isolate all signals? The whole system will be used near a railway, so I’m afraid I could have interferences.
Any kind of suggestion will be welcome.