This year M2M and IoT are two of the most heard keywords in our technological domain. One of these is fresh new, I mean a couple of year old, when the other one is pretty much older. M2M, standing for Machine to Machine is a capability for two machines (or more) to discuss together and take actions. Like a temperature sensor allowing your boiler to decide when to start or stop. In the industry where automatism are in place since many year, M2M is something in place, running, not new. IoT, standing for Internet of Things is part of the M2M technology but actually raising. In IoT the main word is Internet and it stands for the capability of an object to talk over Internet on its own. Why only now ? mostly because now it starts to be possible.
SPI is a frequent way to interface slave device with a micro-controller / cpu. SPI is a 4 wires synchronous connection between a slave and a master. As part of the 4 wires, there is a CLK signal, the speed is not defined and depends on the slave but it is usually > 10Mhz. SPI allows fast data transfer. Two data signals (MISO and MOSI) for Master (In/Out) Slave (In/Out) are used to transfer data in sync with the CLK clock. The last wire is a chip select signal named CEx for Chip Enable. As you can manage multiple slave, you have to manage different CEx signal, one per slave.