i) Serial Peripheral Interface Bus

The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select (chip select) lines. Sometimes SPI is called a "four-wire" serial bus, contrasting with three-, two-, and one-wire serial buses.

Interface

The SPI bus specifies four logic signals:

• SCLK: Serial Clock (output from master);
• MOSI: Master Output, Slave Input (output from master);
• MISO: Master Input, Slave Output (output from slave);
• SS: Slave Select (active low, output from master).

Operation

The SPI bus can operate with a single master device and with one or more slave devices.

If a single slave device is used, the SS pin may be fixed to logic low if the slave permits it. Some slaves require the falling edge (high→low transition) of the slave select to initiate an action such as the Maxim MAX1242 ADC, which starts conversion on said transition. With multiple slave devices, an independent SS signal is required from the master for each slave device.

Most slave devices have tri-state outputs so their MISO signal becomes high impedance ("disconnected") when the device is not selected. Devices without tri-state outputs can't share SPI bus segments with other devices; only one such slave could talk to the master, and only its chip select could be activated. This is as shown in the fig below:

ii) I²C

I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cell-phone

Design

I²C uses only two bidirectional open-drain lines, Serial Data Line (SDA) and Serial Clock (SCL), pulled up with resistors. Typical voltages used are +5 V or +3.3 V although systems with other voltages are permitted.

The I²C reference design has a 7-bit address space with 16 reserved addresses, so a maximum of 112 nodes can communicate on the same bus. Common I²C bus speeds are the 100 kbits/s standard mode and the 10 kbit/s low-speed mode, but arbitrarily low clock frequencies are also allowed. There are also other features, such as 16-bit addressing.

There are four potential modes of operation for a given bus device, although most devices only use a single role and its two modes:

1. master transmit — master node is sending data to a slave
2. master receive — master node is receiving data from a slave
3. slave transmit — slave node is sending data to the master
4. slave receive — slave node is receiving data from the master