Serial Communication

The sequential transmission of bits, one at a time, via some shared medium

VS Parallel

Pros
- Fewer wires
- Works with smaller devices with lower pin counts and power consumption
- Not as affected by noise (and does not have crosstalk)
Cons
- Typically slower

Use a common data rate that is shared between two entities
- Called the baud rate (commonly written in bits per second)
Start and stop signals are used to indicate the beginning or end of a transmission

Universal serial components that can be used to implement some different protocols
Typically use non-negative DC voltages (logical 1 is non-zero)
Don’t have as many strict guidelines and are easier to setup
Can be full or half duplex (typically full)
Transmission
1. Start bit
  - Transition from idle (high) to low
  - Used by receiver to begin synchronization
2. Data Bits
  - Eight data bits is typical
  - Typically LSB is first
3. Parity Bits
  - Optional
  - Can be even or odd parity (for even the total number of 1s should always be even and visa versa)
4. Stop
  - Return to idle state (high)
  - Only use more than 1 stop bit if there are a lot of calculations that need to happen between frames and you don’t want to manually idle
Time to transmit
- $f r am es \cdot \frac{bi t s}{f r am e} \cdot \frac{seco n d s}{bi t}$
Error Types
- Timing
  - Underflow (samples too fast)
  - Overflow (samples too slow)
- Framing
  - The first stop bit was 0
- Overrun
  - Does not get a stop bit

Serial bus standard
Four signals
- V (4.75-5.25V)
- D-
- D+
- GND
Transmitted on a twister pair of data cables, labeled D+ and D-
- Collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines
- Usually operate together (they are not separate simplex connections)

Use a dedicated signal for synchronization that is shared between two entities and controls the rate at which data is transmitted
That signal is often called a clock signal
Flip flops are used to store the data that was sent when the clock signal changed

Typically utilized by components such as sensors, memory, etc.
Generally chosen for applications where short-distance, full-duplex is required
Description
- Single “master” device can communicate with at least one “slave” device
- Only the master device can initiate some transfer of data
- The master device can only communicate with a single slave device at any given time
  - In some unique cases it can transmit to multiple
- Only one master device on a bus
  - I2C does not have this limitation but is slower because of it
Signals
- Chip select (CS)
  - Indicates when an individual slave device should be enabled (accepting data)
  - Unique chip select signal required for every device on the bus (a decoder can be used to squeeze more out of the available master pins)
  - Active low
  - If there is only one slave, it can sometimes be tied to ground (depends on if the slave requires a falling edge or just low)
- Master-out slave-in (MOSI)
  - Data that is sent from the master to slave device
- Master-in Slave-out (MISO)
  - Data that is sent from the slave device to the master device
  - Only input to the master device
- Serial Clock (SCK)
  - Generated by the master device to control when each bit is to be received by the slave device
  - Polarity - Idle voltage level
  - Phase - Which edge is used to sample each bit of data
As each bit gets shifted out of the master device, a bit form the slave device is shifted in

Comprised of two signals
- Serial Data (SDA) - Bidirectional signal for data to propagate
- Serial Clock (SCL) - Dictates when data is valid and can be read by the receiving device
Multiple master and slaves can share the same bus
Each slave device has a slave address associated with it which is what allows the bus to be shared