Serial LCD Display Bus Types

In the world of LCD displays there are numerous options, in this case the serial LCD display, all customizable, to create the right specifications for your product. Of the four types of monochromatic displays there are two types—character (alpha numeric) and graphic (dot matrix) LCD displays—which use the USB, SPI, and I2C bus protocol. In electronic architecture, a bus is a subsystem that transfers data between components and serial LCD displays make use of this protocol.

What does the word serial mean? The word serial (not to be confused with cereal such as Coco Puffs and Lucky Charms) is the type of bus or interface between the LCD and the microprocessor.

The quick and easy description for a serial interface is an electronic configuration that sends and/or receives one bit of data at a time. A parallel bus LCD sends and receives 4 bits, 8 bits, or 16 bits of data at a time. (Parallel bus LCD’s are very popular and are covered in a future article).

Serial LCD displays that make use of the USB bus

The USB (Universal Serial Bus) is the first of three serial LCD display bus types and there is a good reason for that. USB technology was originally introduced in 1996. USB has increased in popularity until it has become the most common standard for interacting external peripherals to a master microprocessor. A microprocessor is another name for the computer chip that controls a customer’s product. This may be the Intel or AMD chip in a computer or the microchip in your cell phone. The symbol below is the industry standard for USB.

The next generation USB, 2.0 was released in 2001. Third generation USB 3.0’s came on the scene in 2009 with a transfer rate of 4.8Gbps, which is hopping fast by any standard today.

Advantages of a serial LCD displays equipped with a USB interface

There are two very good advantages of a USB serial LCD display.

  1. The USB bus is “Plug-and-play” capable. In the past, the LCD module needed to be attached to the microprocessor before power was supplied to the LCD. Otherwise the microprocessor would ‘not see’ the display. The display would not turn on until you rebooted the microprocessor. An example of this is the keyboard and mouse on older computers. If you plugged in the keyboard or mouse after the computer was on, they would not work. You would need to restart your computer before the computer would recognize them. Plug-and-play units allow the user to connect the display to their product while the power is running and the display will work immediately. USB’s do exactly that.
  2. USB bus supplies both the power cable and signal in one interface. All the signal, power and ground, are contained in one plug. In the past the LCD would normally contain two separate connections. One connection was for the signal, this was the data line that ran between the LCD and the microprocessor. The second connection was the power/ground line; this supplied the power necessary for the LCD to operate.

Cost considerations of a USB serial LCD display

Serial LCD display modules with USB interfaces do cost more than the SPI, I2C and/or parallel interfaced displays. Consequently, this makes them an unattractive option for products that are manufactured in large quantities. If the OEM (original equipment manufacturer) or manufacture is cost-driven, they would forego the USB and make use of another type of bus.

Serial LCD displays that make use of the SPI bus

SPI (Serial Peripheral Interface) sometimes called a “four-wire” serial bus is becoming more and more popular as an interface to serial LCD displays.

Serial LCD displays equipped with SPI have become popular as engineers look for ways to reduce the number of connections between their micro-controller and each peripheral device. Generally, peripheral devices are attached to the master controller and are dependent on the controller. An example of this is on a computer. The Intel or AMD processor is the controller, whereas the keyboard, mouse, and LCD monitor are the peripherals. An SPI facilitates adding more devices without adding a large number of new connections for each new device. The fewer connections result in a lower cost to manufacture the product. Lower manufacturing costs benefit everyone—the seller and the buyer of the product.

A block diagram (as seen below) explains the basic structure of a SPI interface when used on a serial LCD display.

SCLK: Serial Clock (output from master);

MOSI; SIMO: Master Output, Slave Input (output from master);

MISO; SOMI: Master Input, Slave Output (output from slave);

SS: Slave Select (active low, output from master).

Serial LCD displays that make use of the I2C bus

Serial LCD displays that use I2C (i-squared cee), sometimes called a “two-wire interface” make use of two bidirectional data lines. Bidirectional is a term describing when data can be sent and received on the same line, but not at the same time. Think of a train track. Trains can go back and forth on the same track, but can experience dire issues if they are on the same track at the same time headed toward each other.. The timing of the send and receiving signals, our proverbial trains, are controlled through the Serial Clock (SCL).

Typical voltage values for I2C used in LCD display modules are 5V and 3.3V. The popularity of 3.3V Serial LCD displays is increasing faster than 5V. This is due to the popularity of battery operated devices that make use of a LCD.

One major advantage of an I2C bus over an SPI is the low number of connections. Each additional device does not require an additional data line that is necessary for the SPI bus.

One main difference between the I2C and USB 3.0 is the data transfer speed. USB 3.0 can operate up to 4.8Gbps. I2C operates much more slowly at 400 kbit/s (Thousand bits per second) but on some devices the transfer rate can go as high as 3.4Mbps (Mega or million bits per second).

ADC = analog-to-digital converter

DAC = digital-to-analog converter

uC = microcontroller (small u is called micro)

Master node — node that issues the clock and addresses slaves. This is the Intel or AMD chip that drives your computer.

Slave node — node that receives the clock line and address. This is the keyboard, mouse, or LCD monitor.

There are four potential modes of operation for a given bus device.

  • master transmit — master node is sending data to a slave
  • master receive — master node is receiving data from a slave
  • slave transmit — slave node is sending data to the master
  • slave receive — slave node is receiving data from the master

Serial LCD displays are available in a variety of bus types. One important item to remember when choosing the interface is to notify the LCD manufacturer as to which serial interface you will use. If you are still uncertain as to which bus type will suit your product and/or what will be the most cost effective while energy conscious, the manufacturer has experts on staff to help you identify the right specs for your product. It can be expensive and painful to switch from one interface to another after the PCB (Printed circuit board) and the LCD glass have been designed and built, so it is best to make it right from the start!

Call us first for your serial LCD design at 480-503-4259 or complete our contact form.