Display Circuits

Driving a bare LCD does not necessarily require specialized interface circuitry or peripherals. This Design Idea describes an alternative drive scheme, which you can easily implement using the general-purpose outputs of a microcontroller. Many embedded-system applications need to interact with a user by displaying simple numeric or alphanumeric characters. Seven- or 14-segment LED displays are readily available at low cost and in many sizes. However, their relatively high power requirements and limited readability in direct sunlight restrict their use in battery-powered, portable devices.

The purpose of this application note is to design a clock while multiplexing the features as much as possible, allowing the circuit to use the 18-pin PIC16C54. Other devices in the Microchip line expand on this part, making it a good starting point for learning the basics. This design is useful because it utilizes every pin for output and switches some of them to inputs briefly to read the keys. For a more extensive clock design, consult application note AN529.

The purpose of this application note is to design a clock while multiplexing the features as much as possible, allowing the circuit to use the 18-pin PIC16C54. Other devices in the Microchip line expand on this part, making it a good starting point for learning the basics.

A tiny sandwich flag connected perpendicular to the motor spindle aids observation of how long it takes to complete one full rotation. This worked initially, as each burst from the capacitor barely moved the flag 1/4 of a rotation. As the circuit variations became more efficient and powerful (the capacitors values became larger), a single burst provided rotational speed faster than a human can count. I took a huge diversion and built a counter circuit. A pushbutton (SW1) was necessary for manual testing, although an infrared interrupter pair would eventually provide input when connected to the BEAM engine project.

Driving these displays takes a little work. Since they are multiplexed only one row/column can be active at a time. I found that lighting up a column at a time is best (less flicker due to only 5 columns per pass as opposed to 7 rows per pass). To get even brightness each column must be left on the same amount of time and each common anode needs a current limited connection to +5. I accomplished this by connecting the output of a 7407 open collector TTL buffer to the anode along with the current limited +5. When the buffer was on it would sink the current and the LED would not light. If the buffer was off then the current flowed through the LED and it would light up.

This is an easy to build, but nevertheless very accurate and useful digital voltmeter. It has been designed as a panel meter and can be used in DC power supplies or anywhere else it is necessary to have an accurate indication of the voltage present. The circuit employs the ADC (Analogue to Digital Converter) I.C. CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal to digital and can drive a series of four seven segment LED displays directly. The circuits built into the IC are an analogue to digital converter, a comparator, a clock, a decoder and a seven segment LED display driver. The circuit as it is described here can display any DC voltage in the range of 0-1999 Volts.

Designers using the Texas Instruments TSS400, a versatile 4-bit sensor-signal processor, may find the circuit in Figure 1 useful as a replacement for, or an adjunct to, the IC's built-in display capabilities. The TSS400 (IC1) combines the capabilities of a four-channel, 12-bit ADC, built-in counter-timer functions, and fixed-point arithmetic functions with direct eight-digit, LCD display-drive ability. In addition, the system is programmable with E2PROM for nonvolatile program and data storage. This design features an application for two separate four-digit displays, but you can easily change the design to a single eight-digit display or four two-digit displays. The circuit selects code B decoding at pin 9 of IC2. Code B decoding allows the display of characters 0 to 9, "- E, H, L, P" and blanking of digits. Pulling pin 9 to ground (when both inputs to IC3 are active high) results in the blanking of all digits. Pin 7 on IC2 is the active-low decimal-point input.

The LTC3205 is a highly integrated multidisplay LED controller. The part contains a high efficiency, low noise fractional step-up/step-down charge pump to provide power for both main and sub white LED displays plus an RGB color LED display. The LTC3205 requires only four small ceramic capacitors plus two resistors to form a complete 3-display LED power supply and current controller. Maximum currents for the main/sub and RGB displays are set independently with a single resistor. Current for each LED is controlled with an internal current source. Dimming and ON/OFF control for all displays are achieved via a 3-wire serial interface.

This circuit is intended for precision centigrade temperature measurement, with a transmitter section converting to frequency the sensor's output voltage, which is proportional to the measured temperature. The output frequency bursts are conveyed into the mains supply cables. The receiver section counts the bursts coming from mains supply and shows the counting on three 7-segment LED displays. The least significant digit displays tenths of degree and then a 00.0 to 99.9 °C range is obtained.

Features • High efficiency (>82%) with universal range input. • Various Protection functions against over load, over voltage, over current and over temperature.

Figure 1 shows a simple, cost-effective way of providing these bias voltages. A step-up regulator, IC1, forms the heart of the circuit. This regulator switches at a constant frequency of 1 MHz and a fixed duty cycle of 70%. IC1 steps up the input voltage to 5V by storing the energy in the inductor when the internal MOSFET, M1, is on and transferring this energy to C1 when M1 is off. IC1's hysteretic gated-oscillator control scheme achieves the regulation.

The circuit uses a serial-in-parallel out shift register, 74HC595 for receiving serial data from uController board. See example of U5 in the schematic, SER is for data input, SRCLK is shift clock and RCLK is Latch clock. Each data bit is shifted into the register on rising edge of the shift clock. When all data bits are shifted into the 8-bit register, the rising edge of RCLK will clock the data to be latched at each output bit, i.e. QA - QH.

There are several ways that "Caller's Name" is used in public networks. ANSI has standardized two methods; one which sends the name during call setup and another which uses a database dip to look it up at the called end. The former utilizes an optional information element in an SS7 ISUP initial address message (IAM). The later uses SS7 TCAP messages to query an SCP.

I recently needed to control nine seven-segment displays for a microcontroller's serial port. The complication I faced was the need to provide a continuous brightness adjustment for all the digits—from completely dark to fully bright. I couldn't easily use the obvious solution of a string of 74HC595 serial-to-parallel converters driving the segments through series resistors, because I would have needed a variable power supply for the displays—an inefficient and inelegant approach.

A very simple circuit to experiment with AT90S2313, 2x16 LCD display and 4x4 keypad. The clock based on 4 MHz crystal, but you can use anyone crystal between 1-4 MHz. The keys with the name "A", "B" ... "F" are typed to the LCD with numbers 10-16. Because the AVR have only 15 I/O pins we are working the LCD display with 4-bit databus. The 4 resistors (10K) are to protect the AVR from the shortcuts as the coloumn of the keypad is change.

Flat-panel LCD monitors offer excellent image quality and more compact form factor than CRTs—hence, their steadily increasing popularity. Unfortunately, the complexity of their manufacturing process makes LCD monitors considerably more expensive than CRTs. The amplifier that drives VCOM, the voltage on the backplane of the LCD panel, must be able to drive large capacitive loads, deliver high peak output currents, and maintain a constant output voltage.

This application note describes how LEDs (including white LEDs) work. The note also explains how to drive them in battery-powered LED applications, including lithium-ion (Li+ or Li-ion), nickel-cadmium (NiCd), and nickel metal-hydride (NiMH) rechargeable handheld devices where power consumption is important. LED brightness-matching and the value of series vs. parallel LEDs are discussed. Application information is also presented for several LED drivers that can efficiently drive and control LEDs.

The PIC16C71 is a member of the mid-range family of 8-bit, high-speed microcontrollers, namely the PIC16CXXX. The salient features of the PIC16C71 are: ? Improved and enhanced instruction set ? 14-bit instruction word ? Interrupt capability ? On-chip four channel, 8-bit A/D converter This application note demonstrates the capability of the PIC16C71. This application note has been broken down into four subsections: Multiplexing Four 7-Segment LED Displays Multiplexing Four 7-Segment LED Displays and Scanning a 4x4 Keypad Multiplexing Four 7-Segment LED Displays and the A/D Channel Multiplexing Four 7-Segment LED Displays with a 4x4 Keypad and 4 A/D Channels

Although in abundant use, mini µPs suffer from a low I/O-pin count, which places each pin at a premium. Interfacing a bus-type device, such as an alphanumeric LCD, can use most of the I/O pins. Even placing the display in the 4-bit transfer mode still can cost as many as seven lines. An easier way to save pins uses a lowly 74HC164 shift register to cut the pin count to four I/O lines, PA0 to PA3 (Figure 1).

the circuit in Figure 1a implements a clocked serial input for an LCD module and allows a µC to communicate with the LCD over just two signal lines. You can fit the circuit onto a small pc board and mount the board directly behind the LCD module. The connection between the board and LCD module comprises just four wires, including VCC and ground.