Digital Logic and Embedded Systems: Smart People Use Logic To Craft Machines

Logic gates are physical devices that use combinational logic to switch an electrical one (“1”) or zero (“0”) to downstream blocks in digital design. Combinational logic uses those bits to send or receive data within embedded systems. Data bits build into digital words used to communicate with other design blocks within the system. Digital bits and words do this with logic gates in an organized fashion using dedicated address, data, or control signal nodes. Logic gates are the physical devices that enable processing of many 1’s and 0’s.

Logic families are collections of integrated circuits containing logic gates that perform functions needed by embedded systems to communicate with one another to drive the design. Logic gates are organized into families relative to the type of material and its operational characteristics.

Most logic gates are made from silicon, although some utilize gallium arsenide or other semiconductor materials. The semiconductor material is doped for organization into layers. The doped layers drive power capabilities and typical impedances at input or outputs of each gate. Logic gates used together must employ the same, or complementary, material properties. Knowledge of material properties for logic gates will drive selection of parts within design blocks.

Logic Families Develop From Transistor Invention

Embedded systems’ evolution was built from combinational logic families made possible from the discovery of the transistor. The transistor is made from semiconductor material and is compact. It is able to handle large amounts of power quickly. The transistor employs three terminals to activate electron flow for use in downstream devices as electricity.

Electricity represented as 1’s and 0’s combines to communicate information throughout an embedded system. Because of its compact size, many millions of transistors combine within very small spaces. This allows millions of gates to operate in compact areas while transmitting and receiving mind-boggling amounts of intelligence through combinational logic. This is all accomplished within a minimal power budget.

Embedded systems process enormous amounts of information using logic gates

Combinational logic continues to be in high demand for embedded systems to use for processing strength. More information requires more gates, and more gates means increasing power demands. Power is both limited in terms of electricity and in terms of heat tolerance by the semiconductor material. Therefore semiconductor laboratories continue to develop material that provides more computational ability at lower power needs. This continues to lead evolution of semiconductor material that is able to compute from much lower source voltages to keep systems cool. By minimizing power needs of the semiconductor material, more gate logic is available to drive computing strength.

Transistor Doping Drives Logic Family Groupings

With changes in semiconductor material to drive logic gates from lower power supplies, the industry now has families of logic. Families of logic are needed to provide multiple integrated circuits (ICs) with compatible switching and power needs. Compatibility is required for glueing microprocessors and memories together into a cohesive design. The types of logic families typically used throughout the industry are identified by material and by defined logic levels.

Compatible gates are those that agree on defined logic states for either a digital “1” or a digital “0.” Logic states use voltage level to define either binary state, a digital “1” as opposed to a digital “0.” Voltage levels are consistent within each logic family to enable complementary use with one another. Mixing families could cause design blocks to miscommunicate information. Some of the generic families each semiconductor vendor offers are:

ECL: Emitter-coupled logic. A family of logic developed for high-speed applications using an overdriven BJT differential amplifier, typically with a small voltage swing of about 0.8V. A logical “1” is typically driven from a negative rail to avoid generating switching noise on the power plane.

TTL: Transistor-transistor logic. A logic family built from bipolar junction transistors. One of the first totem-pole configurations and is typically driven from a 5V voltage rail.

CMOS: Complementary Metal Oxide Semiconductor. A pair of metal oxide transistors, one n-type and one p-type used in a stacked totem with one of the pair activated during operation.

BiCMOS: Bipolar complementary metal-oxide-semiconductor. Combination of bipolar junction transistor and complementary metal oxide semiconductor transistor. This is a combination useful for constructing low-power logic gates.

Embedded system: A computer system with dedicated function within a larger mechanical or electrical system. Ninety-eight percent of all microprocessors are manufactured as components of embedded systems.

Embedded systems use logic families to glue computational ICs together

Each logic family uses a different voltage level to define a digital “1” or “0.” Semiconductor materials to produce each family of logic are somewhat standard throughout the electronic industry. This means that multiple vendors offer logic families with characteristics that are compatible such as TTL, CMOS, or BiCMOS. Knowing core and I/O voltages of the microprocessor, or microprocessors, used within your embedded system will guide selection of vendor families.

Given the vast array of computational operations, logic families offer a multitude of ICs to cover various functions required by an embedded system. Along with gate functions, there are buffers and registers, level translators, and switches. Every function from Schmitt triggers to flip-flops, multiplexors to level translators can be found within families that offer both compatible switching levels and varying switching speeds.

Logic Families Grow Combinational Logic Possibilities

Within each family of semiconductor material there are ICs offered that are useful in building embedded systems. Some of the functional ICs within the logic families offer the following operations:

Arithmetic Logic Unit (ALU): A combinational digital electronic circuit that performs arithmetic and bitwise operations on integer binary numbers.

Buffers and Line Drivers: A line driver is an electronic amplifier circuit designed for driving a load such as a transmission line. Used together with buffers to communicate digital signals across circuit board traces and cables.

Comparators: A device that compares two voltages or currents and outputs a digital signal indicating which is larger.

Counter: Constructed of a number of flip-flops connected in cascade.

Decoders / Demultiplexers: A decoder is a many input to many output device, whereas a demultiplexer is a one input to many output device.

FIFOs: FIFO indicates first-in, first-out and is used to indicate flow of digital bits; shift registers may be used to accomplish delivery into a processor.

Flip-Flops or Latches: A data storage element.

Gates: A physical device implementing a Boolean function.

Multiplexers: A device that selects one of several analog or digital input signals and forwards the selected input into a single line.

Parity Generator / Checker: A combined device used in digital systems to detect single bit errors in the transmitted data word.

Shift Register: A cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the ‘data’ input of the next flip-flop.

Transceivers: A transceiver is a device comprising both a transmitter and a receiver that are combined and share common circuitry in a single housing.

Video Support: Video display controller integrated circuits are devices responsible for generating display signals for embedded systems.

Voltage Translators: A device that can take one voltage input and deliver a different voltage output for the same signal.

Computers are complex and sophisticated arrangements of digital logic

Presently, semiconductor vendors offer voltage level translators that can be used within your embedded system to “glue” complementary logic ICs together. Knowing how your digital design blocks identify 1’s and 0’s with respect to voltage and with respect to timing will help you select level translators for application within your design.

Level Translators and Bus Transceivers Considered

Level translators are devices for use when mixing logic families is desirable. In today’s world of increasing bandwidth needs for processing data, logic devices are needed that consume less power. These devices are used with both new and with legacy designs requiring the ability to mix logic families. One technique available to combine new and old logic is use of translators to recognize switching levels between different families of logic.

Both single-supply and dual-supply translators are available. These translators are offered in various power and timing configurations with the ability to translate as either unidirectional or bidirectional signal paths. Some are configurable and some are available for application-specific configurations.

I2C: Inter-Integrated Circuit is a synchronous, multi-master, multi-slave, packet switched, single-ended, serial computer bus. It is widely used for attaching lower-speed peripheral ICs to processors and microcontrollers.

GPIO: General-Purpose Input/Output is an uncommitted digital signal pin on an integrated circuit or electronic circuit board whose behavior is controllable by the user at run time.

MDIO: Management Data Input/Output, also known as Serial Management Interface (SMI) or Media Independent Interface Management (MIIM), is a serial bus defined for the Ethernet family of IEEE 802.3 standards for the Media Independent Interface (MII).

PMBus: The Power Management Bus is a variant of the System Management Bus (SMBus) which is targeted at digital management of power supplies.

SDIO: Secure Digital Input Output is a type of secure digital card interface used for input or output devices.

SMbus: The System Management Bus is a single-ended simple two-wire bus for the purpose of lightweight communication. It is most commonly found in computer motherboards for communication with the power source for ON/OFF instructions.

SPDT: Single-pull double-throw switch.

UART: Universal Asynchronous Receiver-Transmitter is a device for asynchronous serial communication in which the data format and transmission speeds are configurable.

In addition, for address and data lines, there are bus transceivers that provide level translation as required for specific bus lengths, whether 8-, 16-, 32-, or 64-bit. We look at two level translators and one bus transceiver below.

_LSF010x 1/2/8 Channel Bidirectional Multi-Voltage Level Translator

This part supports open drain and push-pull applications such as I2C, PMBus, or SMbus. It supports hot insertion and is 5V tolerant for I/O ports to support TTL voltage levels. It enables bidirectional voltage level translation between 0.95V, 1.2V, 1.8V, 2.5V, and 3.3V to respective 1.8V, 2.5V, 3.3V, and 5V ports.

LSF family supports up to 100MHz up translation and greater than 100MHz down translation at < 30pF capacitive load and up to 40MHz up/down translation at 50pF capacitive load which allows the LSF family to support more consumer or telecom interfaces (MDIO or SDIO). The LSF family has bidirectional voltage translation without the need for DIR pin which minimizes system effort.

It is great for applications that use GPIO, MDIO, PMBus, SMBus, SDIO, UART, I2C, and other interfaces within the telecom infrastructure. These types of interfaces are commonly found in industrial, automotive and personal computing. If you have need for bidirectional multi-voltage level translation, take a look at the LSF0102YZTR.

TXS02612 SDIO Port Expander With Voltage-Level Translation

A port expander is used with logic family devices to expand one port into two or more for communication with downstream devices.

The TXS02612 is designed to interface the cell phone baseband with external SDIO peripherals. The device includes a 6-channel SPDT switch with voltage-level translation capability. This allows a single SDIO port to be interfaced with two SDIO peripherals. The TXS02612 has three separate supply rails that operate over the full range of 1.1V to 3.6V. This allows the baseband and SDIO peripherals to operate at different supply voltages if required.

If you have need for a port expander for interface with a cell phone, take a look at the TXS02612.

SN74AXC8T245 8-Bit Dual-Supply Bus Transceiver With Configurable Voltage Translation and Tri-State Outputs

Frequently there is need to translate either address or data buses within an embedded system using different logic families. If the system contains 8-bit buses, the SN74AXC8T245 can be a good choice for providing the translation.

The SN74AXC8T245 device is an 8-bit non-inverting bus transceiver that resolves voltage level mismatch between devices operating at the latest voltage nodes (0.7V, 0.8V, and 0.9V) and devices operating at industry standard voltage nodes (1.8V, 2.5V, 3.3V) and vice versa.

This device operates by using two independent power-supply rails (VCCA and VCCB) that operate as low as 0.65V. Data pins A1 through A8 are designed to track VCCA , which accepts any supply voltage from 0.65V to 3.6V. Data pins B1 through B8 are designed to track VCCB, which accepts any supply voltage from 0.65V to 3.6V.

Typical application schematic

This bus transceiver with configurable voltage translation is suitable for use in enterprise and communications systems, in industrial, personal electronics, wireless infrastructure, building automation, and point of sale embedded systems. For any of these types of applications, take a look at the SN74AXC8T245.

Embedded systems are prolific within many applications used by individuals and by industry including personal computers and appliances, enterprise and network computing systems, and within industrial applications throughout the world. Over time, semiconductor material has continued to evolve by providing faster computing speeds along with more switching gates. This has resulted in semiconductors driven from lower voltage levels in order to conserve power and allow realistic thermal management. Level translators are available to arbitrate voltage and switch characteristics between logic family parts.

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