Digital logic families

Digital logic families Digital integrated circuits are classified not only by their complexity or logical operation, but also by the specific circuit technology to which they belong. A logic family is a collection of different integrated-circuit chips that have similar input, output, and internal circuit characteristics, but they perform different logic functions (AND, OR, NOT, etc.). The electronic components used in the construction of the basic circuit are usually used as the name of the technology. The following are the most popular: RTL resistor-transistor logic (obsolete) DTL diode-transistor logic (obsolete) TTL transistor-transistor logic (widespread, standard) ECL emiter-coupled logic (high speed) MOS PMOS, NMOS metal-oxide semiconductor (high component density) CMOS complementary metal-oxide semiconductor (low power consumption)

Various series of the TTL Logic family

Various series of the CMOS Logic family

Why NAND and NOR are so popular? Logical inversion comes free as a result an inverting gate needs smaller number of transistors compared to the non-inverting one. In CMOS, and in most other logic families, the simples gates are inverters, and the next simplest are NAND and Nor gates.

CMOS NAND Gates Use 2n transistors for n-input gate

2-input AND gate:

Electrical Characteristics The characteristics of digital logic families are usually compared by analyzing the circuit of the basic gate in each family: the most important parameters are: fan-out specifies the number of standard loads that the output can drive without impairing its normal operation. A standard load is usually defined as the amount of current needed by an input of another similar gate of the same family. Power dissipation is the power consumed by the gate propagation delay is the average transition delay time for the signal to propagate from input to output. Noise margin is the minimum external noise vo,ltage that causes an undesirable change in the circuit output.

Data sheet for 74HC00 CMOS NAND gates

Logic Levels and Noise Margin for CMOS devices

Logic Levels and Noise Margin for CMOS devices VOHmin the minimum output voltage in the HIGH state VIHmin the minimum input voltage in the HIGH state VILmax the maximum input voltage in the LOW state VOLmax the maximum output voltage in the LOW state

Logic Levels and Noise Margin for CMOS devices

Logic Levels and Noise Margin for CMOS devices

Circuit behaviour with resistive loads An output must sink current from a load when the output is in the LOW state. An output must source current to a load when the output is in the HIGH state.

loading calculation Need to know “on” and “off” resistances of output transistors, and know the characteristics of the load.

Calculate for LOW and HIGH state

Output-voltage drops Resistance of “off” transistor is > 1 Megaohm, but resistance of “on” transistor is nonzero, Voltage drops across “on” transistor, V = IR For “CMOS” loads, current and voltage drop are negligible. For TTL inputs, LEDs, terminations, or other resistive loads, current and voltage drop are significant and must be calculated.

Calculate for LOW and HIGH state

Limitation on DC load If too much load, output voltage will go outside of valid logic-voltage range. VOHmin, VIHmin VOLmax, VILmax

Output-drive specs VOLmax and VOHmin are specified for certain output-current values, IOLmax and IOHmax. No need to know details about the output circuit, only the load.

Input-loading specs Each gate input requires a certain amount of current to drive it in the LOW state and in the HIGH state. IIL and IIH These amounts are specified by the manufacturer. Fanout calculation (LOW state) The sum of the IIL values of the driven inputs may not exceed IOLmax of the driving output. (HIGH state) The sum of the IIH values of the driven inputs may not exceed IOHmax of the driving output. Need to do Thevenin-equivalent calculation for non-gate loads (LEDs, termination resistors, etc.)

TTL Electrical Characteristics

TTL LOW-State Behavior

TTL HIGH-State Behavior

TTL Logic Levels and Noise Margins Asymmetric, unlike CMOS CMOS can be made compatible with TTL “T” CMOS logic families

CMOS vs. TTL Levels TTL levels CMOS levels CMOS with TTL Levels -- HCT, FCT, VHCT, etc.

TTL differences from CMOS Asymmetric input and output characteristics. Inputs source significant current in the LOW state, leakage current in the HIGH state. Output can handle much more current in the LOW state (saturated transistor). Output can source only limited current in the HIGH state (resistor plus partially-on transistor). TTL has difficulty driving “pure” CMOS inputs because VOH = 2.4 V (except “T” CMOS).

AC Loading AC loading has become a critical design factor as industry has moved to pure CMOS systems. CMOS inputs have very high impedance, DC loading is negligible. CMOS inputs and related packaging and wiring have significant capacitance. Time to charge and discharge capacitance is a major component of delay.

Transition times

Circuit for transition-time analysis

HIGH-to-LOW transition

Exponential rise time

LOW-to-HIGH transition

Exponential fall time t = RC time constant exponential formulas, e-t/RC

Transition-time considerations Higher capacitance ==> more delay Higher on-resistance ==> more delay Lower on-resistance requires bigger transistors Slower transition times ==> more power dissipation (output stage partially shorted) Faster transition times ==> worse transmission-line effects (Chapter 11) Higher capacitance ==> more power dissipation (CV2f power), regardless of rise and fall time

Open-drain outputs No PMOS transistor, use resistor pull-up

What good is it? Open-drain bus Problem -- really bad rise time

Open-drain transition times Pull-up resistance is larger than a PMOS transistor’s “on” resistance. Can reduce rise time by reducing pull-up resistor value But not too much