ECEN202 Section 2 Characteristics of Digital IC s Part 1: Specification of characteristics An introductory look at digital IC s: Logic families Basic construction and operation Operating characteristics Some variations available Sources of problems. 1 Basic Characteristics of Digital ICs IC chips consist of transistors, diodes and resistors fabricated on a piece of semiconductor material called a substrate. The transistors are used to form the logic gates on the chip Digital ICs may be categorized according to the number of logic gates on the substrate: SSI less than 10 Typical size used in digital lab MSI 10 to 99 LSI 100 to 9999 VLSI 10,000 to 99,999 ULSI 100,000 to 999,999 GSI 1,000,000 or more 2 1
1-1 Basic Characteristics of Digital ICs Two broad families of digital ICs: Transistor-transistor logic (TTL) in which bipolar junction transistor (pnp, npn) form the basic building blocks. CMOS (complementary metal-oxide semiconductor) in which MOSFET devices are the building blocks. Each of these families contain several series of logic IC s. 3 Digital IC Specifications Typical parameters that we may need to know for a digital IC: 1. Power requirements (voltage and current) 2. Logic ranges (NB!) 3. Noise immunity 4. Current parameters fan-out specification 5. Propagation delays 6. Package type and pin-out All this data should be available in the datasheet for that component. We will first briefly look at these specifications before we consider the internal construction of CMOS logic gates and how these specification differ between CMOS and TTL devices (hopefully). Will then also look at some special logic gate configurations. 4 2
Digital IC Specifications 1. Power Requirements All IC s need DC supply power (voltage) to operate. Referred to as V CC for TTL and V DD for CMOS devices V CC for TTL devices is normally +5 V. V DD for CMOS devices can be from +3 to +18 V. 5 Power typically supplied to pin 14 (VCC) and pin 7 in the case of a 14-pin DIP package. What variations can be allowed in supply voltage? For two CMOS devices : 6 3
Power Requirements The amount of power (Watt) required is determined by the amount of current (I CC or I DD ) the IC draws from the supply. Power = Voltage x Current = V CC x I CC or V CC x I DD The power is determined by the logic state of the circuits on the chip. I CC (avg)=(i CCH +I CCL )/2 P(avg)=I CC (avg) x V CC Assuming 50% duty cycle. 7 Power dissipation in a TTL circuit is essentially constant over its range of operating frequencies, Power dissipation in CMOS is frequency dependant, being extremely low under static conditions but rising as the frequency increases. 8 4
Basic Characteristics of Digital ICs Voltage Parameters: Input voltage levels V IH (min) = Minimum high level input voltage the minimum voltage level required for a logical 1 at an input. V IL (max) = Maximum low level input voltage maximum voltage level required for a logical 0 at an input. Voltage Parameters: Output voltage levels V OH (min) = Minimum high level output voltage The minimum voltage level at a logic circuit output in the logical 1 state under predefined load conditions. V OL (max) = Maximum high level output voltage The maximum voltage level at an output in the logical zero state under predefined load conditions. 9 Basic Characteristics of Digital ICs 2. Logic Voltage Ranges Voltages that fall in the indeterminate range will provide unpredictable results and should be avoided. TTL Output TTL Input 10 5
Output and Input voltage ranges for CMOS (a) 5 V (b) 3.3 V 11 3. Invalid voltage levels and noise For proper operation, I/P voltage levels need to be outside the indeterminate range, i.e. < V IL (max) to be LO >V IH (min) to be HI Invalid voltage ranges can be caused by factors such as: exceeding the fan-out (see later discussion) power supply problems noise in the system. 12 6
Noise Immunity Stray electric and magnetic fields can induce spurious signals (noise) on the wiring between logic circuits. A quantitative measure of the noise immunity is called the noise margin Noise on the input signal makes the output briefly change state how much noise can we tolerate before this will happen? Defined by the noise margin of the device 13 Noise Margin The noise margin is a measure of the circuit s noise immunity, expressed in volts. Two measures of the noise margin normally given: High level noise margin V NH = V OH(min) V IH(min) Low level noise margin = V NL = V IL(max) V OL(max) Illustration of noise margin for 5 V CMOS ~0.9 V ~0.9 V 14 7
Basic Characteristics of Digital ICs 4. Current Parameters I IH = High level input current the current that flows into an input when a specified high level voltage is applied to that input. I IL = Low level input current the current that flows into an input when the low level voltage is applied at the input. I OH = High level output current The current that flows from an output in the logical high state under specified load conditions. I OL = Low level output current The current that flows from an input in the logical low state under specified load conditions. 15 Direction of current flow depends on logic level of the output - current sourcing and current sinking 16 8
Fan-Out The output of a logic circuit is often required to drive the input of several further logic circuits. The fan-out or loading factor is defined as the maximum number of logic inputs that an output can drive reliably. Eg. A fan-out of 10 means that 10 logic inputs can be driven from one output. Assume that we are referring to the same logic family, but can also be used in mixed logic families. 17 5. Propagation delays (how long does it take the signal to pass through the logic gate?) A logic signal will experience a delay in going through a circuit. Look at inverter below where the I/P changes from LO to HI and the back to LO. Defined in two ways (imagine inverter): t PLH : Delay from logic 0 to logic 1 t PHL : Delay from logic 1 to logic 0 Shorter propagation delay = faster circuit. 18 9
Speed Power Product A common means for measuring the performance of a digital IC is the speed-power product. Eg. A logic family has an average propagation delay of 10 ns and an average power consumption of 5 mw. Propagation delay = 10 ns x 5 mw = 50 x 10-12 watt.second Smaller is better! = 50 pj 19 6. Package types: DIP vs surface mount packages 20 10