4. TTL Diode Logic AND gate IN1 IN2 OUT L L L L H L H L L H H H = Transistor-Transistor Logic. Uses bipolar transistors and diodes Vcc OUT IN1 IN2 R IN1 IN2 OUT L L L L H L H L L H H H Diode Logic AND gate Problem… defined levels change easily when loaded. E.g. when diode gates are cascaded. Need for transistor buffering IN1 IN2 OUT L L H L H H H L H H H L Vcc IN1 IN2 R Vi OUT R NAND gate! Schottky diodes have a forward drop of 0.25V. They are used to limit the saturation current when a bipolar transistor is turned ON. This allows for faster transistion from ON to OFF state which requires the removal of all charge in the transistor collector-base junction

TTL: practical realisation Dynamic resistance: lower ON (L) voltage, faster switching Limits current in transition Diode AND gate Totem Pole Output Do NOT PRINT Schottky Diodes Clamp diodes

TTL Logic families and specs Vcc=5V±10%, Vohmin=2.7V, Vihmin=2.0V, Volmax=0.5V, Vilmax=0.8V  NMh = 0.7V, NML=0.3V Families: TTL e.g. 7404, 74H04, 74L04 original family Schottky e.g. 74S04: faster, hi power consumption Low Power Schottky e.g. 74LS04: lower Pd, Slower Schottky (common) Advanced Schottky e.g. 74AS04 2x speed of S, same Pd Adv. Low Pwr Sky e.g. 74ALS04 see table 3-11, Wakerly For LS, typically: IILmax=-0.4mA, IIHmax=20uA, IOLmax=8mA, IOHmax=-400uA. FANOUT (LSTTL into LSTTL)=20 NB: TTL outputs can sink more current than they can source. 74FAMnnn and 54FAMnnn TTL and CMOS devices have the same functionality. CMOS devices were given these designations following the wide success of the earlier TTL devices.

TTL vs CMOS TTL CMOS Noise Margins 0.3(high), 0.5 (low) 0.3Vcc Input source currents High in both states: 0.2 to 2mA(L), 20-50uA (H) Typ < 1uA in both states Power Consumption Relatively high, fixed. 2mW for 74LS, 20mW for 74Sxx. Depends on Vcc, frequency. Negligible static dissipation. Very low for FCTT Output drive current Asymmetric: High state: 0.4-2mA Low state: 8 – 20mA Symmetric: Typ 4mA but AC family can drive 24mA Power supply voltage 5V ±10% 3V  Vcc  18V (original 4000 family), 2V  Vcc  6V (newer HC family) Interconnection (CMOS to TTL, TTL to CMOS) Cannot drive CMOS since VOHMIN(TTL)<VIHMIN(CMOS) Pullup resistor needed unless using TTL compatible family e.g. HCT Can directly drive TTL

Applications: CMOS/TTL interfacing 2.7 0.5 VOHMIN, VOLMAX VIHMIN, VILMAX 3.5 1.5 CMOS TTL 4.9 0.1 VOHMIN, VOLMAX VIHMIN, VILMAX 2.0 0.8     Do NOT Print

5. Applications: Unused inputs Floating inputs can lead to unreliable operation!!! Unused (Floating) Inputs [] Tie together and bundle with used inputs OR [] Tie HIGH thru pull up resitor, Rpu OR [] Tie LOW thru pull down resistor, Rpd [] For CMOS use 1K-10K values [] For TTL calculate based on # of inputs tied thru resistor so that: Vcc-RpuIIHmax > VIHmin RpdIILmax < VILmax []Too small Rpu makes TTL susceptible to spikes etc. over 5.5V. See Sec 3.10.4, 3.5.6 Wake. Must ensure that does not affect design function. E.g. tie HIGH for AND/NAND or LOW for OR/NOR

Power supply filtering For each logic IC place a small capacitor (0.01uF tp 0.1uF) across Vcc and ground in close proximity to the IC Reduces transient effect of switching on power supply, particularly when supply source is connected via long circuit path (resistive and inductive effects). Essentially each capacitor provides a local reservoir for fast supply of charge required when the device switches DO NOTPRINT

Applications: Open-drain (CMOS) or open collector (TTL) outputs In CMOS no PMOS transistor, use external pull-up resistor for Vcc drive Vcc Calculate external Rpu so that VOLMAX achieved at IOLMAX. Must include other loads so this gives minimum Rpu. Vcc Rpu IC Z A B Q1 Q2 Z 0 0 open open 1 0 1 open ON 1 1 0 ON open 1 1 1 ON ON 0 A Q1 B Q2 Output stage of Open Drain NAND

Why ? Slightly higher current capability Can form an open-drain/collector bus. Can select data for access to common bus.. E.g for Dataout = Datai set Enablej =0, jI, Enablei =1, DO NOTPRINT Problem -- really bad rise time due to all O/P capacitances in parallel and large pullup.

Applications: Bus Access - Contention and Tristate Logic Common bus Best “fix”…. Tristate logic Vin EN Vout 1 a 1 b ?? EN Vin Vout 0 x HiZ 1 1 0 1 0 1 “regular TTL or CMOS Get bus contention when two outputs try to drive the bus to different states. Value on the bus may be indeterminate; Damage possible (a driving b!!) On a PC data bus, can cause PC to crash TRISTATE LOGIC is widely used in bus systems. They are particularly useful in computer systems where many loic devices must share common signal pathways. Memory devices such as EPROMs, RAMs etc have tristate logic built in, thus saving the user the task of adding external logic for bus control. The ENABLE lines of the internal tristate logic are usually tied to an external OUTPUT ENABLE or CHIP SELECT pin. Available in inverting or non-inverting .. Sec 3.7.3 Wakerly. NO Pull-up needed NO degradation in transition speed

Applications: Digital meets analog Schmitt Trigger Inputs…Sec3.7.2/Wakerly Schmitt trigger devices are used primarily to deal with signal levels which are not at valid logic levels. They can therefore be used for interfacing noisy analogue signals to a logic circuit e.g. signals from switches, RC networks etc. interfacing slow signals (i.e. signals which remain in the invalid range for relatively long periods) regenerating degraded logic signals e.g. signals on a long serial communication line. Schmitt trigger devices do comply with the input thresholds of the respective family. However, they employ a bit of hysterisis (memory!!) to take care of invalid signal levels. The devices are characterised by upper and lower thresholds (UT, LT). When the input exceeds UT it is treated as a logic 1 UNTIL it goes below LT. Then, and only then, is it treated as a logic 0. Vo Exercise: Determine the outptuts for Vi as shown Vi VH VT VL VT VH Vi VL Schmitt Trigger o/p Characteristic Standard logic o/p Characteristic Standard, VO Schmitt, VO

VOL+VLED+(ILED*R)=VCC Applications: Logic Drive ILED is 10mA typically worst case Use formula: VOL+VLED+(ILED*R)=VCC to determine R. NB……. Can assume worst case VOL=VOLMAX for some CMOS as well as TTL at IOL=ILED. Best to use device for which IOLMAX>ILED. Driving a LED with TTL Logic Device Vcc R ILED VLED VOL Low output turns LED ON Drive current typ 5 -10mA Use buffers for extra drive

Applications: Logic Drive Driving a Solenoid or relay with TTL 5V relays do exist. Some incorporate the free wheeling Diode. Most have enough internal resistance to operate directly as shown. Check using LED computation if built in resistance is sufficient or if an external series resitance is needed Logic Device Vcc Free-wheeling diode protects electronics from coil back emf Low output turns activates relay or solenoid DO NOT PRINT