PARTNER FOCUS PROPERTY


Supply Voltage Current Required Switching Speed Fan Out


Figure 1 Figure 1


TTL devices tend to consume an order of magnitude more power compared to CMOS chips especially in their rest state which is why low power consumption applications need to use CMOS (see the comparison in figure 1 above).


The TTL logic family comprises bipolar junction transistors that perform both the delivery of the logic level and an amplification effect. The 5V level originally arose because the logic inputs are derived from the emitters of NPN transistors, and the typical reverse breakdown voltage of an NPN base-emitter junction is slightly higher than 5V.


Due to the characteristics of a higher supply voltage delivering improved noise immunity, 5V was adopted as the highest voltage that could be used safely without causing breakdown problems.


Given that most modern designs are based on lower voltage CMOS logic, does the demise of TTL matter? The answer is yes because many existing and legacy defence and aerospace circuits still use 5V logic in their designs. This factor may cause a good deal of difficulty for engineers as they endeavour to operate their designs at a lower logic level.


It should also be noted that 3.3V levels were originally a compromise between speed and power being close enough to 5V that it was not too challenging to interface with devices that required 5V. A further question for consideration is can you mix TTL and CMOS? While several advances were made in the CMOS technology there are still compatibility issues if you mix the two. The reason for this is because the high voltage figure for each type is different and they are


10 Years 5


4.5 4


3.5 3


2.5 2


1.5 1


0.5 0


Figure 3 10 Years 5V VCC 5V VCC CMOS


3-15V (18V Max) 8µA


Slow 50


TTL


5V (+or -0.25V) 3mA


Fast 10


semiconductor process geometries. As these processes have advanced so the demand for lower voltages has increased as figure 3, (below left) clearly indicates.


A further issue associated with the decision to withdraw 5V chipsets used in oscillators is a reduced drive level capability. With TTL and the higher voltage levels, it was possible to offer a drive capability up to 50pF of load for some applications, but these higher drive versions are now no longer available.


4.44V VOH 3.50V VIH


This limits the output drive of the oscillators to 15pF meaning that to drive more gates and load, multiple oscillators will be required. However, in response to this problem, buffered output oscillators are now available enabling up to four simultaneous 15pF outputs providing up to 60pF of drive capability from a single crystal oscillator, as seen in figure 4 below.


2.4V 2.0V VOH VIH 6 1.5V Vt 1.50V VIL 1 2 3 15pF Output 1 15pF 0.8V


0.4V 0V


5V TTL Figure 2


not generally compatible except in a few cases. An example of the differences in logic levels between TTL at 5V and CMOS at 5V can be seen in figure 2 (above).


The move to ever lower operating voltages in electronic design has gone hand-in-hand with the constant drive for reductions in


10 Years VIL 0.5V VOL Gnd 0V 5V CMOS Gnd VOL


The demise of the 5V crystal oscillator is a relatively recent development and the effects are only just being recognised. The aerospace and defence markets have been slow to understand the implications of the problem despite warnings for the last two to three years preferring to wait until the problem arose. The challenge now is how to adapt existing designs to accommodate a lower voltage part with some even considering using a programmed oscillator as a drop-in replacement which can operate at 5V. The downside of using such devices is that they tend to be unsatisfactory in terms of phase noise/jitter etc.


New designs will obviously not be affected as lower input voltages can be accommodated at the design-planning phase, so it is legacy designs that are likely to suffer most from the demise of 5V crystal oscillators. After a sixty year lifespan during which TTL logic ruled supreme for more than two decades and 5V rails were industry-standard practice, the advent of new technologies and their push for ever lower power consumption mean that it is finally time to sound the death knell for 5V crystal oscillators.


5 3 1 0.6 0.35 Process Geometries nm 0.18 0.13 Euroquartz www.euroquartz.co.uk MARCH 2022 | ELECTRONICS TODAY 29 Figure 4 5 4 15pF Output 2 2.50V Vt 15pF VDD Output 3 Output 4


Voltage


10uF


0.1uF


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