This episode of the UPF in VLSI Series aims to answer the question posed by one of our viewer in the third episode. The episode covers a range of topics, beginning with an introduction to digital buffers and the different types available. The concept of a level shifter, also known as a translator, is then explored, including the various types available. Viewers are then provided with a comparison of buffers and level shifters, including an example to help illustrate the differences between the two. The episode concludes with a summary of the key differences between the two components, with a focus on the SN74LV1T34 logic level shifter.
A digital buffer a.k.a. voltage buffer, provides either "no voltage" or "a output voltage that is same as the input voltage". It draws very little current and will not disturb the original circuitry. To do this, a voltage buffer is constituted with a very high input impedance. As a consequence , the power source will not be affected. It is also called a unity gain buffer because it provides a gain of 1 hence serving no amplification factor. A Digital Buffer is widely used for Registers. A typical Digital Buffer that is used to control multiple data inputs written onto a bus is a Tri-State Digital Buffer, which controls the data flow by "tri- state" pins.
Types of Digital Buffers:
Inverting Buffer : This kind of buffer produces the state opposite to the input. If the input is high, the output is low and vice versa. Graphically, it is often represented as a triangle with a small circle attaching to the tip.
Non-Inverting Buffer : This kind of buffer performs no inversion or decision-making possibilities. A single input digital buffer is different from an inverter. It does not invert or alter its input signal in any way. In other words, Output will be high if and only if Input is HIGH.
Tri-State Digital Buffer : Tri-state digital buffer has two inputs: a data input and a control input. A control works like a valve which controls the data flow. When the control input is active, the output value is the input value.
Active High Tri-State Digital Buffer : An active high tri-state digital buffer is a buffer with control input high as an active state.ONLY when the control input is 1, data transmission occurs.
What is a Level Shifter (a.k.a Translator) ?
A level shifter (a.k.a voltage level translator or logic-level shifter ), in digital electronics, is a circuit used to translate signals from one logic level or voltage domain (VDD/VSS) to another. It allows compatibility between different sub-chip blocks of ICs with different voltage requirements, such as TTL and CMOS. Modern systems use level shifters to bridge domains between processors, logic, sensors, and other circuits. In VLSI most common logic levels have been 1.8V, 3.3V, and 5V. However levels above and below these voltages are also used.
Types of Level Shifters:
Uni-Directional – All input pins are dedicated to one voltage domain, all output pins are dedicated to the other. Bi-Directional with Dedicated Ports – Each voltage domain has both input and output pins, but the data direction of a pin does not change. Bi-Directional with External Direction Indicator – When an external signal is changed, inputs become outputs and vice versa. Bi-Directional with Auto-Sensing – A pair of I/O spanning voltage domains can act as either inputs or outputs depending on external stimulus without the need for a dedicated direction control pin.
Buffer Vs Level Shifter : Comparison
The job of a buffer is to replicate a digital signal and also increase its drive strength to being degraded by other circuitry. As an example , a buffer take the signal from the microcontroller and reproduce it. Unlike the microcontroller, the buffer has a higher tolerance for being loaded down. At he same time the buffer will provide more steady output the with correct voltage for even with fast transition times. A level shifter is similar to a buffer in that it replicates signals and is drive strength is tolerant to being loaded down. However, it contains an additional feature to convert logic levels to to correct voltage value among different Voltage Domains.
Buffer Vs Level Shifter : Example
A micro-controller will use 0v and 3.3v for logic 0 & 1 levels, whereas something you want to connect which will use 0v and 5v instead. What happens when you connect the two? The micro-controller will try to communicate with the external circuit. It will send a 0 at 0v, and the external circuit will recognize the 0v as logic 0. Everything is fine so far. However, when the micro-controller sends a logic 1 at 3.3v, the external circuit may not recognize that as logic 1 since it expects logic 1 to be 5v. A level shifter comes into the picture in this situation. Now we replace the buffer with a level shifter. When the micro-controller outputs logic 0 at 0v, the level shifter will output 0v. When the micro-controller outputs logic 1 at 3.3v, the level shifter will convert 3.3v signal to 5v signal instead. The external circuit sees 0v and 5v and everything works out.
SN74LV1T34 : Logic Level Shifter
Single-Supply Voltage Translator at 5V, 3.3V, 2.5V, 1.8V VCC
Operating Range : 1.65V to 5.5V
Up Translation :
1.2V to 1.8V at 1.8V VCC
1.5V to 2.5V at 2.5V VCC
1.8V to 3.3V at 3.3V VCC
3.3V to 5.0V at 5.0V VCC
Down Translation
3.3V to 1.8V at 1.8V VCC
3.3V to 2.5V at 2.5V VCC
5V to 3.3V at 3.3V VCC
Output Drive
8mA Output Drive at 5.0V
7mA Output Drive at 3.3V
3mA Output Drive at 1.8V
Buffer Vs Level Shifter : Summary
In a nutshell a buffer tries to recreate the input signal as exactly as possible whereas a level shifter looks to alter the signal acting like a language interpreter. A buffer is just an amplifier. It takes a signal and puts out a low- impedance version of the signal. There may or may not be any actual voltage amplification. A a level shifter will translate between different standards of logic 1 & 0 of separate voltage domains.
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