![]() The simplest possible step-down circuit is a resistive divider. Step-down level shifters A simple resistive downshifter. Will it be a step-up, a step-down, or do you need a bi-directional level shifter? If you decide not to use a dedicated part or a 5V tolerant gate in your design, here are a few of the many alternatives. There are many choices available to you including a whole host of dedicated level shifter devices such as these ones from TI, but aside from personal preference some of them will be dictated by your application. If your 3.3V device inputs are not 5V tolerant and your 5V inputs lack 3.3V compatible thresholds then sadly you won’t be able to interface them across voltage levels without a shifter circuit. If there is a moral to this story it is to always read the datasheet carefully, and use the TTL-compatible parts such as in this case 74HCT, when they are available. As you might expect it failed to work, and of course I wasted time looking everywhere else but my defective choice of part. Some CMOS ICs such as the 74HC4053 analogue switch I used in a Raspberry Pi project don’t quite follow this standard and will work from a 3.3V TTL output, so I was lulled into a false sense of security and reached for another 74HC part to connect to my Raspberry Pi with a new design. CMOS logic defines its logic thresholds as a percentage of supply voltage, which with a 5V supply puts the logic 1 threshold of 70% well above the 3.3V logic 1. When directly driving logic you’d normally use at 5V from a 3.3V output there is one cautionary tale of which to take heed, a personal confession of an electronic failure. CMOS And TTL: A Level Shifting Cautionary Tale Comparison of TTL and CMOS logic thresholds with comparison to 3.3V output. For example the 74LVC series contains a range of 5V tolerant 3.3V versions of many 74-series ICs. Many devices are designed to be 5V tolerant, and you might be lucky enough to find that your circuit could use one and avoid the extra circuitry. But before reaching for that shifter it’s worth taking a look at the detailed specifications of your 3.3V input. In the other direction, driving a 3.3V input from a 5V output you might expect that a level shifting circuit would be required, and in many cases you would be right. For example 3.3V TTL logic shares the 0.8V and 2V thresholds for logic 0 and logic 1 transitions with 5V TTL logic, so a 3.3V TTL output can drive a 5V TTL input without any extra hardware required. And you are not likely to encounter a logic input that might demand so much current that it would damage your output (If you do, use a buffer!). If you are lucky the logic voltage ranges of the two devices may even coincide. If the 3.3V part is an output and the 5V one an input, the lower voltage part can hardly damage the higher voltage one with overvoltage. It might seem odd to start a treatise on level shifting this way, but the first question for the designer when looking at making a 3.3V part talk to a 5V part should be this: Do I even need a level shifter? Some means of managing the transition between voltages is required, so we’re going to take a look at the world of level shifters, the circuits we use when interfacing these different voltage logic families. ![]() ![]() When these different families need to coexist as for example when interfacing to the current crop of microcontroller boards, care has to be taken to avoid damage to your silicon. In recent decades the demands of higher speed and lower power have given us successive families of lower voltage devices, and we will now commonly also encounter 3.3V or even sometimes lower voltage devices. This happy state of never encountering anything but 5V logic as a hobbyist has not persisted. If you were a keen reader of electronic text books you might have read about different voltage levels tolerated by 4000 series CMOS gates, but the chances are even with them you’d have still used the familiar 5 volts. Above 2V but usually pretty close to 5V is a logic 1, below 0.8V is a logic 0. If your introduction to digital electronics came more years ago than you’d care to mention, the chances are you did so with 5V TTL logic. ![]()
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