|
|
DC Supply rails in Embedded projectsThis is an important decision and poor choices may lead to poor performance or even device failures. Also an over-complicated supply configuration will add to system complexity. For example consider that the PC-AT power supply "standard" calls for +12V, +5V, -5V and -12V (though the negative rails are low current). This makes for an elaborate power harness. In contrast a typical development board runs off a single supply voltage, often either 5V via a USB connector or 12V via a "barrel" DC connector. Summary of supply rail namingThere's a reason for the double letter convention, I believe it comes from an IEEE paper but the convention has stuck, however some of the labels have changed meanings and can cause conflicts in Schematic CAD tools. Vcc and GND Usually reserved for a 5V supply. The C stands for collector as this was originally used for TTL designs. You might get away with labeling a 3.3V supply Vcc but be sure to document it to avoid confusion. VDD and VSS Originally the Drain and Source supplies of a NMOS design. Older devices had a 12V VDD, an additional VCC pin for a 5V supply, a VBB (below) and VSS meaning ground. Newer devices had a 5V VDD meaning that VDD and VSS were synonyms for VCC and Ground, and since many CAD packages object to connecting supply symbols together this has been causing problems for at least two decades. The terms VDD and VSS persist in 5V CMOS devices even though both supplies go to the source of a FET. VBB A negative supply, the B probably stood for "bias" not "base" as this connected to the substrate of a NMOS device and VSS was already taken. VBB was usually -5V. VEE Another misleading name, it is found on a small number of CMOS devices mostly switches and some LCD drivers. VEE is an optional negative supply (ground if not used) that allows the IV to switch signals below ground. The E probably once stood for emitter but it is now just a label. Keep it simple with a single 3.3V supplyMany current generation microcontrollers and embedded peripherals have a maximum supply voltage of 3.3V, so an all 3.3V design makes sense. Many USB-powered development boards regulate the nominal 5V bus supply down to a steady 3.3V supply to supply the microcontroller. A 3.3V supply might be derived from a 4V lithium cell using a 3.3V low dropout regulator. Examples of 3.3V devices:
An all 3.3V design avoids the need to translate voltages which may simplify the design process. Go old school with 5VOlder generation PIC microcontrollers are designed for maximum performance at 5V. They will operate at lower supply voltages but the maximum recommended clock frequency will be lower at 3.3V. A 5V supply gives good compatibility with older 5V logic parts such as LSTTL. A 5V supply may help in interfacing with some analogue circuits, since by default interfaces such as ADCs and DACs will have a voltage span of 0-5V. 3.3V powered devices obviously support a lower 0-3.3V span. 5V circuits may be powered directly from a "USB" 5v power supply. Go dual voltage with 3.3V and 5VA dual voltage system means checking device compatibility and implementing level converters where required. An example of where conversion may not be needed in a dual voltage system is the ENC28J60 Ethernet interface. The inputs that need to be driven by a microcontroller: CS, SCK and SI are 5V tolerant meaning they will not be adversely affected by being driven 1.8V above the device's supply rail. The outputs that need to go to the microcontroller: SO, Clockout and INT will only be driven to 3.3V which is too low for reliable operation of a 5V CMOS input, however if it can be arranged that the microcontroller pins used have "TTL" input thresholds then direct connection is possible. I've made a list showing logic compatibility and a list of resources for voltage conversion. Be warned that It is easy to "sleepwalk" into a mixed voltage system, particularly when using development kits, as an example the Arduino Uno R4 is primarily a 5V platform but the WiFi function is provided by a 3.3V powered ESP32 device. The Mikroelektronika "click" platform specifies dual supplies of 3.3V and 5.0V. Many click boards have configuration links for 3.3V or 5.0V compatibility, though the factory default is usually 3.3V. Alternatively you may intentionally choose to go dual voltage in order to support a mix of 3.3V and 5V devices running at their specified supply voltages. Add a -5V, -3.3V or -1.8V supplyA low current supply may be inexpensively derived using a charge pump IC such as the ICL7660 or one of its numerous equivalents. A negative supply may be required for some types of LCD display, or where not actually required it may be convenient, for example where it is desired to use a 5V LCD in a 3.3V design. A negative supply simplifies processing of analogue signals referenced to ground as some analogue ICs may be powered from +/-5V or +/-3.3V. The supply range of +/-5V is within the supply range of the 74HC4051, 74HC4052 and 74HC4053 analogue switch ICs. A 5V ADC or DAC might be powered from supplies of +3.3V and -1.8V, enabling it to process both positive and negative voltages. It is a challenge to convert the resulting logic levels back to 0-3.3V but a series P-FET might be used. Add +/-15V or +/-12V supplies+/-15V or +/-12V, often described as simply +V and -V, may be used to power "classic" operational amplifiers such as the 741 and its derivatives, also analogue multipliers such as the AD633. Historically a +/-15V supply allowed for a +/-10V signal range without clipping. More modern devices do not require as much "overhead" and can reliably achieve +/-10V from +/-12V supplies. Another use for +/-12V was to power RS232 buffers. The MC1488 quad buffer requires +/-12V supplies, though it has mostly been superceded by parts with inbuilt charge pumps running from 5V or 3.3V. For modest current drains of around 50mA it may be worth considering using a DC-DC converter to derive the supplies from 5V. It is worth checking if you can get away with lower supply voltages, for example +/-5v signals might be handled adequately by "rail to rail" operational amplifiers running from +/-5V. Run from a 12V or 24V supplySometimes it is advantageous to fit a regulator or step-down DC to DC converter to allow a board to accept a 12V or 24V supply. This can go a long way to "foolproof" a design as it is relatively simple to protect against reverse polarity and many converter circuits will accept a wide range of voltages. This may be desirable in systems incorporating DC motors or other high power loads. A higher input voltage means a lower current. Many broadband routers run from 12V external supplies, even though internally the supply must be converted down. One explanation I have read is that meeting energy efficiency regulations at 5V demands a thicker cable compared to 12V.
|