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That depends on the microcontroller and the signal source. In many cases you can connect the data directly, but sometimes you need a buffer. Some people always use buffers because they consider it to be good practice.
If you can't draw much current from the signal source and for some reason the input impedance is (too) low (because of capacitance etc.), then you need a buffer to drive the uP input.
By the way.. 74LS347 is not a buffer circuit. It is a "BCD To 7 Segment Decoder/Driver".
I hope somebody with better English can explain those.. Sorry if my English was confusing. Meanwhile you could read about output and input impedances and buffer circuits:
I'll try: "good practice" = cheaper to have an input booboo burn a $.30 buffer than a $X. microprocessor. A lot of sensors have too high an output impedance (not enough current) to properly drive a micros' input directly. That also Requires an input amplifier/buffer. If inputs can be Really Strange (i.e. engine tach), I use an optocoupler for the isolation.
Outputs: "most" micros can't drive the currents the circuit design calls for. This Requires an output buffer/driver. If it's even close, see: cheaper to.... HIH (hope it helps) <<<)))
Sticking a TTL chip in front of a CMOS isn't 'buffering' anything - and there's really little point in doing it, it's going to impose a FAR greater load on the source.
There's certainly no common practice of doing so, the OP needs to tell us EXACTLY what the application he's talking about is, although it's entirely possible the IC was left in place from a previous design, or something similar?.
@Nigel Sir,
Well,basically its input interfacing with 8086 microprocessor.The inputs are set of 8 switches (DIP) and connected to pull up at the input of buffer(74ls244) and the other end of DIP grounded.So here they have used buffer for interfacing..
also the book mentions that usually a buffer is always neccesary when interfacing output to uP.
I agree with your point that if the uP has CMOS inputs which are high impedance ,it doesnt make sense connecting the buffer.
And I guess that 8086uP is also CMOS based,so is there any reason for the book to mention otherwise?
See the second para in the attached image from the book.
hi,
For a micro processor buffering of the data and address lines is important.
A uP has all of its external peripherals connected to the data and address buses, so to avoid overloading the data/addr lines of the uP, buffers are used.
The other very important point about the 374 IC you asked about is that the output pins can be set in the Hi Z state as they are tri-state devices.
When in the HiZ state they are in effect disconnected from the uP bus lines, so leaving other peripherals to use the buses.
OK.
EDIT:
With regard to a PIC or micro-controller, most of the peripherals are built within the PIC, so the PORTS do not see such heavy loading as micro-processor, so buffers are not used.
@Nigel Sir,
Well,basically its input interfacing with 8086 microprocessor.The inputs are set of 8 switches (DIP) and connected to pull up at the input of buffer(74ls244) and the other end of DIP grounded.So here they have used buffer for interfacing..
also the book mentions that usually a buffer is always neccesary when interfacing output to uP.
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