inputmicrocontrollerpulldownpullupstm32
I am working with STM32 micro-controllers. These micro-controllers have the option to set the input pin as pull up or pull down:
Earlier I used to give external pull up or pull down resistor wherever required but now I am wondering whether I can do away with external components and use the pull down feature of the micro-controller itself.
Will that be a good thing to do?
Is there some use case where I should take extra precaution or not use this feature at all?
A Leopardboard 368 plus a LI-VI365 video input board (connected to its optional camera interface) allows you to do both composite video input and composite video output. As indicated by the links in the previous sentence, both boards are available from Mouser.
The Leopardboard has a 432 MHz ARM plus a video processing subsystem. It is open-source and well documented.
From a TI datasheet, this is the equivalent circuit for a 7407 buffer:
Note that the input is relatively low impedance (having a 6k pullup to Vcc through the emitter-base junction (which effectively looks like a diode) of the transistor shown). The datasheet says it requires 1.6mA to pull the input low (parameter IIL). This is a lot of current compared with the few microamps, nanoamps, or picoamps typical of CMOS or other high-impedance inputs.
Q: Since the pull-down resistor is supposed to create a low-level at the buffer's input, why doesn't it show something closer to 0V at A?
A 5.1k pulldown resistor is too large to pull the low impedance input down. With 1.6mA flowing through 5.1k, the voltage drop (calculated via Ohm's law) would be 8.16 volts. To pull the input down below 0.8 volts, you would need a 500 Ohm or smaller resistor. I would suggest trying 330 Ohms.
Q: If a buffer's output is supposed to mirror its input, why does it not read the same 1.9V as A?
The output is floating. The voltage you are measuring is just stray charge. Unless the output is actively pulled down by the 7407 or pulled up by an external circuit, it will float.
Also, it should only "mirror" its input in terms of logic state, not voltage. When the input is pulled to a logic low level (below 0.8V), the output transistor is fully switched on. When the input is at a high logic level (above 2V), the output transistor is fully switched off. At input voltages between 0.8V and 2V, the output transistor may be fully on or fully off or partially switched on.
Best Answer
Yes... You can... It's a proven feature. If you see pin structure in STM32 Reference manual, you can see that it has required components --
If this satisfies your requirements, then, you can do away with external components.
Edit Remember that at Power up, Pins will be tristated. If it does not hamper your application, it is good! That's why it's good practise to init IO section first and then go ahead with other peripherals.