Which is the default state of MCU's GPIO before initialization?

Question

I have a microcontroller that enables an IC. This IC would be enabled when a logic 0 is set in its enable input pin. So, one microcontrollers' GPIO is connected to that pin. My question is, before initialization of all the GPIOs, they are not set to any value. So, maybe, before setting a logic 1 in that GPIO to start with disabled state of my system before GPIO initialization there is another default value. If that value is a low level value, the MCU would enable the IC during a short period of time. How can I know which is the default state of GPIO (before initialization)? How can I find this in the MCU's datasheet? And how I avoid this to happen in case the default state is a logic 0.

Datasheet

Answer 1

Figure 8.5 on page 62 of your data sheet shows 'GPIO pins as inputs with pullups disabled' after power up when \$\overline{XRS}\$ (the active low system reset) is asserted, before the user code starts running.

However, immediately following power up, there is a short period when the I/O pins are undefined, shown by the cross hatching. This does not guarrantee that they are Hi-Z, or outputting any particular state, so this diagram does not give you the assurance you want. However ...

\$\overline{XRS}\$ is stated on page 34 to be an output, and this diagram shows it staying low for the whole duration of power being applied. You can therefore use this low state to qualify the I/O, so that it's in a known state at all times.

Use a pulldown on your MCU output pin 'GPIO_peripheral_enable' so that it's low when powered off, low during most of the MCU powering-on state, and low when still not yet configured as an output. It only goes high when your code configures it as an output, and sets it high.

Now put \$\overline{XRS}\$ and 'GPIO_peripheral_enable' into a NAND gate, with its output going to your peripheral's \$\overline{ENABLE}\$ pin.

Answer 2

How can I find this in the MCU's datasheet?

You could try Ctrl + F for 'initial state' or 'startup state' and similar terms.

I think it is usual for pins to come up tristated or as inputs, i.e. neither high nor low, but driven from the periphery.

And how I avoid this to happen in case the default state is a logic 0.

You could place a pull-up resistor. Also consider an additional pull-up capacitor. The latter helps keeping static current consumption low when on, because it will guarantee that the line stays high during startup even with very large pull-up resistors (e.g. >1 MΩ).

Answer 3

All registers in all microcontrollers have a default value out of reset. This is described in the specific register documentation. You cannot possibly miss it if you ever read any MCU manual about any specific register. Here's from a random manual for an SAM Cortex M:

The "Reset: 0" unsurprisingly meaning that the corresponding bit is set to zero out of reset. 0 is the overwhelmingly most common reset value for GPIO ports and data direction registers, because it is the setup leading to the lowest current consumption (even though leaving the pins as inputs make them more vulnerable to EMI).

---

How can I find this in the MCU's datasheet?

Please note that it is not uncommon that the manufacturer creates two documents:

• The datasheet, intended for the electrical engineer doing the PCB design
• The user manual, intended for the software engineer writing the program.

In case the manufacturer of your specific part chose the above way of documentation, then you can stare at the datasheet as long as you like because the information isn't there - it is in the user manual.

Generally it is the software engineer's responsibility to make signal pin-outs and determine which pins that need external pull resistors, external ground etc.

Answer 4

You don't say which microcontroller you use, but there must be a section in the datasheet that explains how GPIO pins work and what are the defaults, not all pins are equal as some may be used for e.g. debugging so they are already initialized to certain use during and after reset. Sometimed the detailed settings are in another document such as reference manual.

Generally you would look for GPIO pin section in the data sheet index and failing to find that you use the search functionality of the PDF reader to find keywords such as "gpio" or the specific pin name.

Most general pins are inputs so they don't have any state unless defined by external circuit so pins are not outputs that drive high or low. So floating pins are undefined.

So if you want to be sure what the state is before the MCU initializes the pin as output with correct state, you add external circuit such as a resistor to keep the pin at required level until MCU takes control of the pin.