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gpio_pads_control.md

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GPIO pads control

This page explains the current capabilities of the Raspberry Pi GPIO pins. It is applicable to all models up to and including the 3B+ model. The main thing to note is that the GPIO drive strengths do not indicate a maximum current, but a maximum current under which the pad will still meet the specification. You should set the GPIO drive strengths to match the device being attached in order for the device to work correctly.

How drive strength is controlled

Inside the pad are a number of drivers in parallel. If the drive strength is set low (000) most of these are tri-stated so they do not add anything to the output current. If the drive strength is increased, more and more drivers are put in parallel. The following diagram shows that behaviour:

GPIO Drive Strength Diagram

What does the current value mean?

The current value specifies the maximum current under which the pad will still meet the specification.

  1. It is not the current that the pad will deliver
  2. It is not a current limit so the pad will not blow up

The pad output is a voltage source:

  • If set high,the pad will try to drive the output to the rail voltage, which on the Raspberry Pi is 3V3 (3.3 volts)
  • If set low, the pad will try to drive the output to ground (0 volts)

The pad will try to drive the output high or low. Success will depend on the requirements of what is connected. If the pad is shorted to ground, it will not be able to drive high. It will actually try to deliver as much current as it can, and the current is only limited by the internal resistance.

If the pad is driven high and it is shorted to ground, in due time it will fail. The same holds true if you connect it to 3V3 and drive it low.

Meeting the specification is determined by the guaranteed voltage levels. Because the pads are digital, there are two voltage levels, high and low. The I/O ports have two parameters which deal with the output level:

  • VIL, the maximum low-level voltage (0.9V at 3V3 VDD IO)
  • VIH, the minimum high-level voltage (1.6V at 3V3 VDD IO)

VIL=0.9V means that if the output is Low, it will be <= 0.9V. VIH=1.6V means that if the output is High, it will be >= 1.6V.

Thus a drive strength of 16mA means:

If you set the pad high, you can draw up to 16mA, and the output voltage is guaranteed to be >=VIH. This also means that if you set a drive strength of 2mA and you draw 16mA, the voltage will not be VIH but lower. In fact, it may not be high enough to be seen as high by an external device.

There is more information on the physical characteristics of the GPIO pins here. Note that on the Compute Module devices, it is possible to change the VDD IO from the standard 3V3. In this case, VIL and VIH will change according to the table on the linked page.

Why don't I set all my pads to the maximum current?

Two reasons:

  1. The Raspberry Pi 3V3 supply was designed with a maximum current of ~3mA per GPIO pin. If you load each pin with 16mA, the total current is 272mA. The 3V3 supply will collapse under that level of load.
  2. Big current spikes will happen, especially if you have a capacitive load. That will "bounce" around all the other pins near it. It is likely to cause interference with the SD card or even the SDRAM behaviour.

What is a safe current?

All the electronics of the pads are designed for 16mA. That is a safe value under which you will not damage the device. Even if you set the drive strength to 2mA and then load it so 16mA comes out, this will not damage the device. Other than that, there is no guaranteed maximum safe current.

GPIO Addresses

  • 0x 7e10 002c PADS (GPIO 0-27)
  • 0x 7e10 0030 PADS (GPIO 28-45)
  • 0x 7e10 0034 PADS (GPIO 46-53)
Bits Field name Description Type Reset
31:24 PASSWRD Must be 5A when writing; accidental write protect password W 0
23:5 Reserved - Write as 0, read as don't care
4 SLEW Slew rate; 0 = slew rate limited; 1 = slew rate not limited RW 0x1
3 HYST Enable input hysteresis; 0 = disabled; 1 = enabled RW 0x1
2:0 DRIVE Drive strength, see breakdown list below RW 0x3

Beware of SSO (Simultaneous Switching Outputs) limitations which are device-dependent as well as dependent on the quality and layout of the PCB, the amount and quality of the decoupling capacitors, the type of load on the pads (resistance, capacitance), and other factors beyond the control of Raspberry Pi.

Drive strength list

  • 0 = 2mA
  • 1 = 4mA
  • 2 = 6mA
  • 3 = 8mA
  • 4 = 10mA
  • 5 = 12mA
  • 6 = 14mA
  • 7 = 16mA