GPIO Cross Reference

Configuring Pull-Up Resistors

The Broadcom procedure may seem confusing because of the word clock. Writing to GPPUP and GPPUDCLK0/1 registers by using the preceding procedure is designed to provide a pulse to the internal pull-up resistor flip-flops (their data clock input). First a state is established in step 1, and then the configured 1 bits are clocked high in step 3 (for selected GPIO pins). Step 5 establishes a zero state, which is then sent to the flip-flop clock inputs in step 7.

The documentation also states that the current settings for the pull-up drivers cannot be read (there is no register access available to read these flip-flops). This makes sense when you consider that the state is held by these internal flip-flops that were changed by the procedure. Fortunately, when configuring the state of a particular GPIO pin, you change only the pins that you select by the GPPUDCLK0/1 register. The others remain unchanged. Chapter 16 will demonstrate how to change the pull-up resistors in a C program.

Sourcing Current

Figure 11-2 illustrates how a GPIO port sources current into its load (illustrated as a resistor). Current flows from the +3.3 V supply, through transistor M₁, out the GPIO pin, into the load and then to ground. Because of this, it takes a high (logic 1) to send current into the load. This is an example of an active high configuration.

Sinking Current

Figure 11-3 illustrates how the GPIO output can sink current through the load to ground using transistor M2. Because the load is connected to the +3.3 V supply, current flows from the supply into the load and then into GPIO output pin to ground through M₂. To send current through the load, a logic 0 is written to the output port making this the active low configuration.

Driving LEDs

When an LED is hooked up to the GPIO output port, the load becomes the LED and current limiting resistor. Figure 11-4 illustrates the two ways of driving an LED from a GPIO.

The MOSFETs used in the GPIO output driver, are complementary. Notice how the arrow in the symbol for M1 differs from M2. These transistors act as switches. The drive signal is inverted from the GPIO output bit written. The drive signal to the gate of M1 and M2 is low when a 1-bit is written to the output. A low turns M1 on while simultaneously turning off M2. In this manner, only the upper or lower transistor is on for a given drive signal.

When the GPIO is written with a 1-bit, LED1 is lit because the GPIO transistor M1 is sourcing current through LED1 (review Figure 11-2). Because a 1-bit turns on the LED, this is known as the active high configuration.

When a 0-bit is written out to the GPIO, LED2 is lit because transistor M2 is conducting, sinking the current to ground (review Figure 11-3). Because a 0-bit turns the LED on, this is known as the active low configuration.

To limit the current flow through the LED and to protect the output transistors, a limiting resistor (R) should be used. Calculate the resistor using Ohm’s law:

The math is complicated slightly by the fact that the LED has a forward voltage drop (V_F), as all diodes do. This forward voltage drop should be subtracted from the supply voltage (V_CC) for the resistor calculation. For red LEDs, the voltage drop is usually between 1.63 and 2.03 V.

Knowing the desired current draw for the LED, the resistance needed can be calculated from the following:

where:

V_CC is the supply voltage (+3.3 V).

V_LED is the forward voltage drop for the LED.

I_LED is the required current draw for the LED.

For V_LED it is best to assume the worst case and assume the lowest voltage drop of 1.63 V. About 8 mA is reasonable for brightness from a 5 mm LED, allowing us to calculate the resistance of the limiting resistor:

Since resistors come in standard values, we round up to the nearest standard 10% component value of 220 Ω.

The LED and the 220 Ω limiting resistor can be wired according to Figure 11-4, either in the active high (A) or low (B) configuration.

Other LED Colors

Some pure-green, blue, white and UV (ultra violet) LEDs have a V_F of about 3.3 V. These LEDs would have you compute a resistance at or near zero ohms. In this case, no limiting resistor is required.

Yellow LEDs, on the other hand, can have a V_F of about 1.8 V. It normally happens that you don’t have the datasheet when working with LEDs. Especially when you’ve pulled them out from a junk box. It is best to measure V_F, using the breadboard circuit of Figure 11-5. In this measurement, use 5 V or more as the supply. That way, if your V_F measures near or above 3.3 V, you can get a good reading. Attach your DMM (digital multi-meter) probes at the points indicated and measure the voltage. Assume a resistor of about 220 to 330 ohms (higher when using 3 mm or smaller LEDs).

Despite all the precaution of measuring the forward voltage, you may still compute the 10% resistor value of 220 or 330 ohms. But this gives you reassurance that no harm will occur to the GPIO. For higher voltage LEDs, the limiting resistor can be safely eliminated. If there is any doubt, measure the current consumed by the LED when it is on. It should not exceed 16 mA to stay within the Pi’s driving limit.

Driving Logic Interfaces

For LEDs, the requirements of the interface are rather simple. The interface is a success if the LED is lit when the output port is in one state, and the LED is dark in the other. The precise voltage appearing at the GPIO output pin in these two states is of little concern if the maximum current limits are respected.

When interfacing to logic, the output voltage is critical. For the receiving logic, the output level must be at least V_IH to reliably register a 1 bit (for the BCM2835, this is 1.3 V). Likewise, the output should present less than V_IL to reliably register a 0 in the receiver (for the BCM2835, this is 0.8V). Any voltage level between these limits is ambiguous and can cause the receiver to randomly see a 0 or a 1.

There are a large number of approaches to interfacing between different logic families. A good source of information is provided by the document “Microchip 3V Tips’n Tricks.”¹² Another document titled “Interfacing 3V and 5V Applications, AN240” describes the issues and challenges of interfacing between systems.¹³ It describes, for example, how a 5 V system can end up raising the 3.3 V supply voltage if precautions are not taken.

Approaches to interfacing include direct connections (when safe), voltage-dividing resistors, diode resistor networks, and the more-complex op-amp comparators. There is a whole chapter devoted to the subject in “Custom Raspberry Pi Interfaces.”¹⁴ When choosing an approach, remember to consider the necessary switching speed of the interface.

Driving Bi-color LEDs

This is a good place to mention driving bi-color LEDs. Some of these are configured so that one LED is forward biased while the other is reversed biased but using two leads. Or you can just use a pair of LEDs connected together as shown in Figure 11-6. This has the advantage of needing only two GPIO outputs. To change colors, you simply change the polarity of the pair of GPIO outputs.

The one complication to be aware of is that the V_F of the different colored LEDs are likely to be quite different. You will need to compromise on the limiting resistor. Be sure that the GPIO output never needs to source or sink more than 16 mA.

Output Pins

The output state of the GPIO pins can either be set as a 32-bit word affecting 32 GPIOs at a time or set or cleared individually. Having individual set/clear operations allows the host to change individual bits without disturbing the state of others, or having to know their state.

Input Pins

Once these choices have been made, it is possible to receive data related to input signal changes, or simply query the pin’s current state.

Floating Potentials

The unconnected GPIO input can “float” if there is no pull-up or pull-down resistor provided or configured. When the input is connected to a driving circuit, that circuit will provide a non-floating voltage level. The GPIO input uses a MOSFET transistor. The nature of that is that it is sensitive only to voltage (not current, like a bipolar transistor). So when the input is left unconnected, the GPIO input can sense voltages, including static electricity nearby (like a cat).

Output GPIO pins are clamped to the output level, which leaves the internal input transistor in a safe state. When a GPIO is configured as an input, it is normally best to configure it with a pull-up or pull-down resistance. This will pull the signal high or to ground. When otherwise left to float, the static electricity will be random and require the ESD (electrostatic discharge diodes) protection diodes to bleed away the charge.