General-Purpose Peripherals

Modern microcontrollers integrate several features that help in building stable and reliable embedded systems. Once the system is up and running, memory and peripherals can be accessed, and basic functionalities are in place. Only then can all the components of the system be initialized by activating the associated peripherals through the system registers, setting the correct frequencies for the clock lines, and configuring and activating interrupts. In this chapter, we will describe the interface exposed by the microcontroller to access built-in peripherals and some basic system functionalities. We will focus on the following topics:

• The interrupt controller
• System time
• Generic timers
• General-purpose input/output (GPIO)
• The watchdog

While these peripherals are often accessible through the hardware-support libraries implemented and distributed by chip manufacturers, our approach here involves fully understanding the hardware...

Technical requirements

You can find the code files for this chapter on GitHub at https://github.com/PacktPublishing/Embedded-Systems-Architecture-Second-Edition/tree/main/Chapter6.

Bitwise operations

The examples associated with this chapter make extensive use of bitwise operations for checking, setting, and clearing single bits within larger registers (in most cases, 32-bit long). You should already be familiar with bitwise logic operations in C.

The operations commonly used in the examples are the following:

• Setting the Nth bit in the register R via the assignment R |= (1 << N): The new value of the register R will contain the result of the bitwise OR operation between its original value and a bitmask containing all zeros, except the bit corresponding to the value we want to set, which is set to the value one
• Clearing (resetting) the Nth bit in the register R via the assignment R &= ~(1 << N): The new value of the register is the result of a bitwise...

The interrupt controller

Real-time systems have improved their accuracy thanks to the rapid evolution of modern embedded systems, in particular from the research on interrupt controllers. Assigning different priorities to interrupt lines guarantees a lower interrupt latency for higher-priority interrupt sources and makes the system react faster to prioritized events. Interrupts may, however, occur at any time while the system is running, including during the execution of another interrupt service routine. In this case, the interrupt controller provides a way to chain the interrupt handlers, and the order of execution depends on the priority levels assigned to the interrupt source.

One of the reasons for the popularity of the Cortex-M family of microprocessors among real-time and low-power embedded applications is perhaps the design of its programmable real-time controller—namely, the Nested Vector Interrupt Controller, or NVIC for short. The NVIC supports up to 240 interrupt...

System time

Timekeeping is a basic requirement for almost any embedded system. A microcontroller can be programmed to trigger an interrupt at regular intervals, which is commonly used to increment the monotonic system clock. To do so, a few configuration steps must be performed at startup in order to have a stable tick interrupt. Many processors can run at custom frequencies while using the same oscillator as the source. The input frequency of the oscillator, which can be internal or external to the CPU, is used to derive the processor’s main clock. The configurable logic integrated into the CPU is implemented by a phase-locked loop (PLL) that multiplies the input clock from an external stable source and produces the desired frequencies used by the CPU and integrated peripherals.

Adjusting the flash wait states

If the initialization code is running from flash, it might be necessary to set the wait state for the flash memory before altering the system clocks. If the microprocessor...

Generic timers

Providing a SysTick timer is not mandatory for low-end microcontrollers. Some targets may not have a system timer, but all of them expose some kind of interface to program several general-purpose timers for the program to be able to implement time-driven operations. Timers in general are very flexible and easy to configure and are generally capable of triggering interrupts at regular intervals. The STM32F4 provides up to 17 timers, each with different characteristics. Timers are in general independent from each other, as each of them has its own interrupt line and a separate peripheral clock gate. On the STM32F4, for example, these are the steps needed to enable the clock source and the interrupt line for timer 2. The timer interface is based on a counter that is incremented or decremented at every tick. The interface exposed on this platform is very flexible and supports several features, including the selection of a different clock source for input, the possibility...

GPIO

The majority of the pins of a microcontroller chip represent configurable I/O lines. Each pin can be configured to represent a logic level by driving the voltage of the pin as a digital output or to sense the logic state by comparing the voltage as a digital input. Some of the generic pins, though, can be associated with alternate functions, such as analog input, a serial interface, or the output pulse from a timer. Pins may have several possible configurations, but only one is activated at a time. The GPIO controller exposes the configuration of all the pins and manages the association of the pins with the subsystems when alternate functions are in use.

Pin configuration

Depending on the logic of the GPIO controller, the pins can be activated all together, separately, or in groups. In order to implement a driver to set up the pins and use them as needed, it is possible to refer to the datasheet of the microcontroller or any example implementation provided by the silicon...

The watchdog

A common feature in many microcontrollers is the presence of a watchdog timer. A watchdog ensures that the system is not stuck within an endless loop or any other blocking situation within the code. This is particularly useful in bare-metal applications that rely on an event-driven loop, where calls are required not to block, and to return to the main event loop within the allowed amount of time.

The watchdog must be seen as the very last resort to recover an unresponsive system, by triggering a forced reboot regardless of the current state of execution in the CPU.

The reference platform provides one independent watchdog timer, with a counter similar to those of the generic timers, with a 12-bit granularity and a prescaler factor. The prescaler of the watchdog, however, is expressed in multiples of 2 and has a range between 4 (represented by the value 0) and 256 (value 6).

The clock source is connected to a lower-speed oscillator, through an independent branch...

Summary

The clock configuration, timers, and I/O lines are the general-purpose peripherals shown in this chapter, commonly supported by a wide range of microcontrollers. Although implementation details such as register names and placement may differ on other targets, the proposed approach is valid on most embedded platforms, and the general-purpose peripherals are the bricks for building the most basic system functionalities as well as providing a means of interaction with sensors and actuators.

In the next chapter, we will focus on serial communication channels provided by most microprocessors as communication interfaces toward other devices, and peripherals in the proximity of the target system.