| AVR Optimization #1: Introduction to Register-Level (Mid-Level) Coding of the ATmega328P (Last Update: January 2023) |
Your comprehensive CHUMP journey has prepared you for the second half of your ICS4U experience: the deepest possible drilling into the hardware and software of modern microcontroller technology. The carefully designed sequence of register-level coding exercises below, exploiting familiar ACES' optical devices, is the final preparation for the final push in the Spring: Pure Assembly Language Programming of the AVR ATtiny84 on the Dolgin Development Platform (DDP).
Although these C coding exercises can be considered high-level, they call for a deeper understanding of the underlying architecture of the AVR family of MCUs. For now, this is the familiar ATmega328P, the MCU of choice for the UNO, Nano and other Arduino development boards. I often refer to this coding as mid-level, as it lies in-between high-level C (that hides much of the functionality and potential optimization) and low-level Assembly language (that you've had a brief introduction to in your 7 CHUMP instrucitons). The magic behind functions such as pinMode(), digitalWrite(), digitalRead(), and shiftOut() for example, are exposed and rewritten, in some cases, more efficiently. Direct port bit manipulation of the MCUs' General Purpose Input/Output (GPIO) Registers are called for to achieve improved performance. Conveniently, your CHUMP experience has given you the necessary insights that were lacking at the end of Grade 11 to guide these next steps.
(Informative) Sparkfun Video Series: Level-Up Your Arduino Code: Registers
ATmega328P GPIO Register Reference
Register-Level Exercises
You may wish to create a folder entitled RegisterLevelCoding or something along those lines, to house this unique set of sketches I am asking you to develop.
References
The Arduino Language Reference web page includes links to resources in support of bit manipulation.
Two of these categories include the Bitwise Binary Operators and convenient Bitwise Functions that employ these and other operators to facilitate binary manipulation (aka bit-banging). It serves our immediate purposes with respect to knowledge and skill to code the bodies of these bitwise functions to produce the same result, ourselves.
Task.
B. General Purpose Input/Output (GPIO) Tasks.
Note. It's not how many lines of code that matters, it's often how few.
With the awareness that the AVR's GPIO registers are accessible by name to your Arduino C sketches, develop the most efficient register-level statements you can to achieve each of the following tasks on your UNO or Nano. The employment of register-level coding strategies precludes the use of high-level statements such as pinMode(), digitalRead(), or digitalWrite(), etc.
Curious? See §13.2.2...
(2-2) Place a bicolor LED in pins 4 and 5 and alternate flashing red and green, in a complete cycle every second. |
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A Closer (Register-Level) Look at the shiftOut() Function. Click on the animated timing diagram of the shiftOut() function and study the digital signal forms, carefully. Since the orchestrated Clock, Data, and Latch forms are very similar to other protocols such as SPI and I2C, it is well worth the effort today to become familiar with their behaviour.
Task.
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DIY Charlieplexed 6-LED Stripboard VU Meter. The layout for a simple 6-LED stripboard VU meter that supports a charlieplex strategy appears above, right. In this exercise you are asked solder one together using the components you have been provided with in this arrangment, with L0 starting from the bottom. Three of your AVR's tri-stated I/O pins can drive this display with pins 9-11 working well if you ever intend to apply PWM the LEDs. Obtain ADC readings mapped appropriately to project onto your device.
CharlieStick DOT/BAR Display. Here's an interesting challenge for your nascent register-level coding skills and to best prepare yourself for the next section devoted to a register-level manipulation of the ATmega328P's Interrupt Vector Table.
| LM3914 Circuit | Register-Level (Charlieplexed) LM3914 Emulation |
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for 2022/2023. Over the 22/23 Christmas holidays S. Appleyard and J. Rogan answered the call to design an encasement for an ACES 12-LED Charlieplexed bargragh to support an interesting Register-Level coding project we have in mind. Within two days an initial order to JLCPCB was placed for 15 units and the result was terrific. The photos below shown the first unit curing after the LEDs were epoxied in. Click to enlarge. |
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C. AVR Optimization: Create a Custom Library (Mega328P.h)
You've employed a number of libraries over the past year or so, from the Arduino Core libraries (transparently included in every project) to third party libraries (through the explicit use of the #include directive) that hide the functionality of complex devices and tasks through convenient object instantiation and subsequent member function calls. Well, it's time to establish and exploit your own custom library. For the first attempt, you'll pack useful ATmega328P resources (constants, data structures, functions, etc.) of your own design and selection. You'll gain some familiarity with the process from completing the directed exercises below. In doing so, future projects can reuse and enhance this code to yield the desired outcomes in less time, and with improved confidence.
Task 1. Separation of Code. First, let's consider some code that is generic enough to be reusable in future projects and confirm that it can exist in a separate file.
Task 2. Centralized Access to Common Code. Now, we should place your library (the Mega328P.h file) in a folder that is accessible to all future projects.
Task 3. Syntax Highlighting: keywords.txt. Syntax highlightig is a useful feature that enhances code development. The key to ensuring the personal assets of our custom libraries are supported by the Arduino IDE's conventional syntax highligting scheme is a separate text file called keywords.txt.
Congratulations! You can now proceed to Interrupts Using Register-Level Coding on the ATmega328P.
