{"id":39968,"date":"2021-01-25T16:27:18","date_gmt":"2021-01-25T08:27:18","guid":{"rendered":"\/blog\/?p=39968"},"modified":"2021-09-29T16:25:15","modified_gmt":"2021-09-29T08:25:15","slug":"programmable-io-with-raspberry-pi-pico","status":"publish","type":"post","link":"https:\/\/www.seeedstudio.com\/blog\/2021\/01\/25\/programmable-io-with-raspberry-pi-pico\/","title":{"rendered":"Programmable I\/O with Raspberry Pi Pico"},"content":{"rendered":"\n

The Raspberry Pi Foundation has just announced the brand new Raspberry Pi Pico<\/a> microcontroller board, with programmable I\/O as one of its highlight features. Read our full coverage on the Raspberry Pi Pico release at Raspberry Pi Pico, the first microcontroller released by Raspberry Pi Foundation, based on the new RP2040 MCU<\/a>.<\/p>\n\n\n\n

In this article, we will focus on the programmable I\/O feature on the Raspberry Pi Pico. We\u2019ll cover what it is, how it works, and what it can be used for.<\/p>\n\n\n\n

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Raspberry Pi Pico and Programmable I\/O<\/strong><\/h2>\n\n\n\n
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Programmable I\/O or PIO stands for \u201cprogrammable input-output\u201d. Programmable I\/O refers to input-output pins which can process data ingoing or outgoing data without requiring a CPU.<\/p>\n\n\n\n

The Raspberry Pi Pico features 8 Raspberry Pi Programmable I\/O (PIO) state machines. State machines are essentially simple processors that can be programmed with a special form of assembly language to separately handle data.<\/p>\n\n\n\n

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What can Programmable I\/O be used for?<\/strong><\/h3>\n\n\n\n
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Microcontroller units (MCUs) like the Raspberry Pi Pico are small and perfect for many embedded electronics purposes. However, because they are small, they also have limited processing power and memory. Hence, resource management is very important for microcontrollers.<\/p>\n\n\n\n

The programmable I\/O on any MCU becomes extremely useful when we have a process that requires continuous instructions. For example, fading an LED, while a simple task, requires constant input to vary the PWM (pulse width modulation) duty cycle gradually. Yet, this can also be done easily with a state machine!<\/p>\n\n\n\n

Thus, programmable I\/O can transfer the burden of simple but resource-intensive tasks from the CPU to the state machine. That way, we can make better use of our microcontroller\u2019s main processing power and resources to improve our devices!<\/p>\n\n\n\n

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State Machine Infrastructure<\/strong><\/h3>\n\n\n\n
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Each state machine on the Raspberry Pi Pico has 2 First-In-First-Out (FIFO) structures: One for incoming data, and another for outgoing data. In FIFO structures, the data that is written in first will then be the first to be read out. FIFO structures are also known as queues, since they work similarly to a queue of people.<\/p>\n\n\n\n

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The input FIFO is also known as the RX FIFO, since it receives data to be read by the state machine. On the other hand, the output FIFO is also known as the TX FIFO, since it holds data to be transmitted out.<\/p>\n\n\n\n

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Programmable I\/O Benefits<\/strong><\/h3>\n\n\n\n
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When we want to send data to a pin via a state machine, we first push the data to the FIFO input. When the state machine is ready to process the data, it will pull it from the queue and perform the instruction.<\/p>\n\n\n\n

The key benefit here is that we can decouple the need for the central CPU from the execution of the instruction, since it has been \u201cdelegated\u201d to the PIO\u2019s state machine.<\/p>\n\n\n\n

Although each FIFO can only hold up to four words of data (each of 32 bits), we can link them with direct memory access (DMA) to transmit larger amounts. This way, we can once again free up the CPU from having to \u201cbabysit\u201d the process.<\/p>\n\n\n\n

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Shift & Scratch Registers<\/strong><\/h3>\n\n\n\n
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The FIFOs are linked to the state machine via the input and output shift registers (ISR \/ OSR).<\/p>\n\n\n\n

Separately, each state machine also has two scratch registers called X and Y. Scratch registers are for storing temporary data. Think of them as variables for use when programming our PIO.<\/p>\n\n\n\n

Adjusting our previous schematic, the complete structure becomes as follows. Cool!<\/p>\n\n\n\n

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Using Programmable I\/O with MicroPython<\/strong><\/h2>\n\n\n\n
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The Raspberry Pi Pico comes with its official MicroPython port, so we can use the beginner-friendly Python programming language to program our Raspberry Pi Pico\u2019s state machines.<\/p>\n\n\n\n

The following section is based on Ben Everad\u2019s Flashing lights with MicroPython and Programmable I\/O part 1 Tutorial<\/a>. We\u2019ll use it to demonstrate how we can program a state machine to fade an LED with MicroPython – working all without the help of our CPU!<\/p>\n\n\n\n

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State Machine Instructions<\/strong><\/h3>\n\n\n\n
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We can issue nine different instructions to the state machines:<\/p>\n\n\n\n