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This example highlights usage of PIC MCU as I/O expander using UART interface. The PIC16F15276 MCU acts as host and is used to control seven segment display. Compatible PIC16F152xx family of MCUs are: PIC16F15244, PIC16F15254, PIC16F15276.

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MCHP

UART I/O Expander: Host Implementation Using PIC16F15276

This code example showcases how the PIC16F15276 microcontroller s used as an I/O expander in embedded applications. in this example, the PIC16F15276 microcontroller provides additional I/O pins to the main microcontroller through a serial communication interface like I2C, SPI, or UART.

Introduction

The PIC16F152xx family of microcontrollers are available in packages for various embedded applications. The PIC16F15276 simplified feature set includes Peripheral Pin Select (PPS), digital communication peripherals, timers, and Enhanced Universal Synchronous Asynchronous Receiver Transmitter (EUSART). In real time applications, EUSART can be configured as a full-duplex asynchronous system or half-duplex synchronous system. Full-Duplex mode is useful for communications with peripheral systems, such as CRT terminals and personal computers. Half-Duplex Synchronous mode is intended for communications with peripheral devices, such as A/D or D/A integrated circuits, serial EEPROMs or other microcontrollers.

Several embedded applications use I/O expander to add the additional I/O pins required for the main microcontroller while interfacing standalone IC modules display modules for various feature implementation. This code example demonstrates implementation of I/O Expander using EUSART peripheral and I/O pins of the PIC16F15276 microcontroller.

For more details about the host device implementation see the UART I/O Expander: Host Implementation using PIC16F15276 code example.

Related Documentation

Software Used

Microchip’s free IDE, compiler and graphical code generators are used throughout the application firmware development. The tools used for this demo are:

Note: For running the demo, the installed tool versions must be the same or later. This example is not tested with the previous versions.

Hardware Used

Description

In this code example, the host microcontroller expects to display the numbers on Seven Segment Display (SSD). To interface SSD with the host device, it requires minimum seven general purpose I/O pins. To minimize the usage of I/O pins on host, a secondary microcontroller is used as a client for driving the SSD and UART serial communication interface to communicate between the host and client device. To establish communication between the host and client device, the UART serial communication interface is used.

For the ease of demonstration, this code example uses two microcontrollers, PIC16F15276 as a host, and PIC16F15276 as a client. Also, the code example uses PIC16F15276 Curiosity Nano Development boards for the demonstration.

On host device, on-board switch of the PIC16F15276 Curiosity Nano board is used to send command to the client device through the UART interface. The on-board LED notifies the user about successful switch press event detection and subsequent UART command transmission by the host. After receiving the command from host, the client device drives the respective I/O pins connected to the SSD in a pre-defined sequence, to display numbers from zero to nine in ascending order.


Figure 1: Block Diagram

  • In this example, PIC16F15276 CNANO boards, PROTO Xplained Pro extension kit with SSD circuitry populated over it and few connecting jumper cables are used for demonstrating UART I/O Expander.
  • The on-board switch of host device is used to initiate the communication between the host and client devices over UART interface. The HLT mode of timer2 module is used for switch debounce implementation.
  • The client receives the command from the host device over UART interface.
  • The client microcontroller verifies the commands upon reception, if it matches then initiates the number display from 0 to 9 on SSD. It is followed up with SSD value resetting to zero.

Seven Segment Display (SSD)

The 7-segment display consists of seven LEDs arranged in a rectangular fashion. Each of the seven LEDs is called a segment because when illuminated the segment forms part of a numerical to be displayed.

An additional 8th LED is sometimes used within the same package thus allowing the indication of a decimal point, (DP) when two or more 7-segment displays are connected to display numbers greater than ten.

Each one of the seven LEDs in the display is given a positional segment with one of its connection pins being brought straight out of the rectangular plastic package. These individual LED pins are labelled from a through to g representing each individual LED. The other LED pins are connected and wired to form a common pin. An additional 8th LED is sometimes used within the same package thus allowing the indication of a decimal point, (DP) when two or more 7-segment displays are connected to display numbers greater than ten.


Figure 2: SSD Segment Naming Conventions

Seven Segment Hex values:

Number HGFEDCBA Hexadecimal
0 11000000 0xC0
1 11111001 0xF9
2 10100100 0xA4
3 10110000 0xB0
4 10011001 0x99
5 10010010 0x92
6 10000010 0x82
7 11111000 0xF8
8 10000000 0x80
9 10010000 0x90

Resistor Value Calculations

If forward current (If) is 20 mA If forward voltage (Vf) is 2.2V Input voltage (Vin) is 5V

Resistor ( R ) = (Vin – Vf ) / ( If ) = 140 ohms

Note: 330 or 470 ohms resistor will be used.


Figure 3: SSD LED Resistor Driver Circuit

Application Firmware

This example uses two standalone firmware named host and client firmware.

The host firmware communicates with the client through the EUSART peripheral. When a Valid Switch Press event is detected, the host sends a command to the client over UART interface. Also, the host blinks on-board LED each time a command is successfully sent to the client.

The client firmware is comprised of two sections. First section is verifying the command received from the host through UART interface. The second section consists of driving SSD, so the SSD start displaying digits from zero to nine in an incrementing order each time a valid command is received from the host. Additionally, the client firmware toggles the on-board LED when digits are displayed on the SSD and resets display value to zero when the cycle is completed.

Hardware Setup

The following figure consists of populated PROTO Xplained Pro extension kit, Curiosity Nano Adapter Board and PIC16F15276 Curiosity Nano Evaluation Kit (host and client devices). The figure shows information about the hardware setup. The populated PROTO Xplained Pro extension kit is interfaced with PIC16F15276 microcontroller using an extension header.


Figure 4: Hardware Setup

Note: To use on-board mechanical switch on host device timer is used to avoid the debouncing. Thereby, timer input pin RC3 and pin RB5 (on-board switch), need to be shorted using a jumper. The usage of jumper is required as RB5 pin cannot be selected as a timer input through PPS feature of the PIC16F15276 microcontroller.

Operation

  • Make the hardware connections as shown in the Hardware Setup. Power up the Curiosity Nano board using micro-USB cable.
  • Download the firmware available from the GitHub code example page.
  • Build the project using latest version of tools as mentioned in the Software Tools section and flash the generated file on the PIC16F15276 microcontroller.
  • On the host side, the on-board Switch Press event transmits the command to the client through UART interface.
  • On the host side, the on-board LED indicates the Switch Press event and indicates which command to be sent to the client.
  • On the client side, after receiving the command from the host, client device verifies the command and drives the seven segment LED to display numbers from zero to nine. On-board LED of CNANO board toggles for every displayed number.

Conclusion

In many real-world embedded applications, the microcontroller requires to interface with multi-pin standalone IC modules and display units to perform various functionalities. So, the developers have a challenge in effectively using microcontrollers I/O pins for interfacing the external modules without opting an expensive, higher pin and, memory variant. This code example demonstrates how cost effective and entry level PIC16F15726 microcontroller can be used as an I/O expander using UART interface.

Appendix

MPLAB® Code Configurator is a graphical programming environment that generates seamless, easy to understand C code to give a head start to the project, saving the designer’s time to initialize and configure all the modules, and to go through the datasheets. Using an instructive interface, it enables and configures all peripherals and functions specific to the application requirements.

Start by creating a new Project and open MCC

  • Go to File and click New Project
  • Select Microchip Embedded and click Standalone Project
  • Enter the device name, in this case, PIC16F15276
  • Name the project
  • Launch MCC tool by navigating to “Tools -> Embedded -> MPLAB Code Configurator v4: Open/Close” . Alternatively, click the MCC icon to launch the MCC tool.

System Configuration

  • Configure Clock

    Open Clock Control setup present under "System" dropdown menu in Project Resources tab. Host and the client device will be configured with same configuration as given below.

    • Set Clock Source as HFINTOSC
    • Set HF Internal Clock as 8_MHz

The Configurations Bits (Project Resources -> System) window in MCC is used for MCU oscillator, Watchdog timer (WDT) and, low voltage programming configuration. The WDT is disabled in the application.

The following figure shows the clock configuration setting in MCC tool.


Figure 5: Clock Configuration

  • Timer 2 Configuration

    Configure Timer2 in HLT mode for switch debouncing with the following configuration:

  • Enable Timer checkbox

  • Control Mode – Monostable

  • Ext Reset – T2INPPS

  • Start/Reset Option – Start on rising edge on TMR2_ers

  • Clock Source – MFINTOSC 31.25 kHz

  • Polarity – Rising Edge

  • Prescaler – 1:16, Postscaler – 1:1

  • Time Period – 100 ms

  • Enable Timer Interrupt checkbox


Figure 6: Timer 2 Configuration

  • EUSART Configuration


Figure 7: EUSART Configuration

Pin Mapping

The following images informs about the pin usage in the project.

  1. Host Device


Figure 8: Pin Configuration (Host)

  1. Client Device


Figure 9: Pin Configuration (Client)

About

This example highlights usage of PIC MCU as I/O expander using UART interface. The PIC16F15276 MCU acts as host and is used to control seven segment display. Compatible PIC16F152xx family of MCUs are: PIC16F15244, PIC16F15254, PIC16F15276.

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