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.
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.
- PIC16F15276 Product Page
- PIC16F152xx MCU Family Product Brief
- How to Use the PIC16F15244 Family as an I/O Expander
- SPI I/O Expander Using PIC16F15276
- PIC16F15244 Code Examples on GitHub
- PIC16F152xx MCU Family Video
Microchip’s free IDE, compiler and graphical code generators are used throughout the application firmware development. The tools used for this demo are:
- MPLAB® X IDE 6.15.0 or newer
- MPLAB® XC8 Compiler 2.45.0 or newer
- MPLAB® Code Configurator (MCC) 5.3.7 or newer
- Microchip PIC16F1xxxx_DFP Device Support 1.21.368 or newer pack
- ESUART MCC Melody driver 7.1.5
Note: For running the demo, the installed tool versions must be the same or later. This example is not tested with the previous versions.
- PIC16F15276 Curiosity Nano Board
- Curiosity Nano Base for Click BoardsTM
- PROTO Xplained Pro Extension Kit
- Seven Segment Display
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.
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
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.
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.
- 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.
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.
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.
-
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
The following images informs about the pin usage in the project.
- Host Device
Figure 8: Pin Configuration (Host)
- Client Device
Figure 9: Pin Configuration (Client)
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