Objective

The objective of this laboratory is to demonstrate the CMOS logic levels associated with an MCU.

Preparation

The following schematic details the hardware setup.

Table 1 lists the components used in the schematic.

IDComponentManufacturer Part No.ValueQty.
Base Components
IC1MCUPIC24FJ256GA702-I/SP1
IC2RegulatorLM1117T-3.3/NOPB3.3V / 800mA1
C1 & C2Capacitor (tantalum)TAP106K025SRW10uF / 25V2
C3 & C4Capacitor (ceramic)SR155C103KARTR10.01uF / 50V2
C5 & C6Capacitor (ceramic)SR155C104KARTR10.1uF / 50V2
C7 & C8Capacitor (ceramic)SR151A150JARTR115pF / 100V2
C9Capacitor (ceramic)FG16X7R1E106KRT0610uF / 25V1
Y1CrystalABL-16.000MHZ-B216MHz1
R1ResistorSFR2500001002FR50010kΩ1
R2ResistorSFR2500001004FR5001MΩ1
J1Header (6-way)
(PICkit 5)
22-27-20611
Additional Components
R3 – R4ResistorSFR2500001400FR500140Ω2
D1LED (green)WP132XGD10mA1
D2LED (red)WP132XID10mA1
Table 1: Components

Refer to the following source code.

/*                         Logic_Levels (source code)                         */  
/*                            MCU: PIC24FJ256GA702                            */
/*                              Author: Michael                               */

/**************************** Configuration Bits ******************************/

// FSEC
#pragma config BWRP = OFF                                                       // Boot Segment Write-Protect bit (Boot Segment may be written)
#pragma config BSS = DISABLED                                                   // Boot Segment Code-Protect Level bits (No Protection (other than BWRP))
#pragma config BSEN = OFF                                                       // Boot Segment Control bit (No Boot Segment)
#pragma config GWRP = OFF                                                       // General Segment Write-Protect bit (General Segment may be written)
#pragma config GSS = DISABLED                                                   // General Segment Code-Protect Level bits (No Protection (other than GWRP))
#pragma config CWRP = OFF                                                       // Configuration Segment Write-Protect bit (Configuration Segment may be written)
#pragma config CSS = DISABLED                                                   // Configuration Segment Code-Protect Level bits (No Protection (other than CWRP))
#pragma config AIVTDIS = OFF                                                    // Alternate Interrupt Vector Table bit (Disabled AIVT)

// FBSLIM
#pragma config BSLIM = 0x1FFF                                                   // Boot Segment Flash Page Address Limit bits (Enter Hexadecimal value)

// FOSCSEL
#pragma config FNOSC = PRIPLL                                                   // Oscillator Source Selection (Primary Oscillator with PLL module (XT + PLL, HS + PLL, EC + PLL))
#pragma config PLLMODE = PLL96DIV4                                              // PLL Mode Selection (96 MHz PLL. Oscillator input is divided by 4 (16 MHz input))
#pragma config IESO = OFF                                                       // Two-speed Oscillator Start-up Enable bit (Start up with user-selected oscillator source)

// FOSC
#pragma config POSCMD = HS                                                      // Primary Oscillator Mode Select bits (HS Crystal Oscillator Mode)
#pragma config OSCIOFCN = ON                                                    // OSC2 Pin Function bit (OSC2 is general purpose digital I/O pin)
#pragma config SOSCSEL = OFF                                                    // SOSC Power Selection Configuration bits (Digital (SCLKI) mode)
#pragma config PLLSS = PLL_PRI                                                  // PLL Secondary Selection Configuration bit (PLL is fed by the Primary oscillator)
#pragma config IOL1WAY = OFF                                                    // Peripheral pin select configuration bit (Allow multiple reconfigurations)
#pragma config FCKSM = CSDCMD                                                   // Clock Switching Mode bits (Both Clock switching and Fail-safe Clock Monitor are disabled)

// FWDT
#pragma config WDTPS = PS1                                                      // Watchdog Timer Postscaler bits (1:1)
#pragma config FWPSA = PR32                                                     // Watchdog Timer Prescaler bit (1:32)
#pragma config FWDTEN = OFF                                                     // Watchdog Timer Enable bits (WDT and SWDTEN disabled)
#pragma config WINDIS = OFF                                                     // Watchdog Timer Window Enable bit (Watchdog Timer in Non-Window mode)
#pragma config WDTWIN = WIN50                                                   // Watchdog Timer Window Select bits (WDT Window is 50% of WDT period)
#pragma config WDTCMX = WDTCLK                                                  // WDT MUX Source Select bits (WDT clock source is determined by the WDTCLK Configuration bits)
#pragma config WDTCLK = SYSCLK                                                  // WDT Clock Source Select bits (WDT uses system clock when active, LPRC while in Sleep mode)

// FPOR
#pragma config BOREN = OFF                                                      // Brown Out Enable bit (Brown Out Disabled)
#pragma config LPCFG = OFF                                                      // Low power regulator control (No Retention Sleep)
#pragma config DNVPEN = DISABLE                                                 // Downside Voltage Protection Enable bit (Downside protection disabled when BOR is inactive)

// FICD
#pragma config ICS = PGD1                                                       // ICD Communication Channel Select bits (Communicate on PGEC1 and PGED1)
#pragma config JTAGEN = OFF                                                     // JTAG Enable bit (JTAG is disabled)

// FDEVOPT1
#pragma config ALTCMPI = DISABLE                                                // Alternate Comparator Input Enable bit (C1INC, C2INC, and C3INC are on their standard pin locations)
#pragma config TMPRPIN = OFF                                                    // Tamper Pin Enable bit (TMPRN pin function is disabled)
#pragma config SOSCHP = ON                                                      // SOSC High Power Enable bit (valid only when SOSCSEL = 1 (Enable SOSC high power mode (default))
#pragma config ALTI2C1 = ALTI2CEN                                               // Alternate I2C pin Location (SDA1 and SCL1 on RB9 and RB8)

/************************* Configuration Bits (end) ***************************/

#define FCY 16000000                                                            // FCY = FOSC / 2 (FCY: Instruction clock cycle) (FOSC: System clock cycle)
#define LED_Green_1 LATBbits.LATB6                                              // LED_Green_1 associated with MCU Pin #15
#define LED_Red_2 LATBbits.LATB7                                                // LED_Red_2 associated with MCU Pin #16
#define Analog_In PORTBbits.RB10                                                // Analog_In associated with MCU Pin #21

#include <libpic30.h>                                                           // Delay functions
#include <xc.h>                                                                 // MCU pin mapping

int main(void)                           
{
    
    // Set pin direction
    TRISBbits.TRISB6 = 0;                                                       // MCU Pin #15 output (LED_Green_1)        
    TRISBbits.TRISB7 = 0;                                                       // MCU Pin #16 output (LED_Red_2)
    TRISBbits.TRISB10 = 1;                                                      // MCU Pin #21 input (Analog_In)
     
    /************************* configure MCU modules **************************/
    
    // Analog Ports
    ANSA = 0;                                                                   // Disable A (PIC24FJ256GA702 datasheet p.126 Table 11-1)
    ANSB = 0;           
    
    // Comparators
    CM1CONbits.CEN = 0;                                                         // Disable #1 (PIC24FJ256GA702 datasheet p.310 Register 25-1)
    CM2CONbits.CEN = 0;                                                         
    CM3CONbits.CEN = 0;                                                                                                                 
    
    // ADC
    AD1CON1bits.ADON = 0;                                                       // Disable module (PIC24FJ256GA702 datasheet p.291 Register 24-1)
    
    /********************** configure MCU modules (end) ***********************/
    
    while (1)
    {    
        
        if (Analog_In == 1)                                                     // Logic HIGH
        {
            LED_Green_1 = 1;                                                    // LED_Green_1 ON ON
            LED_Red_2 = 0;                                                      // LED_Red_2 OFF
        }
        
        else                                                                    // Logic LOW
        {
            LED_Green_1 = 0;                                                      
            LED_Red_2 = 1;                                                                                           
        }
           
    }
    
    return 0;
    
}
Testing

Turn the voltage knob of the analog PSU slowly clockwise from 0V to 3.3V, paying attention to the absolute maximum allowable voltages on pin #21 (-0.3V to +6.0V). Notice the red LED illuminating only, due to low voltage levels being fed into pin #21, followed by the green LED illuminating only, from approximately the 1.95V level upwards. Even though the green LED first illuminates at the 1.95V level, it does not mean this voltage level is reliable, at which the MCU reads HIGH. Turning the voltage knob anticlockwise reveals the red LED illuminating only, from approximately the 1.25V level down to 0V. Even though the red LED first illuminates at the 1.1V level, it does not mean this voltage level is reliable at which the MCU reads LOW. As per the MCU datasheet, page 364 Table 32-8, it can be seen that the VIL(max) and VIH(min) voltage levels are 0.2VDD (0.66V) and 0.7VDD (2.31V), respectively. These levels are well below and above the 1.1V LOW and 2.0V HIGH levels, respectively.

Conclusion

Based on measurements and the behaviour of the LEDs, the CMOS levels outlined in the MCU datasheet are functional.