Embedded Lab

USB CDC

With USB CDC mode we can directly transfer data to and from a PC/mobile device without needing an external USB-to-Serial converter like FT232, CP2102, PL2303, etc. We’ll simply make a virtual COM port. The idea in this demo is to send out on-chip temperature sensor’s temperature reading upon request from a host PC. The request is nothing special but just an ASCII character. The key that was pressed in the host PC’s keyboard is then displayed on an LCD connected with the MSP430.

Code Example

#include "driverlib.h"
#include "delay.h"
#include "lcd.h"
#include "lcd_print.h"

#include "USB_config/descriptors.h"
#include "USB_API/USB_Common/device.h"
#include "USB_API/USB_Common/usb.h"
#include "USB_API/USB_CDC_API/UsbCdc.h"
#include "USB_app/usbConstructs.h"

#include "hal.h"

volatile uint8_t bCDCDataReceived_event = FALSE;

#define RX_BUFFER_SIZE  1
#define TX_BUFFER_SIZE  4

char dataBuffer[RX_BUFFER_SIZE] = "";
char nl[2] = "\n";
uint16_t count;

void ADC12_init(void);
void REF_init(void);

#pragma vector = UNMI_VECTOR
__interrupt void UNMI_ISR (void)
{
    switch (__even_in_range(SYSUNIV, SYSUNIV_BUSIFG ))
    {
        case SYSUNIV_NONE:
            __no_operation();
            break;
        case SYSUNIV_NMIIFG:
            __no_operation();
            break;
        case SYSUNIV_OFIFG:
            UCS_clearFaultFlag(UCS_XT2OFFG);
            UCS_clearFaultFlag(UCS_DCOFFG);
            SFR_clearInterrupt(SFR_OSCILLATOR_FAULT_INTERRUPT);
            break;
        case SYSUNIV_ACCVIFG:
            __no_operation();
            break;
        case SYSUNIV_BUSIFG:
            SYSBERRIV = 0;
            USB_disable();
    }
}

void main (void)
{
    float tmp = 0.0;
    signed int ADC_Count = 0;
    signed long temp = 0;
    uint8_t tc[TX_BUFFER_SIZE] = {0x00, 0x00, '\r', '\n'};

    WDT_A_hold(WDT_A_BASE);

    PMM_setVCore(PMM_CORE_LEVEL_2);

    REF_init();

    ADC12_init();

    LCD_init();
    LCD_clear_home();

    LCD_goto(1, 0);
    LCD_putstr("MSP430 USB CDC");

    LCD_goto(0, 1);
    LCD_putstr("T/'C:");

    LCD_goto(11, 1);
    LCD_putstr("RX:");

    USBHAL_initPorts();
    USBHAL_initClocks(8000000);
    USB_setup(TRUE, TRUE);

    __enable_interrupt();

    while (1)
    {
        uint8_t ReceiveError = 0;
        uint8_t SendError = 0;

        ADC_Count = ADC12_A_getResults(ADC12_A_BASE,
                                       ADC12_A_MEMORY_0);

        tmp = (((float)ADC_Count * 1.5) / 4095.0);
        temp = ((tmp - 0.688) / 0.00252);

        print_C(6, 1, temp);

        switch (USB_getConnectionState())
        {
            case ST_ENUM_ACTIVE:
            {
                __disable_interrupt();
                if (!USBCDC_getBytesInUSBBuffer(CDC0_INTFNUM))
                {
                    __bis_SR_register(LPM0_bits + GIE);
                }

                __enable_interrupt();

                if (bCDCDataReceived_event)
                {
                    bCDCDataReceived_event = FALSE;

                    count = USBCDC_receiveDataInBuffer((uint8_t*)dataBuffer,
                                                       RX_BUFFER_SIZE,
                                                       CDC0_INTFNUM);
                    LCD_goto(15, 1);
                    LCD_putstr(dataBuffer);
                }

                tc[0] = ((temp / 10) + 0x30);
                tc[1] = ((temp % 10) + 0x30);

                if(USBCDC_sendDataInBackground((uint8_t*)tc,
                                               TX_BUFFER_SIZE,
                                               CDC0_INTFNUM,
                                               1))
                {
                    SendError = 0x01;
                    break;
                }

                break;

            }
            case ST_PHYS_DISCONNECTED:
            case ST_ENUM_SUSPENDED:
            case ST_PHYS_CONNECTED_NOENUM_SUSP:
                __bis_SR_register(LPM3_bits + GIE);
                _NOP();
                break;

            case ST_ENUM_IN_PROGRESS:
            default:;
        }

        if (ReceiveError || SendError)
        {
        }
    }
}

void ADC12_init(void)
{
    ADC12_A_configureMemoryParam configureMemoryParam = {0};

    ADC12_A_init(ADC12_A_BASE,
                 ADC12_A_SAMPLEHOLDSOURCE_SC,
                 ADC12_A_CLOCKSOURCE_ACLK,
                 ADC12_A_CLOCKDIVIDER_1);

    ADC12_A_setupSamplingTimer(ADC12_A_BASE,
                               ADC12_A_CYCLEHOLD_768_CYCLES,
                               ADC12_A_CYCLEHOLD_4_CYCLES,
                               ADC12_A_MULTIPLESAMPLESENABLE);

    ADC12_A_setResolution(ADC12_A_BASE,
                          ADC12_A_RESOLUTION_12BIT);

    configureMemoryParam.memoryBufferControlIndex = ADC12_A_MEMORY_0;
    configureMemoryParam.inputSourceSelect = ADC12_A_INPUT_TEMPSENSOR;
    configureMemoryParam.positiveRefVoltageSourceSelect = ADC12_A_VREFPOS_INT;
    configureMemoryParam.negativeRefVoltageSourceSelect = ADC12_A_VREFNEG_AVSS;
    configureMemoryParam.endOfSequence = ADC12_A_NOTENDOFSEQUENCE;

    ADC12_A_configureMemory(ADC12_A_BASE,
                            &configureMemoryParam);

    ADC12_A_enable(ADC12_A_BASE);

    ADC12_A_startConversion(ADC12_A_BASE,
                            ADC12_A_MEMORY_0,
                            ADC12_A_REPEATED_SINGLECHANNEL);
}

void REF_init(void)
{
    while(REF_ACTIVE == Ref_isRefGenBusy(REF_BASE));

    Ref_setReferenceVoltage(REF_BASE,
                            REF_VREF1_5V);

    Ref_enableReferenceVoltage(REF_BASE);

    __delay_cycles(100);
}

Hardware Setup

Explanation

The setup for using the ADC and on-chip temperature sensor have already been discussed and demoed previously. Likewise, the basic USB setup has been discussed in the USB module overview section. Therefore, I’ll skip these parts. The main loop is the part that is of our interest. As can be seen, temperature read from ADC is shown in the display first. The switch-case statement acts according to the state of the USB module’s connection state. Of these states, the first one (ST_ENUM_ACTIVE) is of most importance. In this state, data is transferred by a method called enumeration. The rest of the states have other importance but these won’t be needed here. The whole system is interrupt driven and so it is necessary to disable interrupts upon a reception event and reenable interrupts after processing them.

while (1)
{
    uint8_t ReceiveError = 0;
    uint8_t SendError = 0;

    ADC_Count = ADC12_A_getResults(ADC12_A_BASE, ADC12_A_MEMORY_0);

    tmp = (((float)ADC_Count * 1.5) / 4095.0);
    temp = ((tmp - 0.688) / 0.00252);

    print_C(6, 1, temp);

    switch (USB_getConnectionState())
    {
        case ST_ENUM_ACTIVE:
        {
            __disable_interrupt();
            if (!USBCDC_getBytesInUSBBuffer(CDC0_INTFNUM))
            {
                __bis_SR_register(LPM0_bits + GIE);
            }

            __enable_interrupt();

            if (bCDCDataReceived_event)
            {
                bCDCDataReceived_event = FALSE;
	
                count = USBCDC_receiveDataInBuffer((uint8_t*)dataBuffer,
                                                    RX_BUFFER_SIZE,
                                                    CDC0_INTFNUM);
                LCD_goto(15, 1);
                LCD_putstr(dataBuffer);
            }

            tc[0] = ((temp / 10) + 0x30);
            tc[1] = ((temp % 10) + 0x30);

            if(USBCDC_sendDataInBackground((uint8_t*)tc, 
                                            TX_BUFFER_SIZE,
                                            CDC0_INTFNUM,
                                            1))
            {
                SendError = 0x01;
                break;
            }

            break;

        }
        case ST_PHYS_DISCONNECTED:
        case ST_ENUM_SUSPENDED:
        case ST_PHYS_CONNECTED_NOENUM_SUSP:
             __bis_SR_register(LPM3_bits + GIE);
             NOP();
             break;

        case ST_ENUM_IN_PROGRESS:
        default:;
    }

    if (ReceiveError || SendError)
    {
    }
}

Out of the all the jargon, the following codes are what that we need to receive and transmit data respectively.

USBCDC_receiveDataInBuffer((uint8_t*)dataBuffer, RX_BUFFER_SIZE, CDC0_INTFNUM);

USBCDC_sendDataInBackground((uint8_t*)tc, TX_BUFFER_SIZE, CDC0_INTFNUM, 1))

The receive function reads received data from CDC0_INTFNUM buffer. This is simply a buffer location that acts bidirectionally. We have to mention the array to which received data needs to be transferred. We also have to mention the size of this array. The transmit function is almost the same as the receive function. Note that all data transactions are done in chunks of bytes. For the sake of simplicity, errors and other USB states are not used. However, when developing a good real-life USB device these should not be ignored.

Since it is a USB example, our MSP430 launchpad board needs to be connected with a host PC. Note that no driver installation would be needed for this COM port device as it is a virtual/non-physical COM port and the port number can be any port number.

We’ll need a port monitoring terminal software like the Eltima Soft’s Serial Port Monitor.

The demo here works by sending out on-chip temperature sensor’s reading when a keyboard key of the host PC is pressed. The key that was pressed is echoed back.

Demo

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