Problem in PWM and UART (T89C51AC2)

#include <t89c51ac2.h>
//#include <math.h>

void serialInterrupt (void);
volatile char uart_data;

char duty;

void main(void)
{

        // Set up UART mode
        SCON = 0x50;                    // uart in mode 1 (8 bit), REN=1
        TMOD = TMOD | 0x20 ;    // Timer 1 in mode 2
        TH1 = 0xFB;                     // 9600 Bds at 18.432MHz
        TL1 = 0xFB;                     // 9600 Bds at 18.432MHz
        ES = 1;                                 // Enable serial interrupt
        EA = 1;                                 // Enable global interrupt      */
        TR1 = 1;

        // Set up PWM mode
        CMOD = 0x02;                    // Setup PCA timer
        CL = 0x00;
        CH = 0x00;
        CCAP0L = 0x40;                  // Set the initial value same as CCAP0H
        CCAP0H = 0x40;                  // 75% Duty Cycle
        CCAPM0 = 0x42;                  // Setup PCA module 0 in PWM mode.
        CR = 1;                                 // Start PCA Timer.*/



        // Init variable

        uart_data = 128;                        // 50% Duty Cycle


        //Program main routime, do forever
        while (1)
        {
                P2 = 0xFF;                      // turn off all lamps (for test)
                P2_2=0;                         // Turn on
                CCAP0H = (int) uart_data;

        }
}

void serialInterrupt (void) interrupt 4 using 1{
        if (RI)
        {                                                               // Byte received on UART.

                uart_data = SBUF;

                if (uart_data == '1')           // For Testing
                        P2_0 = 0;

                SBUF = uart_data;                       // Initiate transmit of the byte received.

                RI = 0;                                 // Clear interrupt flag.

        }
        else if (TI)
        {                               // Finished transmitting a byte.
                //P2_4 = 0;
                TI = 0;

        }
}

I try to control position of motor. So I need receive new position from computer through RS232 and control motor by PWM mode 1.

My PWM code (without UART) controlling a motor.
Result: SUCCESS!

My UART code (without PWM) echoing characters it receives.
Result: SUCCESS!

I have 2 questions:

1. Why in UART code, in serialInterrupt function I have to put this code:

if (uart_data == '1')           // For Testing
                        P2_0 = 0;

and in main loop :

P2 = 0xFF;                      // turn off all LED (for test)
                P2_2=0;                         // Turn on

If I use this code (first time I only think it only use for test) UART is well done, but when I close it, UART can not work correct.

2. If I combine UART and PWM projects, the new project (above code) only has PWM but can not send/receive.

One body said that: "If that indicates a PCA interrupt (I don't know because I'm not familiar with how uVision debugger presents the PCA subsystem) and you don't have an ISR for it, then that could cause a lot of problems."

But I don't understand much.

Did I miss somethings in my code?

Kind Regards,
Mr. Huy