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twi out of work

I've a problem using the twi-interface from the AT91RM9200 controller. At the moment, I'm not able to transmit any kind of signals (startbyte and so on)... both pins (clock and data) stay always high.

/* crystal frequency */
#define AT91_CRYSTAL    18420000

/* pll param -> processor-frequency */
#define AT91_MASTER_CLK ((AT91_CRYSTAL / 4) * 38

#define MCK   (AT91_MASTER_CLK / 3)

/* twi clock 100kHz */
#define AT91C_TWI_CLOCK         100000


#define I2C_ADDR                        0x48



void AT91F_SetTwiClock(void)

{
unsigned int sclock;

/* Here, CKDIV = 1 and CHDIV=CLDIV  ==> CLDIV = CHDIV = 1/4*((Fmclk/FTWI) -6)*/




sclock = (10*MCK /AT91C_TWI_CLOCK);

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if (sclock % 10 >= 5)
    sclock = (sclock /10) - 5;

else
    sclock = (sclock /10)- 6;




sclock = (sclock + (4 - sclock %4)) >> 2; // div 4




AT91C_BASE_TWI->TWI_CWGR  = ( 1<<16 ) | (sclock << 8) | sclock;




}

AT91F_TWI_CfgPIO();
AT91F_TWI_CfgPMC();

//disable all interrupts, reset twi, set master mode
AT91F_TWI_Configure(AT91C_BASE_TWI);

AT91F_SetTwiClock();


//read operation

*AT91C_TWI_CR = AT91C_TWI_MSEN | AT91C_TWI_SVDIS;

//internal device addr
*AT91C_TWI_IADR = 0x01;
*AT91C_TWI_MMR = ((I2C_ADDR<<16)  //slave addr
                 | AT91C_TWI_MREAD
                 | AT91C_TWI_IADRSZ_1_BYTE);

*AT91C_TWI_CR = AT91C_TWI_START | AT91C_TWI_STOP;

I can't find the error causing this problem. Maybe someone of you could give me some useful hints.

best regards
Stefan