Die Kommunikation über I2C funktioniert nur manchmal. Ich benutze das Programm von Peter Fleury In main wird i2c_start_wait aufgerufen Aus der while Schleife über Ende kommt er nicht mehr raus. Die beiden Widerstände 4,7k sind drin. Könnte das ein HW-Problem sein? in main: i2c_init(); i2c_start_wait(DS1621+I2C_WRITE); // set device address and write ... In Twimaster von Peter Fleury: void i2c_start_wait(unsigned char address) { uint8_t twst; while ( 1 ) { // send START condition TWCR = (1<<TWINT) | (1<<TWSTA) | (1<<TWEN); // wait until transmission completed while(!(TWCR & (1<<TWINT))); #########Ende######### In Doku: • Bit 7 – TWINT: TWI Interrupt Flag This bit is set by hardware when the TWI has finished its current job and expects application software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by software by writing a logic one to it. Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag.