what is the difference between 'twi_sw_master.c' and 'twi_hw_master.c' ?

Question

Hi. 
 I'm using nRF51 DK for developing a device. 
 and, I'm trying to use 24LC01B(external eeprom) by i2c. 
 Looking for code used for i2c, there are two *.c files, sw and hw. 
 There are same functions in two *.c files. (like 'bool twi_master_init(void)' and so on...) 
 what's the difference between them?
and what should I use?

Ole Bauck · Accepted Answer

twi_sw_master.c is a software controlled twi driver, which means that SCL and SDA are set manually in software and timing is done with busy wait. twi_hw_master.c is a hardware controlled twi driver which use the twi peripheral on the chip. 
 Use the hardware controlled twi driver, twi_hw_master.c. I think the software controlled driver is to be used if the twi peripheral is already occupied. 
 If you are using newer SDK, I would recommend to use nrf_drv_twi.