If this is a one off copy/write operation then a ch340 adapter or ftdi232h would work. if you intend to do more poking about then i would say a bus pirate (does uart,i2c,spi,jtag, swim, SWD and more) ,
the bus pirate has flashrom(to copy spi flash roms like w25q64 etc ) support and can also be used with openocd and other programs for debug and more.
while slow compared to some specific tools its more universal in it's support.
also other way of doing it is any arduino device (check you chip voltages as arm based arduino is normally 3.3v and atmel are 5v )
darkspr1te