But 555 timer is only capable of about 300kHz
Where do you get that number from?
There are lots of different makes of 555 compatible timers. You need the datasheet for the specific type you are going to use.
Assuming it's the 555 from the TI datasheet you linked, there is no explicit mentioning of a max. frequency. There is, however, figure 17 on page 9 which suggests a max. frequency of 100kHz. But the diagram showing not more than 100kHz doesn't mean the chip will not operate above that frequency.
A quick off the cuff calculation would take into consideration teh follwong parameters from the datasheet:
output rise time: 100ns
output fall time: 100ns
minimum pulse width for triggering: ~100ns (figure 2)
output propagation delay: ~200ns (figure 8+9)
Assuming all signals need to be stable before the next switching operaation starts, these numbers add up to ~500ns, the reciprocal of which is 2MHz. This would be the upper limit.
We are tasked to create a pulse code modulation generator with 1MHz max frequency
If the 555 doesn't fit the bill, there are many other ways, even using discrete transistors, to achieve this goal.
