If you get around to reading about the fast parts, some of which operate at 100 MHz with one clock per instruction cycle, so you could generate a 50 MHz rate on a given bit, you may find a solution. However, with an MCU, you have to consider the number of instructions necessary to effect the signal changes you require.

The reason microprocessors were invented was because most applications don't demand transitions as fast as what logic allows. Sequentially executing instructions was plenty fast. That hasn't really changed. MCU's use less logic than discrete designs.

IF you were to use an external RAM, for example, and load a bit sequence into it, then use logic to count through the RAM and load the RAM content into a PISO shift register, and clock the logic from a numerically controlled oscillator or a synthesiser, then the top end for your signal generation setup would be the rate at which you can count, and the access time of the RAM. If you have a really fast MCU, and use, say, a synchronous serial port in the MCU to shovel out the data, then you can drive the MCU clock with a synthesiser as well. The top end then becomes the fastest bit rate of the serial channel.

There will, of course, be other issues ... since you no longer can use firmware timing or the internal timer to measure time without considerable fiddling with your constants, but it can be managed.

RE