RCON for AES 192 and 256 This is question about how many RCON values are used in AES 192 and 256.
I read the NIST Publication introducing AES and in Appendix A.2 and A.3 (Key Expansion for 192 and 256-bit keys), only $rcon_1$ to $rcon_8$ were used for 192 bits and upto $rcon_7$ for 256 bits. My question is how can only 7 or 8 values of $rcon$ be used when we have more rounds in AES 192 and 256.
To my understanding, $rcon$ is used every round so should't we have more $rcon$ values (owing to the increased no. of rounds in  AES192 and 256) instead of less?
You have 10, 12, and 14 rounds respectively; however, they are not linear increments depending on the key schedule.  For AES-192, you have 8 different values and you have 7 values for AES-256.  The RCON value only changes with every round for AES-128.
The RCON value (in hardware) is generated by a register that is 8-bits and increments every round. It wraps around with the count, and left alone for encryption, you'd get:
0: 00000001 
1: 00000010 
2: 00000100 
3: 00001000 
4: 00010000 
5: 00100000 
6: 01000000 
7: 10000000 
8: 00011011 
9: 00110110 
10: 01101100 
11: 11011000 
12: 10101011 
13: 01001101 
14: 10011010 

This is not what you'll actually use, but it's necessary to have a complete discussion.  In the above list, for AES-128, you'd start at round 0, and the final round, 9 (the 10th round), you will have 0x36.
For AES-192 and AES-256, you end up skipping constants due to the key schedule.  For AES-192:
0: 00000001 
1: 00000001 
2: 00000001
3: 00000010 
4: 00000100 
5: 00000100 
6: 00001000 
7: 00010000 
8: 00010000 
9: 00100000 
10: 01000000 
11: 01000000 
12: 10000000 

For AES-256:
0: 00000001 
1: 00000001 
2: 00000001
3: 00000001 
4: 00000010
5: 00000010 
6: 00000100 
7: 00000100 
8: 00001000 
9: 00001000 
10: 00010000 
11: 00010000 
12: 00100000 
13: 00100000 
14: 01000000 

You can implement this in many ways. The naive approach that you'd have in class, but is easy to follow (also, FPGAs would do this generally):

The way that I generally do it (note, this is encryption, you move tap 4 for decryption):

The clock control increments the value.