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Ripple Counter

- The most common counter
- The problem is that, because more than one output is changing at once, the signal is glichy
- To avoid this problem, use Gray or Johnson code

Johnson Counter

- The Johnson counter is type of shift counter
- Put an inverted MSB back to LSB
- Glitch output free

Linear Feedback Shift Registers

- A small number of taps are recycled
- An LFSR can operate at high speed compared to a binary counter because the feedback logic is very simple
- Reduce clock noise

Maximal length LFSR

- With maximal length logic (taps selected to give the maximal count), a small number of register can create sequence up to 2n-1

Divide by N LFSR Counter

- An example of the use of a LFSR
- A terminal count is provided as an input to be compared to

Divide by N LFSR Counter

- Test fixture

4-Bit LFSR One-to-Many Code

- One-to-Many variant splits the XOR into 2-input gates and distributes them throughout the register array

Cyclic Redundancy Checksums

- Error detection
- The data packet is looked at as a huge binary number
- A polynomial divide this number in GF
- Reminder is checksum

ROM

- ROM stands for Read-Only Memory
- This memory is initialized when the FPGA is configured and cannot be changed after configuration

ROM Version of LFSR

- We can implement four-bit LFSR counter with a ROM

RAM

- RAM stands for Random Access Memory
- A RAM is an array of cells, addressable in groups N element wide and M elements deep

RAM

- Unless the FPGA support embedded RAM blocks, it will consume a huge amount of logic

- Jovan Popovic [email protected]
- Milos Milovanovic [email protected]
- Veljko Milutinovic [email protected]
- Nobelovac?

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