More Digital circuits

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# More Digital circuits - PowerPoint PPT Presentation

More Digital circuits. 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

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Presentation Transcript
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
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]