I have been searching about programmable priority encoder but have not find a good explanation on it. I know about priority encoder. But generally what is 'programmable' priority encoder are? Can somebody explain it?
Electronic – Programmable Priority Encoder
cmosdigital-logiclogic-gates
Related Solutions
The solution is found by searching for "encoder" on Wikipedia:
Key point, the only valid inputs are when exactly one of the inputs is high. That means there are lots of don't cares in the output columns of the truth table.
One approach would be for each of the 25 inputs to either show 0 if deselected, or it's own 5-bit priority value P(i) if selected. Throw all the intermediate values into a sorting network that returns the maximum priority shown in a particular cycle. If you then have a comparator for each input to see if it's show priority equals the maximum as a single bit per input, then use a classic priority encoder technique (e.g. bitscan) on those bits.
Total storage for all priority bits is 25*5 as @Dave_Tweed mentions.
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Best Answer
Wikipedia article for Priority Encoders is nice and simple. In Priority encoders, We have 4 Input Channels, 1 Enable Input and 2 outputs - Considering a 4 to 2 Priority Encoder. Here, 0th channel is at Lowest priority. If 0th and 2nd Channel is High, Output will be 2 and will completely ignore 0th Channel.
To avoid this, Programmable Priority Encoders (PPE) are used. In PPE, we can specify which input should have lowest priority. Generally PPE has additional inputs to specify Lowest Priority (LOWP). Number of lines for LOWP are same as Output Lines.
In case of 4 to 2 Programmable Priority Encoder - 4 Input channels, 2 LOWP lines (Input), 1 Enable Input and 2 Output lines.
If you set LOWP lines as b010 - 2nd input channel will have lowest priority (Instead of 0th in normal Priority Encoder).
Priority order will change to (From lowest to Highest) 2nd,3rd,0th,1st If 3rd and 1st Input channel is enabled, output will be b01 instead b11