hysteresis_switch.cdl File Reference

A hystersis detector using counter pairs. More...

Detailed Description

A hystersis detector using counter pairs.

Copyright

(C) 2016-2017, Gavin J Stark. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

CDL implementation of a module that takes an input signal and notionally keeps a count of cycles that the input is low, and cycles that the input is high; using these counters it makes a decision on the real value of the output, using hysteresis.

Since infinite history is not sensible and the counters cannot run indefinitely without overflow anyway, the counters divide by 2 on a configurable divider (effectively filtering the input stream).

The two notional counters are cycles_low and cycles_high.

To switch to a 'high' output from a current 'low' output requires the cycles_high - cycles_low to be greater than half of the filter period.

To switch to a 'low' output from a current 'high' output requires the cycles_high - cycles_low to be less than minus half of the filter period.

Hence a n+1 bit difference would need to be maintained for cycles_high and cycles_low. This difference would increase by 1 if the input is high, and decrease by 1 if the input is low.

Hence an actual implementation can maintain an up/down counter cycles_diff, which is divided by 2 every filter period, and which is incremented on input of 1, and decremented on input of 0.

When the output is low and the cycles_diff is > half the filter period the output shifts to high.

When the output is high and the cycles_diff is < -half the filter period the output shifts to low.

Data Structures
struct	hysteresis_switch::t_hystersis_state
struct	hysteresis_switch::t_hysteresis_combs

Namespaces
	hysteresis_switch