hysteresis_switch

Files
file	hysteresis_switch.cdl
	A hystersis detector using counter pairs.

Detailed Description

Modules

module hysteresis_switch::hysteresis_switch	(	clock	clk,
		input bit	reset_n,
		input bit	clk_enable,
		input bit	input_value,
		output bit	output_value,
		input bit	filter_period[16],
		input bit	filter_level[16]
	)

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.

Clock for all the logic, based on an enable in

Parameters

[in]	clk	Clock for the module
[in]	reset_n	Active low reset
[in]	clk_enable	Assert to enable the internal clock; this permits I/O switches to easily use a slower clock
[in]	input_value	Input pin level, to apply hysteresis to
[out]	output_value	Output level, after hysteresis
[in]	filter_period	Period over which to filter the input - the larger the value, the longer it takes to switch, but the more glitches are removed
[in]	filter_level	Value to exceed to switch output levels - the larger the value, the larger the hysteresis; must be less than 2*filter_period