Information

• The comparator itself
• The window function
• The filter function
The filter, CR0[FILTER_CNT], can be clocked from an internal or external clock source.
The filter is programmable with respect to the number of samples that must agree before
a change in the output is registered. In the simplest case, only one sample must agree. In
this case, the filter acts as a simple sampler.
The external sample input is enabled using CR1[SE]. When set, the output of the
comparator is sampled only on rising edges of the sample input.
The "windowing mode" is enabled by setting CR1[WE]. When set, the comparator output
is sampled only when WINDOW=1. This feature can be used to ignore the comparator
output during time periods in which the input voltages are not valid. This is especially
useful when implementing zero-crossing-detection for certain PWM applications.
The comparator filter and sampling features can be combined as shown in the following
table. Individual modes are discussed below.
Table 32-22. Comparator sample/filter controls
Mode # CR1[EN] CR1[WE] CR1[SE]
CR0[FILTER_C
NT]
FPR[FILT_PER] Operation
1 0 X X X X Disabled
See the Disabled mode (# 1).
2A 1 0 0 0x00 X Continuous Mode
See the Continuous mode (#s 2A &
2B).
2B 1 0 0 X 0x00
3A 1 0 1 0x01 X Sampled, Non-Filtered mode
See the Sampled, Non-Filtered
mode (#s 3A & 3B).
3B 1 0 0 0x01 > 0x00
4A 1 0 1 > 0x01 X Sampled, Filtered mode
See the Sampled, Filtered mode (#s
4A & 4B).
4B 1 0 0 > 0x01 > 0x00
5A 1 1 0 0x00 X Windowed mode
Comparator output is sampled on
every rising bus clock edge when
SAMPLE=1 to generate COUTA.
See the Windowed mode (#s 5A &
5B).
5B 1 1 0 X 0x00
Table continues on the next page...
Chapter 32 Comparator (CMP)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 645