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Steps include the following (refer to Figure 17-16):
1. When the first edge having the selected polarity is detected, the 16-bit counter bus
value is latched into the 16-bit data register A,
2. Data in register B1 is transferred to data register B2 and
3. Finally the data in register A is transferred to register B1.
On this first capture the FLAG line is not activated, and the value in register B2 is
meaningless. On the second and subsequent captures, the FLAG line is activated when the
data in register A is transferred to register B1.
When the second edge of the same polarity is detected, the counter bus value is latched into
data register A, the data in register B1 is transferred to data register B2, the FLAG line is
activated to signify that the beginning and end points of a complete period have been
captured, and finally the data in register A is transferred to register B1. This sequence of
events is repeated for each subsequent capture. Reading data register B returns the value in
register B2.
If a 32-bit coherent operation is in progress when an edge (except for the first edge) is
detected, the transfer of data from B1 to B2 is deferred until the coherent operation is
completed. At any time, the input level present on the input signal can be read on the PIN
bit.
The input pulse period is calculated by subtracting the value in data register B from the
value in data register A.
Figure 17-16 provides an example of how the MDASM can be used for input period
measurement.
MOTOROLA
Chapter 17. Modular Input/Output Subsystem (MIOS14)
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
MIOS14 Double Action Submodule (MDASM)
17-31
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