MAC7200 datasheet(782/976 Pages) FREESCALE

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Functional Description

MAC7200 Microcontroller Family Reference Manual, Rev. 2

704

Freescale Semiconductor

Reading UCAn followed by UCBn always provide coherent data. If not coherent data is required for any

reason, the sequence of reads should be inverted, therefore UCBn should be read prior to UCAn register.

Note that even in this case B1 register updates will be blocked after UCAn read, thus a second UCBn is

required in order to release B1 register updates.

32.13.1.1.5

Input Period Measurement (IPM) Mode

The IPM mode allows the measurement of the period of an input signal by capturing two consecutive rising

edges or two consecutive falling edges. Successive input captures are done on consecutive edges of the

same polarity. The edge polarity is defined by the EDPOL bit in the UCCRn register.

When the first edge of selected polarity is detected, the selected time base is latched into the registers A2

and B2, and the data previously held in register B2 is transferred to register B1. On this first capture the

FLAG line is not set, and the values in registers B1 is meaningless. On the second and subsequent captures,

the FLAG line is set and data in register B2 is transferred to register B1.

When the second edge of the same polarity is detected, the counter bus value is latched into registers A2

and B2, the data previously held in register B2 is transferred to data register B1 and to register A1. The

FLAG bit is set to indicate the start and end points of a complete period have been captured. This sequence

of events is repeated for each subsequent capture. Registers UCAn and UCBn return the values in register

A2 and B1, respectively.

In order to allow coherent data, reading UCAn forces A1 content be transferred to B1 register and disables

transfers between B2 and B1. These transfers are disabled until the next read of the UCBn register.

Reading UCBn register forces A1 content to be transferred to B1 and re-enables transfers from B2 to B1,

to take effect at the next edge capture.

The input pulse period is calculated by subtracting the value in B1 from A2.

Figure 32-19 shows how the Unified Channel can be used for input period measurement.

Figure 32-19. Input Period Measurement example

Figure 32-20 describes the A1 and B1 register updates when UCAn and UCBn read operations are

performed. When UCAn read occurs the content of A1 is transferred to B1 thus providing coherent data

selected counter bus

$000500

$001000

$001100

$001250

$001525

$0016A0

A2(captured) value2

A1 value

B2 (captured) value

$xxxxxx

$001000

$001250

$xxxxxx

$001000

$001250

$0016A0

$xxxxxx

$001000

$001250

$0016A0

Input signal1

EDPOL = 1

FLAG set event

Notes: 1. After input filter

2. UCAn = A2

3. UCBn = B1

B1 value3

$xxxxxx

$001000

$001250

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