Owner's manual

73M1866B/73M1966B Implementer’s Guide UG_1x66B_016
20 Rev. 1.3
4.3 Ring Detection and Line Voltage Reversal
When the 73M1966 is in on-hook mode, and when the ring detect interrupt is enabled (ENRGDT), a ring
signal can be detected. Figure 3 shows the possible scenarios that can be encountered on the line.
The threshold voltage for detecting the ring signal is programmable in the 73M1x66B (RGTH[1:0]). When
the line voltage exceeds the programmed threshold, the 73M1x66B asserts the hardware interrupt output
pin (INT) and asserts both RGDT and RGMON in the interrupt register, as illustrated by “1” and “3” in
Figure 3. Note that the behavior is identical when the line voltage goes below the programmed threshold,
as shown by “2” and “7” in Figure 3.
When the host reads the 73M1x66B interrupt register, RGDT is cleared, while RGMON remains asserted
as long as:
1. The line voltage is above (or below) the upper (lower) threshold level shown in Figure 3 and
2. A time period of 25 ms has not elapsed after the line voltage crossed back over the threshold level.
The activity of the timer ruling RGMON is indicated by “4” and “5” (as well as “8” and “9”) in Figure 3.
Threshold
Threshold
25msec
25msec
Delay
Line
Voltage
RGMON
RGDT
INT
time
1
2
3
4
6
7
5
8
8
9
Figure 3: Ring Detection
If the line voltage does not exceed the threshold within the 25 ms period, RGMON becomes de-asserted,
as illustrated by “6” and “9” in Figure 3.
Upon the first reception of the first RGDT interrupt, if no other ring interrupt was received during a
sufficiently long period then it can be assumed that a Line Polarity Reversal has occurred. To filter out
spurious ring events (from a parallel device going off hook or battery loss), the driver should check that
the line voltage is the same as previously before determining that the single ring event was, in fact, a line
polarity reversal.