Maxim Integrated 73M1822/73M1922 Implementers Guide - Manual (Page 16)

73M1822/73M1922 Implementer’s Guide UG_1x22_052
16 Rev. 1.0
4.3 Ring Detection and Line Voltage Reversal
When the 73M1922 is in on-hook mode, and when the ring detect interrupt is enabled (ENRGDT), a ring
signal can be detected.
Figure 1 shows the possible scenarios that can be encountered on the line.
The threshold voltage for detecting the ring signal is programmable in the 73M1x22 (RGTH[1:0]). When
the line voltage exceeds the programmed threshold, the 73M1x22 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 1. Note that the behavior is identical when the line voltage goes below the programmed threshold,
as shown by “2” and “7” in
Figure 1 .
When the host reads the 73M1x22 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 1 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 1 .
Threshold
Threshold
25msec 25msec
Delay
Line
Voltage
RGMON
RGDT
INT
time
1
2
3
4
6
7
5
8
8 9
Figure 1: Ring Detection
1M
TIP
RNG
73M1x22B
RGP
RGN
1M
0.022uF 250V
0.022uF 250V
10K
10K
8Bit
A/D
Figure 2 : Ring Circuit