Silicon Laboratories SI2494/39 manual Overcurrent Detection, Pulse/Tone Dial Decision

AN93

The following steps will allow the spurious data and bit errors to be eliminated while preserving the valid data.

1.Ignore 19 B0.

2.Use 19 B2 to discard all collected receive data.

The filtered version of the HDLC frames, based on this algorithm, is shown below with the valid data in bold.

0D 0A 43 4F 4E 4E 45 43 54 20 31 32 30 30 0D 0A 19 BE 20 20 19 B1 19 B0 19 B2 30 93 19 B1 19 B2 30 93 19 B1 19 B2 30 93 19 B1 19 B2 30 93 19 B1 19 B2 19 B2 B6 9E F7 46 19 B0 19 B2 29 C6 19 B0 19 B2 FF 98 89 18 19 B0 19 B2 92 6E EF 14 65 19 B0 19 B2 DA BE C6 07 EA D8 31 C2 05 3C FA C8 86 C4 40 E6 19 A0 CA EA A8 F9 19 B2 8D 00 57 A5 43 29 19 B0 19 B2 05 CB 14 9F 7C 2D 19 B0 19 B2 19 B2 19 BA 0D 0A 4E 4F 20 43 41 52 52 49 45 52 0D 0A

6.9. Overcurrent Detection

The ISOmodem has a built-in overcurrent detection feature (disabled by default) that measures loop current a programmable amount of time after going off-hook. This allows the modem to detect an improper line condition. The overcurrent detect feature is enabled by setting U70 [11] (OCDM) = 1. During the time after the modem goes off-hook, loop current is measured and set by U77 [8:0] (OHT). The default delay is 16 ms. After the delay, current is sampled every 1 ms. An overcurrent is detected if two consecutive samples indicate an overcurrent condition. If this feature is enabled and excessive current is detected, the ISOmodem sends the X result code and triggers an interrupt by asserting the INT pin or by setting the INT bit in the parallel or SPI mode. After an interrupt is received, the host issues the AT:I command to verify the OCD interrupt and clear the OCD bit. The delay between modem off-hook and loop current measurement is set by the OHT bits. OHT is a 9-bit register with 1 ms units. The default delay is 16 ms. When the modem is off-hook in an overload condition, LVCS = 11111 (full scale—overload error condition), an X is sent to the DTE, and the OCD bit is set.

The Overcurrent Detection feature is controlled by changing U-register settings. The registers and bits that control these features are shown in Table 105.

Table 105. Overcurrent Detection

Register	Bit	Value	Function
U67	7	DCR	DC Impedance Select
U70	11	OCDM	Overcurrent Detect Mask
U70	3	OCD	Overcurrent Detect
U77	8:0	OHT	Off-Hook Time
U79	4:0	LVCS	Line Voltage Current Sense

6.10. Pulse/Tone Dial Decision

There are three methods to detect whether a telephone line supports DTMF dialing or pulse dialing only. The first method, which is the simplest, may require the modem to go off-hook more than once. The second method is slightly more complicated but does not require the modem to go off-hook multiple times.

6.10.1. Method 1: Multiple Off-Hook Transitions

Use DTMF to dial the desired number with the ATDT command. If the line accepts tone dialing, the call is completed, and connection to the remote modem proceeds as usual.

If the line only allows pulse dialing, the modem hangs up and reports UN-OBTAINABLE NUMBER. This indicates that the modem detected a dial tone after the DTMF dial attempt. Dial the number again using the ATDP command instead of ATDT to use pulse dialing.

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