Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 169)

AN93
Rev. 1.3 169
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.
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.
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