Silicon Laboratories SI2494/39, SI2493/57/34/15/04 Register Address Name Description Default Hex

AN93

5.7. U Registers

U registers (user-access registers) are 16-bit registers written by the AT:Uhh command and read by the AT:R (read all U registers) command or AT:Rhh (read U-register hh) command. See the AT command list in Table 39 on page 59. All values associated with the U registers, the address, and the value written to or read from the register are hexadecimal.

Some U registers are reserved and not available to the user. Therefore, there are gaps in the available U-register address sequence. Additionally, some bits within available U registers are reserved. Any attempt to write to a non- listed U register or to write a reserved bit can cause unpredictable modem operation.

There are two types of U registers. The first represents a single 16-bit term, such as a filter coefficient, threshold, delay, or other quantity. These registers can be read from or written to as a single 16-bit value. The second type of U register is bit mapped. Bit-mapped registers are also written and/or read in hexadecimal, but each bit or combination of bits in the register represents an independent value. These individual bits are used to enable or disable features and indicate states. Bits in these registers can be read/write, read only, reserved, or they may be required to always be set to a certain value. Pay particular attention when writing to bit-mapped registers to ensure no reserved bits are overwritten. When changing bits in a U register with reserved bits, use a read-modify-write procedure: read the register value with AT:R; modify only the desired bits, then write the new value with AT:U. This will ensure the reserved bits are not altered. All U registers revert to their default settings after a reset.

The U registers can be broken into three groups: call progress (U0–U33, U49–U4C), dialing (U37–U48), line interface, and extended functions (U4D–UA9). Table 46 lists the available U registers, a brief description, and their default values. Table 47 summarizes the signals and values available in the bit-mapped registers. Country-specific register values are presented in "6.2. Country-Dependent Setup" on page 133. All default settings are chosen to meet FCC requirements.

Table 46. U-Register Descriptions

Register	Address	Name	Description	Default
	(Hex)			Value
U00	0x0000	DT1A0	Dial-tone detection filters stage 1 biquad coefficients.	0x0800
U01	0x0001	DT1B1		0x0000
U02	0x0002	DT1B2		0x0000
U03	0x0003	DT1A2		0x0000
U04	0x0004	DT1A1		0x0000
U05	0x0005	DT2A0	Dial-tone detection filters stage 2 biquad coefficients.	0x00A0
U06	0x0006	DT2B1		0x6EF1
U07	0x0007	DT2B2		0xC4F4
U08	0x0008	DT2A2		0xC000
U09	0x0009	DT2A1		0x0000
U0A	0x000A	DT3A0	Dial-tone detection filters stage 3 biquad coefficients.	0x00A0
U0B	0x000B	DT3B1		0x78B0
U0C	0x000C	DT3B2		0xC305
U0D	0x000D	DT3A2		0x4000
U0E	0x000E	DT3A1		0xB50A

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