Method and apparatus for dynamically adapting the clock rate and data transfer rate of phone-line powered modems.

Methods and apparatus are disclosed for detecting the amount of the power available from a phone line and for adjusting the clock rate and data transfer rate of a modem or a device that transfers data over the phone line to provide the best possible performance for the available phone-line power. A method of operating a device for transferring data over a phone line includes the following steps: setting the device off hook; measuring the voltage level of the phone line; determining the amount of the power available from the phone line; selecting a clock rate for the device; setting the device to operate at the clock rate; placing a call; selecting a data transfer rate; and transferring the data. An apparatus for transferring data over a phone line includes a clock generator for generating a first clock signal of a first frequency; a frequency divider for generating multiple clock signals of different frequencies and for outputting one of the multiple clock signals; a data pump for modulating and demodulating data; a sensor for sensing the voltage level of the phone line; a microcontroller for controlling the frequency divider, the data pump and the sensor; a line isolator for blocking hazardous electrical signals from coming into the apparatus; an interface for coupling the microcontroller to a computer host; and a power converter for converting a DC voltage of the phone line to an appropriate DC power supply voltage to be used by the apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to modems or devices used to transfer data 
over a phone line and, in particular, to the modems or devices that derive 
power from the phone line. 
2. Background Information: 
Definition 
A central office exchange is a place where a communication common carrier 
locates the equipment which interconnects subscribers and circuits. A 
private branch exchange is a local place where a communication common 
carrier locates the equipment which interconnects subscribers and 
circuits. A private branch exchange typically serves a smaller community 
than a central office exchange. 
A subscriber loop length is defined as the service distance between a 
user's modem and the central office exchange or the private branch 
exchange. A short subscriber loop length indicates a short distance 
between the user's modem and the central office exchange or the private 
branch exchange. 
An "off-hook" state is equivalent to lifting a receiver off of a phone. A 
modem is in an "off-hook" state when the modem is presented to a phone 
line as a load, prior to or during the placement of a call. 
DESCRIPTION OF THE RELATED ART 
While modems used with non-portable computers such as desktop computers 
have essentially "unlimited" power available because they derive power 
from the computers' power supply or wall sockets, modems used in 
conjunction with portable computers have a limited amount of power 
available because they derive power from batteries or phone lines. 
FIGS. 1a-1e illustrate different types of modems that are used in 
conjunction with portable computers. In FIG. 1a, a computer 10a, which may 
be a laptop computer or a notebook computer, has a battery 12a as its 
power supply and a custom-built modem 14a that derives its power from 
battery 12a. The battery capacity determines the amount of power available 
to modem 14a. In FIG. 1b, a computer 10b, which may be a laptop computer 
or a notebook computer, has a battery 12b as its power supply and an 
internal modem 14b having an interface incorporating personal computer 
memory card international association (PCMCIA) standards. Modem 14b 
derives its power from battery 12b. Depending on the battery capacity, 
batteries 12a and 12b in FIGS. 1a and 1b may or may not be able to provide 
enough power for modems 14a and 14b, respectively, to operate at a high 
data transfer rate (e.g., 14400 bits-per-second). 
In FIG. 1c, a portable computer 10c has a battery 12c and is coupled to an 
external pocket modem 14c, in the size of a cigarette pack, having its own 
battery 18c. Battery 18c is typically a 9-volt battery and thus has a 
limited battery capacity. In FIG. 1d, a portable computer 10d has a 
battery 12d and is coupled to an external pocket modem 14d that derives 
most of its power from a phone line. Battery 12d provides only minimal 
power to start pocket modem 14d. During operation, the phone line provides 
power to pocket modem 14d. 
FIG. 1e shows a personal digital assistance (PDA) 10e, such as a palmtop 
computer, having a battery 12e and a PCMCIA modem 14e. Because battery 12e 
is typically an AA battery, the amount of power available to modem 14e is 
almost non-existent. Battery 12e provides only enough power for PCMCIA 
modem 14e to receive initial commands from PDA 10e. Once in operation, 
PCMCIA modem 14e derives its power from the phone line. 
Although modems are currently capable of operating at 14400 
bits-per-second, a modem that derives most of its power from a phone line, 
such as those shown in FIGS. 1c-1e, typically operates at 2400 
bits-per-second because of the limited power availability. As the data 
transfer rate of the modem becomes higher, the modem requires more 
computing power, and hence more battery power. Thus, unless the phone line 
provides enough power for 14400 bits-per-second data transfer rate, one 
cannot operate a modem at that rate. 
FIG. 2 is a plot showing the available power from a phone line versus the 
line resistance. As the distance (or subscriber loop length) between the 
modem and a central office exchange or a private branch exchange office 
becomes longer, the line resistance becomes larger. As the line resistance 
increases, the amount of power available from the phone line decreases, as 
shown in FIG. 2. As the power decreases, the modem's computing power 
decreases, thus the potential data transfer rate also decreases. While 
transferring data at 14400 bits-per-second typically requires 0.5-0.6 
watts, transferring data at 2400 bits-per-second requires only 0.3 watts. 
Thus if there are only 0.3 watts available from the phone line, then the 
modem deriving power from the phone line can transfer data only at a low 
data transfer rate such as 2400 bits-per-second. Although a short 
subscriber loop provides power sufficient for 14400 bits-per-second, 
because a long subscriber loop provides power only enough to allow data 
transfer at 2400 bits-per-second, conventional modems that derive most of 
their power from the phone line are fixed to the 2400 bits-per-second so 
that the modems can be used for both long and short subscriber loop 
lengths. 
The drawback of prior art modems that derive power from the phone line is 
that they require a fixed low data transfer rate such as 2400 
bits-per-second. Therefore, prior art modems cannot operate at a higher 
data transfer rate even if the subscriber loop length is short, providing 
power sufficient for the higher data transfer rate. If the subscriber loop 
is long, then there is only low power available to the user, and the modem 
operates at an optimum data transfer rate for the long subscriber loop 
length. However, if the subscriber loop is short, then the modem does not 
operate at an optimum data transfer rate for the power available. While 
the phone line provides high enough power for the modem to transfer data 
at a high rate, since the modem is fixed at the 2400 bits-per-second, the 
modem cannot transfer data at the high rate, and thus the user cannot 
benefit from the short local subscriber loop. 
It would be advantageous, and is therefore an object of the present 
invention, to detect the amount of power available from a phone line and 
to adjust the frequency of the clock signal of the modem and the data 
transfer rate accordingly. 
SUMMARY OF THE INVENTION 
From the foregoing it can be appreciated that it would be desirable to 
provide a mechanism that adjusts the clock rate and the data transfer rate 
of a modem according to the amount of the power available from a phone 
line. Hence, it is an object of the present invention to provide methods 
and apparatus for detecting the amount of the power available from a phone 
line so that a modem or any device that transfers data over the phone line 
can adjust its clock rate and data transfer rate to provide optimal 
performance for the available phone-line power. 
According to the present invention, a method of operating a modem or a 
device for transferring data over a phone line includes the following 
steps: setting the modem off hook; measuring the voltage level of the 
phone line; determining the amount of the power available from the phone 
line; selecting a clock rate for the modem; setting the modem to operate 
at the clock rate; placing a call; selecting a data transfer rate; and 
transferring the data at the selected data transfer rate. 
An apparatus for transferring data over a phone line according to the 
present invention includes a clock generator for generating a first clock 
signal of a first frequency; a frequency divider coupled to the clock 
generator for generating multiple clock signals of different frequencies 
and for outputting one of the multiple clock signals; a data pump coupled 
to the frequency divider for modulating and demodulating data; a sensor 
for coupling to the phone line and for sensing the voltage level of the 
phone line; a microcontroller for controlling the frequency divider, the 
data pump and the sensor; a line isolator for blocking hazardous 
electrical signals from coming into the apparatus; an interface for 
coupling the microcontroller to a computer host; and a power converter for 
converting a DC voltage of the phone line to an appropriate DC power 
supply voltage to be used by the apparatus. 
The microcontroller of the present invention operates as follows: The 
microcontroller directs the sensor to measure the voltage level of the 
phone line. Once the sensor sends the measured data to the 
microcontroller, the microcontroller determines the appropriate clock rate 
to be used for the data pump. The microcontroller sends a control signal 
to the frequency divider to output the clock signal corresponding to the 
appropriate clock rate and another control signal to the data pump so that 
the data pump can be set up to operate at that clock rate.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a method and apparatus for implementing a 
phone-line powered modem that can dynamically adapt the clock rate and the 
data transfer rate. In the following detailed description, numerous 
specific details are set forth such as particular hardware configurations, 
devices, programming codes, and data control structures in order to 
provide a thorough understanding of the present invention. It will be 
appreciated, however, by one having ordinary skill in the art that the 
present invention may be practiced without such specific details. In other 
instances, well-known structures and data are shown in block diagram form 
in order not to obscure the present invention unnecessarily. 
Now referring to FIG. 3, a block diagram of a modem is presented according 
to one embodiment of the present invention. The modem includes a clock 
generator such as a crystal oscillator 50 for providing a first clock 
signal of one frequency (F.sub.0) and a programmable frequency divider 52 
for generating multiple clock signals of different frequencies from the 
first clock signal and for outputting one of the multiple clock signals 
(F.sub.1), a data pump 54 for converting the data signals coming from a 
computer host 66 into a form compatible with a phone network and for 
converting the signals coming from the phone network into a form 
compatible with computer host 66, and a line isolator 56 for providing 
isolation to prevent hazardous electrical signals from being carried into 
the modem. Phone line 74 has two wires: a tip 70 and a ring 72. 
The modem further includes a data terminal equipment (DTE) interface 58 for 
interfacing the modem to a DTE such as computer host 66, a microcontroller 
60 for controlling and sequencing various components of the modem, an 
isolated voltage sensor 62 for sensing the voltage level of phone line 74 
to determine the amount of power available from phone line 74, and an 
isolated power converter 64 for converting a direct-current (DC) voltage 
of phone line 74 into an appropriate DC power supply voltage to be used by 
the modem components. 
Continuing to refer to FIG. 3, in operation, after computer host 66 directs 
the modem to send data, the modem goes "off hook." Microcontroller 60 
sends an appropriate signal to phone line 74 so that current flows into 
the modem from phone line 74. Isolated voltage sensor 62 measures the 
voltage level of phone line 74 and provides a digital signal to 
microcontroller 60 using an analog-to-digital (A/D) converter. Upon 
receiving the digital signal from isolated voltage sensor 62, 
microcontroller 60 determines the appropriate frequency for the clock 
signal to be used in the modem corresponding to the power available at 
phone line 74. Microcontroller 60 sends a control signal to frequency 
divider 52 so that frequency divider 52 can send a clock signal having the 
appropriate frequency to data pump 54. Microcontroller 60 also sends a 
control signal to data pump 54 so that data pump 54 can be set up to 
operate at the frequency of the clock signal coming from frequency divider 
52. If the voltage at phone line 74 is high, then the frequency of the 
clock signal is high, and the data transfer rate can be set high or low 
depending on the phone line quality. Since the mechanisms for adjusting 
the data transfer rate according to the phone line quality are well-known 
in the art, the details of such description is omitted. If the voltage at 
phone line 74 is low, then the frequency of the clock signal is set low, 
and the data transfer rate is set low. Once the frequency of the clock 
signal and the data transfer rate are set, data pump 54 accepts data from 
computer host 66, modulates and converts the data into analog signals and 
sends them to phone line 74. 
In FIG. 3, once the modem goes "off hook," isolated power converter 64 
receives DC power from phone line 74, converts it to an appropriate DC 
power supply voltage, and provides the DC power supply voltage to the 
various components of the modem. All components of the modem except 
microcontroller 60 derive their power entirely from phone line 74. 
Although microcontroller 60 derives its power from phone line 74 during 
the modem operation, it requires a small amount of power from computer 
battery 68 to perform a modem initiation procedure. The modem initiation 
procedure includes the following: accepting modem initiation commands from 
computer host 66 and sending an appropriate signal to the phone line so 
that current can flow from the phone line to the modem. Microcontroller 60 
typically draws a current in the range of a micro-amp to a few milli-amps 
during the modem initiation procedure and does not drain computer battery 
68 in any significant amount. 
It should be noted that the data transfer rate is determined not only by 
the amount of the power available to the modem but also by the quality of 
the phone line. If the amount of the available power is high, and the 
quality of the phone line is good, then the modem can transfer the data at 
a high rate. If the amount of the available power is high, but the quality 
of the phone line is bad, then the data transfer occurs at a low data 
transfer rate. In addition, if the quality of the phone line is good, but 
the amount of the available power is low, then the data transfer again 
occurs at a low data transfer rate. 
FIG. 4a is a flow chart showing the operation of a prior art modem that has 
essentially "unlimited" power available to the modem. Such modems include, 
but are not limited to, those that are used with non-portable computers 
and those shown in FIGS. 1a-1b. In the example shown in FIG. 4a, because 
the amount of the power available to the modem is high (or "unlimited"), 
the data transfer rate is determined only by the quality of the phone 
line. In FIG. 4a, at step 80, the modem places a call. Steps 82-86 are 
used to check the quality of the phone line. At step 82, the modem 
attempts to make a connection at 14400 bits-per-second. If successful, 
then the modem transfers data. 
If, on the other hand, there is a lot of noise, distortion or interference 
in the phone line, the modem determines that the data transfer rate of 
14400 bits-per-second is too high and attempts to establish a connection 
at a lower data transfer rate. It should be noted that the quality of the 
phone line connection is independent of the amount of power available from 
the phone line. The quality of the phone line is determined by the amount 
of noise, distortion and/or interference present in the line and not by 
the amount of the power available. The quality of the phone can be good or 
bad regardless of the amount of the power available. At step 84, the modem 
determines whether the quality of the phone line is good enough for 4800 
bits-per-second. If it is, data is transferred. Otherwise, the modem tries 
to connect at 2400 bits-per-second. If the 2400 bits-per-second rate 
fails, then the process is aborted. 
FIG. 4b is a flow chart showing the operation of a prior art modem powered 
mainly by a phone line, such as those shown in FIGS. 1c-1e. In the case 
shown in FIG. 4b, because the amount of the power available to the modem 
is small for a long subscriber loop--although the amount of the power 
available to the modem is high for a short subscriber loop length, the 
prior art modem has a fixed low data transfer rate (e.g., 2400 
bits-per-second) so that the modem can be used for both short and long 
subscriber loop lengths. In the prior art, the data transfer rate is fixed 
to the lowest data transfer rate since the modem does not have the 
capabilities to determine the amount of the power available from the phone 
line and to set the modem communication rate accordingly. At step 90, the 
modem places a call. At step 92, the modem checks whether the quality of 
the phone line connection is good enough to transfer data at 2400 
bits-per-second. If the modem can make a connection at 2400 
bits-per-second, then data is transferred. Otherwise, the data transfer is 
aborted. 
FIG. 5 illustrates the operation of a modem according to one embodiment of 
the present invention. The present invention is most useful when utilized 
as a phone-line powered modem, such as those shown in FIGS. 1d and 1e or 
any device including, but not limited to, a credit card verifier and an 
answering machine that transfers data over a phone line and needs to be 
powered mainly by the phone line. In addition, the present invention can 
be used in conjunction with other types of portable computers such as 
those shown in FIGS. 1a-1c or with non-portable computers as well. 
Referring to FIG. 5, at step 100, the modem is "off hook." At steps 102 and 
104, isolated voltage sensor 62 measures the voltage level of the phone 
line to determine the amount of the power available from the phone line. 
At step 102, if the voltage is high, indicating a large amount of 
available power, then at step 106, microcontroller 60 signals frequency 
divider 52 to send a high frequency clock signal to data pump 54. 
Microcontroller 60 also sends appropriate parameters to data pump 54 so 
that data pump 54 can be set up to operate at the high frequency clock 
rate (e.g., 30 MHz). Because the phone line provides a large amount of 
power to the modem, the modem is set up to operate at the high frequency 
clock rate and is capable of transferring data at a high data transfer 
rate. If the quality of the phone line is also good, then the modem can 
transfer data at the high data transfer rate. 
At step 112 in FIG. 5, the modem places a call. Step 112 is followed by 
step 82 of FIG. 4a to determine the quality of the phone line. The modem 
attempts to make a connection at 14400 bits-per-second, 4800 
bits-per-second and 2400 bits-per-second successively. Since the phone 
line's available power is high, data pump 54 operates at the high 
frequency clock rate (e.g., 30 MHz), and if the quality of the phone line 
is good, then the data is transferred at the high data transfer rate 
(e.g., 14400 bits-per-second). If, however, the quality of the phone line 
is bad, then while data pump 54 operates at the high frequency clock rate 
(e.g., 30 MHz), the modem transfers data at a low data transfer rate 
(e.g., 2400 bits-per-second). 
Referring back to FIG. 5, at step 102, if the voltage level of the phone 
line is not high, then microcontroller 60 determines whether the voltage 
level is at least medium. If it is, then at step 108, microcontroller 60 
signals frequency divider 52 to send a medium frequency clock signal to 
data pump 54. It also sends appropriate parameters to data pump 54 so that 
data pump 54 can operate at the medium frequency clock rate (e.g., 16 
MHz). At step 114, the modem places a call. Step 114 is followed by step 
84 in FIG. 4a. Depending on the quality of the phone line, a connection is 
made at 4800 bits-per-second (step 84) or 2400 bits-per-second (step 86) 
or is aborted (step 88). 
Referring back to FIG. 5, at step 104, if the voltage level of the phone 
line is lower then the medium voltage level, then at step 110, 
microcontroller 60 signals frequency divider 52 to send a low frequency 
clock signal to data pump 54 and sends appropriate parameters to data pump 
54 so that data pump 54 can operate at the low frequency clock rate (e.g., 
4 MHz). After placing a call at step 116, the modem attempts to make a 
connection at a low data transfer rate (e.g., 2400 bits-per-second) at 
step 86 in FIG. 4a. 
The differences between the capabilities of the present invention and the 
prior art modems are summarized in Tables 1 and 2. Referring to Table 1, 
in the present invention, after determining the amount of the power 
available from the phone line--to be high, medium or low, the present 
invention selects the corresponding frequency of the clock signal to be 
used in the modem. For example, if the amount of the available power is 
high (or the voltage level is high), then the frequency of the clock 
signal is set high. If the amount of the available power is medium, then 
the frequency of the clock signal is set medium. If the amount of the 
available power is low, then the frequency of the clock signal is set low. 
When the amount of the available power is high, the data transfer rate can 
be set to be high, medium or low depending on the quality of the phone 
line. When the amount of the available power is medium, the data transfer 
rate is set to be medium or low depending on the quality of the phone 
line. When the amount of the available power is low, the data transfer 
rate is set low. Although there are only three levels for each of the 
available power, frequencies of the clock signal and data transfer rates 
according one embodiment of the present invention, as shown in Table 1, 
there may be more levels or less levels in alternative embodiments. 
TABLE 1 
______________________________________ 
PRESENT INVENTION 
Available Power Frequency Data transfer 
(or voltage level) 
of Clock rate 
______________________________________ 
High High High 
High High Medium 
High High Low 
Medium Medium Medium 
Medium Medium Low 
Low Low Low 
______________________________________ 
Table 2 shows the capabilities of the prior art modems. Modem A represents 
a modem that has essentially "unlimited" power. Modem B represents a modem 
that has limited available power either because it derives its power from 
a phone line or from a combination of a phone line and a small battery. 
Modem A may represent any of the modems shown in FIGS. 1a-1b or a modem 
used in a non-portable computer. Modem B may be any of the modems shown in 
FIGS. 1c-1e. Referring to Table 2, in modem A, regardless of the amount of 
the power available from the phone line, the frequency of the clock signal 
is fixed. However, the data transfer rate may vary from high to low 
depending on the quality of the phone line. In Modem B, the frequency of 
the clock signal is fixed, and the data transfer occurs at a low data 
transfer rate regardless of the amount of the available power. Thus the 
prior art modems neither detect the amount of the power available from a 
phone line nor have variable frequencies for the clock signals. 
TABLE 2 
______________________________________ 
PRIOR ART 
MODEM A MODEM B 
Data 
Available 
Frequency 
transfer Available 
Frequency 
Data 
Power of Clock rate Power of Clock 
transfer rate 
______________________________________ 
N/A Fixed High N/A Fixed low 
N/A Fixed Medium 
N/A Fixed Low 
______________________________________ 
FIG. 6 illustrates the implementation of data pump 54 of FIG. is according 
to one embodiment of the present invention. Data pump 54 is used to 
modulate the digital signals coming from computer host 66 and convert them 
into analog signals to send them to the phone line and to convert analog 
signals coming from the phone line into digital signals and demodulate the 
signals to send them to computer host 66. 
Continuing to refer to FIG. 6, data pump 54 includes a digital signal 
processor (DSP) 122, a read-only memory (ROM) 126, a random-access memory 
(RAM) 128 and a digital-to-analog (D/A) and analog-to-digital (A/D) 
converter 124. ROM 126 contains a program or different sets of programs to 
operate data pump 54 at a selected clock rate. The pseudo code for such a 
program may be resented as follows: 
______________________________________ 
DATA.sub.-- PUMP(freq.sub.-- clock) 
CASE freq.sub.-- clock DO 
high: GOTO subroutine A; 
medium: GOTO subroutine B; 
low: GOTO subroutine C; 
END CASE; 
______________________________________ 
Each of subroutines A, B and C contains computer instructions for data pump 
54 to modulate or demodulate data. The only difference between the 
subroutines is the frequency of the clock signal at which data pump 
operates. In one embodiment, the present invention may implement the 
pseudo code shown above in a digital signal processor such as TMS320 
manufactured by Texas Instrument and use TMS320 as a digital signal 
processor for data pump 54. Since the details of TMS320 can be found in 
TMS320 Second-Generation Digital Signal Processors data sheet (Texas 
Instruments, Texas 1989), the detailed description on modulation and 
demodulation operations is omitted. 
FIG. 7 illustrates the implementation of microcontroller 60 of FIG. is 
according to one embodiment of the present invention. Microcontroller 60 
includes a microprocessor 132, a ROM 136 and a RAM 138. Microcontroller 60 
uses DTE interface 58 to communicate with computer host 66. DTE interface 
58 may be a serial or parallel I/O port. Microcontroller 60 also has 
peripheral I/O ports for controlling data pump 54, line isolator 56, 
isolated voltage sensor 62 and frequency divider 52. 
In FIG. 7, after microcontroller 60 directs isolated voltage sensor 62 to 
measure the voltage level of the phone line, microcontroller 60 determines 
the amount of the power available from the phone line. Depending on the 
power level, microcontroller 60 signals frequency divider 52 to send a 
high frequency clock signal, a medium frequency clock signal or a low 
frequency clock signal. Microcontroller 60 also sends a signal to data 
pump 54 so that data pump 54 can be configured for the appropriate clock 
rate. After the proper data transfer rate is set, microcontroller 60 
directs data pump to start transferring the data. Microcontroller 60 may 
implement the pseudo code shown below: 
______________________________________ 
BEGIN 
GO off.sub.-- hook; 
MEASURE(line.sub.-- voltage); 
CASE line.sub.-- voltage DO 
high: 
SET.sub.-- FREQ.sub.-- CLOCK(high); 
SET.sub.-- DATA.sub.-- PUMP(high); 
SET.sub.-- BIT.sub.-- RATE; 
medium: 
SET.sub.-- FREQ.sub.-- CLOCK(medium); 
SET.sub.-- DATA.sub.-- PUMP(medium); 
SET.sub.-- BIT.sub.-- RATE; 
low: 
SET.sub.-- FREQ.sub.-- CLOCK(low); 
SET.sub.-- DATA.sub.-- PUMP(low); 
SET.sub.-- BIT.sub.-- RATE; 
END CASE; 
GOTO data.sub.-- mode.sub.-- start; 
END 
______________________________________ 
As discussed earlier, it will be appreciated that although in the above 
pseudo code, the line voltages and clock frequencies each have three 
levels--high, medium and low--according to one embodiment, they may have 
more levels or less levels in alternative embodiments. In addition, 
although the amount of the power available from the phone line is 
determined by measuring the voltage level at the phone line, there are 
other methods of determining the amount of the power available such as 
measuring the current or the power itself. 
The computer instructions of the above pseudo code reside in ROM 136. The 
present invention may implement the pseudo code shown above in a 
microcontroller such as 80C196 manufactured by Intel Corporation. The 
details of Intel microcontroller 80C196 may be found in 
80C196KB10/83C196KB10/80C196KB12/83196KB12 Commercial/Express CHMOS 
Microcontroller data sheet, Order number 270918-002 (Intel Corp. 1991). 
Frequency divider 52 in FIG. is may be implemented in the present invention 
by programming a commercially available product such as SN74HC160 
manufactured by Texas Instrument. In one embodiment, the frequency divider 
52 is programmed to produce three clock signals having different 
frequencies and to output a selected one of the three clock signals 
according to the control signal coming from microcontroller 60. In another 
embodiment, frequency divider 52 may be programmed to produce more or less 
number of clock signals. The details of SN74HC160 can be found in 
SN54HC160, SN54JC162, SN74JC160, SN74HC162 Synchronous 4-Bit Decade 
Counters data sheet (Texas Instrument, Texas). 
FIG. 8 shows a computer system that may utilize a modem in accordance with 
the present invention. A computer host (hereinafter a host) 1000 includes 
a memory 1008 and a central processor 1002. Memory 1008 and central 
processor 1002 are those typically found in most general purpose computers 
and almost all special purpose computers. In fact, these devices contained 
within computer host 1000 are intended to be representative of the broad 
category of data processors and memory. Many commercially available 
computers having different capabilities may be utilized in the present 
invention. 
A system bus 1016 is provided for communicating information. A display 
device 1010 utilized with the computer system of the present invention may 
be a liquid crystal device, cathode ray tube or other display device 
suitable for creating graphic images and/or alphanumeric characters 
recognizable to a user. The computer system may also include an 
alphanumeric input device 1012 including alphanumeric and function keys 
coupled to bus 1016 for communicating information and command selections 
to central processor 1002, and a cursor control device 1018 coupled to bus 
1016 for communicating user input information and command selections to 
central processor 1002 based on a user's hand movement. Cursor control 
device 1018 allows the user to dynamically signal the two-dimensional 
movement of the visual symbol (or cursor) on a display screen of display 
device 1010. Many implementations of cursor control device 1018 are known 
in the art, including a track ball, mouse, pen, joystick or special keys 
on the alphanumeric input device 1012, all capable of signaling movement 
in a given direction or manner of displacement. 
The computer system of FIG. 8 also includes an interface device 1019 
coupled to bus 1016 for communicating information to and from the computer 
system. Interface device 1019 may be coupled to a modem 1020, a 
microphone, a speaker, a network system, other memory devices, other 
computers, etc. Also available for interface with the computer system of 
the present invention is a data storage device 1017 such as a magnetic 
disk or optical disk drive, which may be communicatively coupled with bus 
1016, for storing data and instructions. The computer system of FIG. 8 may 
also include a printer for outputting data. 
While the present invention has been particularly described with reference 
to the various figures and embodiments, it should be understood that these 
are for illustration only and should not be taken as limiting the scope of 
the invention. Many changes and modifications may be made to the 
invention, by one having ordinary skill in the art, without departing from 
the spirit and scope of the invention. It should be noted that although 
specific data transfer rates such as 14400 bits-per-second and 2400 
bits-per-second are used to represent a high and a low data transfer rate, 
respectively, these are mere examples and are not the high and low limits 
of the data transfer rates possible for the present invention.