<PAGE>
                                                                   EXHIBIT 10.21



                                 PROMISSORY NOTE

$20,000,000                                                    December 11, 2001


               FOR VALUE RECEIVED, NEXT LEVEL COMMUNICATIONS, INC., a Delaware
corporation (the "Borrower"), hereby promises to pay to the order of Motorola,
Inc. (the "Lender"), the principal sum of Twenty Million Dollars (or if less,
the outstanding principal amount hereof), in lawful money of the United States
of America and in immediately available funds, on the dates and in the principal
amounts provided in the Credit Agreement, and to pay interest on the unpaid
principal amount of each such Loan, at such office, in like money and funds, for
the period commencing on the date of such Loan until such Loan shall be paid in
full, at the rates per annum and on the dates provided in the Credit Agreement.

               The date, amount, Type, interest rate and duration of Interest
Period (if applicable) of each Loan made by the Lender to the Borrower, and each
payment made on account of the principal thereof, shall be recorded by the
Lender on its books and, prior to any transfer of this Note, endorsed by the
Lender on the schedule attached hereto or any continuation thereof, provided
that the failure of the Lender to make any such recordation or endorsement shall
not affect the obligations of the Borrower to make a payment when due of any
amount owing under the Credit Agreement or hereunder in respect of the Loans
made by the Lender.

               This Note is one of the Notes referred to in the Credit Agreement
dated as of May 16, 2001 (as amended by Amendment No. 1 dated as of July 15,
2001, Amendment No. 2 dated as of September 28, 2001, Amendment No. 3 dated as
of October 15, 2001 and Amendment No. 4 dated concurrently herewith, and as
further modified and supplemented and in effect from time to time, the "Credit
Agreement") between Next Level Communications, Inc. and Motorola, Inc.,
providing for Loans in an aggregate principal amount not to exceed $82,953,626,
and evidences certain Loans made by the Lender to the Borrower thereunder. Terms
used but not defined in this Note have the respective meanings assigned to them
in the Credit Agreement.

               The Credit Agreement provides for the acceleration of the
maturity of this Note upon the occurrence of certain events and for mandatory
prepayments of Loans upon the terms and conditions specified therein.

               This Note may be prepaid as a whole or in part at any time prior
to the Maturity Date upon at least ten Business Days prior written notice to the
Lender, during which time the Lender may exercise the conversion option
described below. Any such prepayment shall be applied first to the payment of
expenses due under this Note, second to interest accrued on this Note and third,
if the amount of prepayment exceeds the amount of all such expenses and accrued
interest, to the payment of principal of this Note.



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<PAGE>

               Conversion

               (a) Conversion into Company Securities. The Lender, at its
option, may convert all or part of the sum of the principal and accrued interest
then outstanding on the Note into (i) common stock of the Borrower, par value
$.01 per share ("Common Stock"), or other securities, upon the occurrence of the
closing of the Borrower's Next Financing (as defined below), upon the same price
and other terms and conditions as are provided to investors in such Next
Financing (e.g., in the case of securities issuable in such Next Financing at a
conversion price equal to the per share purchase price paid for such securities
by the investors in such Next Financing), or (ii) Common Stock, at a conversion
price of $4.29 per share, subject to adjustment as provided below in the
Conversion Price Adjustment provisions below, if a Next Financing has not
occurred and (A) the Borrower delivers a notice of prepayment as described above
or (B) the Maturity Date will occur within thirty days from the date of such
conversion. "Next Financing" means the Borrower's next round of equity financing
after the date of this Note in which at least $15,000,000 is raised. To the
extent that the investors in the Next Financing are provided with other rights
(including, without limitation, warrants, registration rights, antidilution
rights, preemptive rights, future investment rights and discounts, and any other
property, rights or privileges), the Lender, if it exercises its option to
convert, shall be given such other rights on the same terms as such rights are
offered to such investors. The Common Stock or other securities described above
into which the principal and accrued interest then outstanding on the Note may
be converted pursuant to clause (i) or (ii) above is hereinafter called the
"Company Securities". The price at which the principal and accrued interest then
outstanding on the Note may be converted pursuant to clause (i) or (ii) above is
hereinafter called the "Conversion Price".

               The Borrower shall deliver to the Lender any proposed term sheet
or letter of intent containing the material terms of a proposed Next Financing.
The Lender shall thereafter have ten (10) Business Days either (i) to decline to
exercise its conversion option with respect to such Next Financing or (ii)
tentatively to elect to exercise its conversion option with respect to such Next
Financing, subject to the Lender's review and approval of the definitive
documentation for such Next Financing. If the Lender fails to respond within
such ten (10) Business Day period, it shall be deemed to have elected not to
exercise its conversion option with respect to such Next Financing. If the
Lender tentatively elects to exercise its conversion option as described above,
then at any time prior to the closing of such Next Financing, the Lender may
withdraw its election to convert if the material terms of such Next Financing
change from those previously presented to the Lender at the time of its
tentative election to convert or if the definitive documents relating to such
Next Financing are not, in the reasonable judgment of the Lender, acceptable to
the Lender. If the material terms of such Next Financing change after the Lender
has declined to exercise its option to convert, the Borrower shall present a
changed term sheet or letter of intent to the Lender and the decision process
described above shall begin again.

               (b) Mechanics and Effect of Conversion. No fractional shares of
Company Securities shall be issued upon conversion of this Note. Upon the
conversion of all of the principal and accrued interest outstanding under this
Note, in lieu of the



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<PAGE>

Borrower issuing any fractional shares to the Lender, the Borrower shall pay to
the Lender the amount of outstanding principal that is not so converted, such
payment to be in the form as provided below. On partial conversion of this Note,
the Borrower shall issue to the Lender (i) the shares of Company Securities into
which a portion of this Note is converted and (ii) a new convertible promissory
note having identical terms to this Note, except that the principal amount
thereof shall equal the difference between (A) the principal amount of this Note
immediately prior to such conversion minus (B) the portion of such principal
amount converted into Company Securities. Upon conversion of this Note pursuant
to the terms hereof, the Lender shall surrender this Note, duly endorsed, at the
principal office of the Borrower. At its expense, the Borrower shall, as soon as
practicable thereafter, issue and deliver to such the Lender at such principal
office a certificate or certificates for the number of shares of such Company
Securities to which the Lender shall be entitled upon such conversion, together
with any other securities and property to which the Lender is entitled upon such
conversion under the terms of this Note.

               (c) Reservation of Stock Issuable Upon Conversion. The Borrower
shall at all times reserve and keep available out of its authorized but unissued
shares of Common Stock, solely for the purpose of effecting the conversion of
this Note, such number of its shares of Common Stock as shall from time to time
be sufficient to effect the conversion of this Note; and if at any time the
number of authorized but unissued shares of Common Stock shall not be sufficient
to effect the conversion of this Note, the Borrower will take such corporate
action as may, in the opinion of its counsel, be necessary to increase its
authorized but unissued shares of Common Stock to such number of shares as shall
be sufficient for such purpose.

               (d) Payment of Taxes. The Borrower will pay all transfer taxes or
charges that may be imposed with respect to the issue or delivery of shares of
Company Securities upon conversion of this Note, except for any tax or other
charge imposed in connection with any transfer involved in the issue and
delivery of shares of Company Securities in a name other than that in which this
Note was registered.

               Conversion Price Adjustments. The Conversion Price Adjustments
provided below apply only if a Next Financing has not occurred and (A) the
Borrower delivers a notice of prepayment as described above or (B) the Maturity
Date will occur within thirty days and, in either such case, the Lender
exercises its conversion option as described above.

               (a) Adjustment for Stock Splits and Combinations. If the Borrower
shall at any time or from time to time after the date of this Note (the "Date of
Original Issue") effect a stock split or subdivision of the outstanding Common
Stock, the Conversion Price in effect immediately before that subdivision shall
be proportionately decreased, and, conversely, if the Borrower shall at any time
or from time to time after the Date of Original Issue combine the outstanding
shares of Common Stock into a smaller number of shares, the Conversion Price in
effect immediately before the



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<PAGE>

combination shall be proportionately increased. Any adjustment under this
paragraph shall become effective at the close of business on the date the stock
split, subdivision or combination becomes effective.

               (b) Adjustment for Common Stock Dividends and Distributions. If
the Borrower at any time or from time to time after the Date of Original Issue
issues, or fixes a record date for the determination of holders of Common Stock
entitled to receive a dividend or other distribution payable solely in
additional shares of Common Stock, in each such event the Conversion Price that
is then in effect shall be decreased as of the time of such issuance or, in the
event such record date is fixed, as of the close of business on such record
date, by multiplying the Conversion Price then in effect by a fraction (i) the
numerator of which is the total number of shares of Common Stock issued and
outstanding immediately prior to the time of such issuance or the close of
business on such record date, and (ii) the denominator of which is the sum of
the total number of shares of Common Stock issued and outstanding immediately
prior to the time of such issuance or the close of business on such record date
plus the number of shares of Common Stock issuable in payment of such dividend
or distribution; provided, however, that if such record date is fixed and such
dividend is not fully paid or if such distribution is not fully made on the date
fixed therefor, the Conversion Price shall be recomputed accordingly as of the
close of business on such record date and thereafter the Conversion Price shall
be adjusted pursuant to this paragraph to reflect the actual payment of such
dividend or distribution.

               (c) Adjustments for Other Dividends and Distributions. If the
Borrower at any time or from time to time after the Date of Original Issue
issues, or fixes a record date for the determination of holders of Common Stock
entitled to receive a dividend or other distribution payable in securities of
the Borrower other than shares of Common Stock or other property, in each such
event provision shall be made so that the holder of this Note shall receive upon
conversion hereof, in addition to the number of shares of Common Stock
receivable hereupon, the amount of securities of the Borrower or other property
which such holder would have received had this Note been converted into Common
Stock on the date of such event and had they thereafter, during the period from
the date of such event to and including the conversion date, retained such
securities or other property receivable by them as aforesaid during such period,
subject to all other adjustments called for during such period under this Note
with respect to the rights of the holders of this Note or with respect to such
other securities or other property by their terms. As used herein, the term
"other property" does not include cash.

               (d) Adjustment for Reclassification, Exchange and Substitution.
If at any time or from time to time after the Date of Original Issue, the Common
Stock issuable upon the conversion of this Note is changed into the same or a
different number of shares of any class or series of stock, whether by
recapitalization, reclassification or otherwise (other than a subdivision or
combination of shares or stock dividend or a reorganization, merger,
consolidation or sale of assets provided for elsewhere in this Note), then in
any such event the Lender shall have the right thereafter to convert this Note
into the kind and amount of stock and other securities and property receivable
upon such recapitalization, reclassification or other change by holders of the
number of shares



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of Common Stock into which this Note could have been converted immediately prior
to such recapitalization, reclassification or change, all subject to further
adjustment as provided herein or with respect to such other securities or
property by the terms thereof.

               (e) Reorganizations. If at any time or from time to time after
the Date of Original Issue there is a capital reorganization of the Common Stock
(other than a recapitalization, subdivision, combination, reclassification,
exchange or substitution of shares provided for elsewhere in this Note), as a
part of such capital reorganization provision shall be made so that the Lender
shall thereafter be entitled to receive upon conversion of this Note the number
of shares of stock or other securities or property of the Borrower to which a
holder of the number of shares of Common Stock deliverable upon such conversion
would have been entitled on such capital reorganization, subject to adjustment
in respect of such stock or securities by the terms thereof. In any such case,
appropriate adjustment shall be made in the application of the provisions of
this Note with respect to the rights of the holders after such capital
reorganization to the end that the provisions of this Note (including adjustment
of the Conversion Price then in effect and the number of shares issuable upon
conversion of this Note) shall be applicable after that event and be as nearly
equivalent as practicable.

               (f) Sale of Shares Below Fair Market Value.

                      (i) If at any time or from time to time after the Date of
Original issue, the Borrower issues or sells, or is deemed by the provisions of
clause (iii) of this paragraph (f) to have issued or sold, Additional Shares of
Common Stock (as hereinafter defined), other than a subdivision or combination
of shares of Common Stock or as a dividend or other distribution of Common Stock
as provided for elsewhere in this Note, for an Effective Price (as hereinafter
defined) less than the then effective Fair Market Value, then and in each such
case the then existing Conversion Price shall be reduced as of the close of
business on the date of such issue or sale to a price equal to the product of
dividing (A) (1) the number of shares of Common Stock Equivalents Outstanding
(as hereinafter defined) immediately preceding such issue or sale multiplied by
the then existing applicable Fair Market Value, plus (2) the Aggregate
Consideration Received (as hereinafter defined) by the Borrower for the total
number of Additional Shares of Common Stock so issued or sold, by (B) (1) the
number of shares of Common Stock Equivalents Outstanding immediately preceding
such issue or sale plus (2) the number of Additional Shares of Common Stock so
issued or sold.

                      (ii) For the purpose of making any adjustment required
under this paragraph (f):

                             (A) "Additional Shares of Common Stock" means all
shares of Common Stock issued by the Borrower, whether or not subsequently
reacquired or retired by the Borrower, other than Management Common Stock (as
defined below) or Common Stock issued upon the exercise or conversion of
Convertible Securities outstanding on the Date of Original Issue.



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                             (B) "Aggregate Consideration Received" by the
Borrower for any issue or sale of securities shall (1) to the extent it consists
of cash, be computed at the gross amount of cash received by the Borrower before
deduction of any underwriting or similar commissions, compensation or
concessions paid or allowed by the Borrower in connection with such issue or
sale and without deduction of any expenses payable by the Borrower, (2) to the
extent it consists of property other than cash, be computed at the fair value of
that property as determined in good faith by the Board of Directors of the
Borrower, and (3) if Additional Shares of Common Stock or Convertible Securities
are issued or sold together with other stock or securities or other assets of
the Borrower for a consideration which covers both, be computed as the portion
of the consideration so received that may be reasonably determined in good faith
by the Board of Directors of the Borrower to be allocable to such Additional
Shares of Common Stock or Convertible Securities.

                             (C) "Common Stock Equivalents Outstanding" means
all shares of Common Stock that are outstanding plus all shares of Common Stock
issuable upon conversion of this Note or other Convertible Securities.

                             (D) "Convertible Securities" means stock or other
securities (including options, warrants and other rights) of the Borrower
convertible into shares of Common Stock.

                             (E) "Effective Price" of Additional Shares of
Common Stock means the quotient determined by dividing the total number of
Additional Shares of Common Stock issued or sold, or deemed to have been issued
or sold by the Borrower under this paragraph (f), into the Aggregate
Consideration Received, or deemed to have been received by the Borrower for such
issue under this paragraph (f), for such Additional Shares of Common Stock.

                             (F) "Fair Market Value" of a share of Common Stock
as of a particular date (the "Determination Date") shall mean the average of the
Daily Sales Prices (as defined below) of the Common Stock on the primary
securities exchange or market on which it is listed or traded on the last 20
Business Days prior to the Determination Date. The "Daily Sales Price" shall be
the closing price for bona fide transactions of the Common Stock at the end of
each day or, if no such transaction takes place that day, the average of the
closing bid and asked prices for such day. If the Common Stock was, but is no
longer listed or traded on a primary securities exchange or market, then the
Board of Directors of Motorola, Inc. (or a duly constituted committee thereof)
shall determine in good faith the Fair Market Value of the Common Stock on the
basis of such quotations, appraisals and/or evaluations as it considers
appropriate. As used in this paragraph, a primary securities exchange or market
includes national securities exchanges registered under the Securities Exchange
Act of 1934, as amended, the Nasdaq Stock Market National Market, and any
successor to the foregoing. If the Common Stock is listed or traded on more than
one primary securities exchange or market, the Fair Market Value shall be
determined on the basis of the exchange or market on which the largest volume of
shares of the Common Stock is traded during the period in question.



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<PAGE>

                             (G) "Management Common Stock" means all shares of
Common Stock issued or sold, or deemed to have been issued or sold by the
Borrower under this paragraph (f), to employees, officers, directors,
consultants or advisers of the Borrower or any of its subsidiaries pursuant to
any stock purchase plan, stock option plan, stock bonus plan or other plan or
agreement approved by the Board of Directors of the Borrower; provided that the
total number of shares of Management Common Stock shall not exceed 10,000,000.

                      (iii) For the purpose of making any adjustment to the
Conversion Price required under this paragraph (f), if the Borrower issues or
sells any Convertible Securities and if the Effective Price of the shares of
Common Stock issuable upon conversion of the Convertible Securities is less than
the Conversion Price then in effect, the Borrower shall be deemed to have issued
at the time of the issuance of such Convertible Securities that number of
Additional Shares of Common Stock equal to the maximum number of shares of
Common Stock issuable upon conversion thereof and to have received as
consideration for the issuance of such shares an amount equal to the total
amount of the consideration, if any, received by the Borrower for the issuance
of such Convertible Securities, plus the minimum amounts of consideration, if
any, payable to the Borrower (other than by cancellation of liabilities or
obligations evidenced by such Convertible Securities) upon the conversion
thereof, provided that:

                             (A) if the minimum amounts of such consideration
cannot be ascertained, but are a function of antidilution or similar protective
clauses, the Borrower shall be deemed to have received the minimum amounts of
consideration without reference to such clauses;

                             (B) if the minimum amount of consideration payable
to the Borrower upon the conversion of Convertible Securities is reduced over
time or on the occurrence or non-occurrence of specified events other than by
reason of antidilution adjustments, the Effective Price shall be recalculated
using the figure to which such minimum amount of consideration is reduced; and

                             (C) if the minimum amount of consideration payable
to the Borrower upon the conversion of Convertible Securities is subsequently
increased, the Effective Price shall be again recalculated using the increased
minimum amount of consideration payable to the Borrower upon the conversion of
Convertible Securities.

                             (D) no further adjustment of the Conversion Price,
adjusted or subject to adjustment upon the issuance of such Convertible
Securities, shall be made as a result of the actual issuance of shares of Common
Stock on the conversion of any such Convertible Securities. If the conversion
privilege represented by any such Convertible Securities shall expire without
having been exercised, the Conversion Price adjusted upon the issuance of such
Convertible Securities shall be readjusted to the Conversion Price which would
have been in effect had an adjustment been made on the basis that the only
shares of Common Stock so issued were the shares of Common Stock, if any,
actually issued or sold on the exercise of such rights of conversion of such
Convertible Securities, and such shares of Common Stock, if any, were issued or
sold for



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the consideration received for issuing or selling the Convertible Securities
actually converted, plus the consideration, if any, actually received by the
Borrower (other than by cancellation of liabilities or obligations evidenced by
such Convertible Securities) on the conversion of such Convertible Securities,
provided that such readjustment shall not apply to prior conversions of this
Note.

               (g) Certificate of Adjustment. In each case of an adjustment or
readjustment of any Conversion Price for the number of shares of Common Stock or
other securities issuable upon conversion of this Note, the Borrower, at its own
expense, shall cause its Chief Financial Officer to compute such adjustment or
readjustment in accordance with the provisions hereof and prepare a certificate
showing such adjustment or readjustment, and shall mail such certificate, by
first class mail, postage prepaid, to the Lender at the Lender's address as
provided in the Credit Agreement. The certificate shall set forth such
adjustment or readjustment, showing in detail the facts upon which such
adjustment or readjustment is based. No adjustment in the Conversion Price shall
be required to be made unless it would result in an increase or decrease of at
least one cent, but any adjustments not made because of this sentence shall be
carried forward and taken into account in any subsequent adjustment otherwise
required hereunder.

               (h) Notices of Record Date. Upon (i) the establishment by the
Borrower of a record of the holders of any class of securities for the purpose
of determining the holders thereof who are entitled to receive any dividend or
other distribution, or (ii) any capital reorganization of the Borrower, any
reclassification or recapitalization of the capital stock of the Borrower, any
merger or consolidation of the Borrower with or into any other Borrower, or any
transfer of all or substantially all the assets of the Borrower to any other
person or any voluntary or involuntary dissolution, liquidation or winding up of
the Borrower, the Borrower shall mail to the Lender at least twenty days prior
to the record date specified therein a notice specifying (A) the date on which
any such record is to be taken for the purpose of such dividend or distribution
and a description of such dividend or distribution, (B) the date on which any
such reorganization, reclassification, transfer, consolidation, merger,
dissolution, liquidation or winding up is expected to become effective, and (C)
the date, if any, that is to be fixed as to when the holders of record of Common
Stock (or other securities) shall be entitled to exchange their shares of Common
Stock (or other securities) for securities or other property deliverable upon
such reorganization, reclassification transfer, consolidation, merger,
dissolution, liquidation or winding up.

               (i) No Impairment. The Borrower shall not amend its Certificate
of Incorporation or participate in any reorganization, transfer of assets,
consolidation, merger, dissolution, issue or sale of securities or any other
voluntary action for the purpose of avoiding or seeking to avoid the observance
or performance of any of the terms to be observed or performed hereunder by the
Borrower, but shall at all times in good faith assist in carrying out all such
action as may be reasonably necessary or appropriate in order to protect the
conversion rights of the holder of this Note against dilution or other
impairment as provided herein.



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               Except as permitted by Section 10.06 of the Credit Agreement,
this Note may not be assigned by the Lender to any other Person.

               This Note shall be governed by, and construed in accordance with,
the law of the State of New York.

                                          NEXT LEVEL COMMUNICATIONS, INC.


                                          By:/s/ Next Level Communications, Inc.
                                             -----------------------------------
                                             Title: Senior Vice President



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                                SCHEDULE OF LOANS

               This Note evidences Loans made under the within-described Credit
Agreement to the Borrower, on the dates, in the principal amounts, of the Types,
bearing interest at the rates and having Interest Periods (if applicable) of the
durations set forth below, subject to the payments, Continuations, Conversions
and prepayments of principal set forth below:


<TABLE>
<CAPTION>
              Principal                            Maturity     Amount      Unpaid
    Date        Amount       Type      Interest      Date      Paid or    Principal    Notation
    Made       of Loan     of Loan       Rate      of Loan     Prepaid      Amount     Made by
    ----      ---------    -------     --------    -------     -------    ---------    --------
<S>           <C>          <C>         <C>         <C>         <C>        <C>          <C>



</TABLE>



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