Document:

<PAGE>   1

                                 PROMISSORY NOTE

$1,198,800                                                        June 30, 2000

         FOR VALUE RECEIVED, the undersigned, Portsmith LLC, an Idaho limited
liability company ("Maker"), promises to pay to the order of Mobility
Electronics, Inc., a Delaware corporation ("Payee"), at 7955 East Redfield Road,
Scottsdale, Arizona 85260 (or such other address as Payee may advise Maker in
writing), the principal sum of ONE MILLION ONE HUNDRED NINETY EIGHT THOUSAND
EIGHT HUNDRED AND 00/100 DOLLARS ($1,198,800), in lawful currency of the United
States of America, together with interest accrued thereon (calculated on the
basis of a 365-day year) at a rate of 6% per annum from the date hereof until
this Note is paid in full.

         1. Payments. The principal balance of this Note is due and payable in
full on June 30, 2003. Accrued but unpaid interest on this Note shall be due and
payable on each annual anniversary of the date of this Note, commencing on June
30, 2001.

         2. Optional Prepayment. Maker may at its sole option prepay all or any
part of the principal of this Note, or interest thereon, before maturity without
penalty or premium. All such prepayments shall first be applied to accrued
interest under this Note, and the remaining balance of any such prepayments, if
any, shall be applied to principal of this Note.

         3. Security. This Note is secured by a security interest in the Pledged
Securities (as defined in that certain Pledge Agreement, of even date herewith,
by and between Payee and Maker (the "Pledge Agreement")). Payee shall be
entitled to all benefits of the remedies and security set forth in the Pledge
Agreement.

         4. Events of Default and Remedies. At the option of Payee the entire
principal balance of, together with all accrued and unpaid interest on, this
Note shall at once become due and payable, without further notice or demand,
upon the occurrence at any time of any of the following events of default
("Events of Default"):

              (i) Failure of Maker to make any payment of principal or interest
when due hereunder, and such failure continues for a period of 5 business days
after the receipt by Maker of written notice from Payee of the occurrence of
such failure; or

              (ii) Failure of Maker to perform any covenant, agreement or
condition contained herein or in the Pledge Agreement, except the failure of
Maker to make any payment of principal or interest when due hereunder, and such
failure continues for a period of 10 days after the receipt by Maker of written
notice from Payee of the occurrence of such failure; or

              (iii) Maker shall (a) become insolvent, (b) voluntarily seek,
consent to, acquiesce in the benefit or benefits of any Debtor Relief Law (as
hereinafter defined) or (c) become party to (or be made the subject of) any
proceeding provided by any Debtor Relief Law, other than as a creditor or
claimant, that could suspend or otherwise adversely affect the rights of Payee
granted hereunder (unless in the event such proceeding is involuntary, the
petition instituting the same is dismissed within 90 days of the filing of
same). As used herein, the term

<PAGE>   2

"Debtor Relief Law" means the Bankruptcy Code of the United States of America
and all other applicable liquidation, conservatorship, bankruptcy, moratorium,
rearrangement, receivership, insolvency, reorganization or similar debtor relief
laws from time to time in effect affecting the rights of creditors generally.

         In the event any one or more of the Events of Default specified above
shall have happened, the holder of this Note may (a) enforce its rights, if any,
under this Note and or under the Pledge Agreement and/or (b) proceed to protect
and enforce its rights either by suit in equity and/or by action at law, or by
other appropriate proceedings, whether for the specific performance of any
covenant or agreement contained in this Note or in the Pledge Agreement or in
aid of the exercise of any power or right granted by this Note or the Pledge
Agreement, or to enforce any other legal and equitable right of the holder of
this Note or in the Pledge Agreement.

         5. Recourse. Neither Maker nor any of its officers or members shall
have personal liability for the payment of the principal of, or interest on,
this Note, and Payee's source of satisfaction for the payment of the principal
of and interest on this Note shall be limited to the Pledged Securities.

         6. Waiver. Except as expressly provided herein, Maker, and each surety,
endorser, guarantor and other party ever liable for the payment of any sum of
money payable on this Note, jointly and severally waive demand, presentment,
protest, notice of non-payment, notice of intention to accelerate, notice of
protest and any and all lack of due diligence or delay in collection or the
filing of suit hereon which may occur.

         7. Cumulative Rights. No delay on the part of the holder of this Note
in the exercise of any power or right under this Note shall operate as a waiver
thereof, nor shall a single or partial exercise of any other power or right.
Enforcement by the holder of this Note of any security for the payment hereof
shall not constitute any election by it of remedies so as to preclude the
exercise of any other remedy available to it.

         8. Notices. Any notice or demand given hereunder by the holder hereof
shall be deemed to have been given and received (i) when actually received by
Maker, if delivered in person or by facsimile transmission, or (ii) if mailed,
on the earlier of the date actually received or (whether ever received or not)
three Business Days (as hereinafter defined) after a letter containing such
notice, certified or registered, with postage prepaid, addressed to Maker, is
deposited in the United States mail. The address of Maker is 1111 South Orchard,
Suite 109, Boise, Idaho 83705, or such other address as Maker shall advise the
holder hereof by certified or registered letter by this same procedure.
"Business Day" means every day which is not a Saturday, Sunday or legal holiday.

         9. Successors and Assigns. This Note and all covenants, promises and
agreements contained herein shall be binding upon and inure to the benefit of
the respective legal representatives, personal representatives, devisees, heirs,
successors and assigns of Payee and Maker.

         10. Governing Law. THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF DELAWARE. IN

                                       2
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CASE ANY ONE OR MORE OF THE PROVISIONS CONTAINED IN THIS NOTE SHALL FOR ANY
REASON BE HELD TO BE INVALID, ILLEGAL OR UNENFORCEABLE IN ANY RESPECT, SUCH
INVALIDITY, ILLEGALITY OR UNENFORCEABILITY SHALL NOT AFFECT ANY OTHER PROVISION
HEREOF.

         11. Usury Savings Clause. Any provision in this Note or in any other
document executed in connection herewith, or in any other agreement or
commitment, whether written or oral, express or implied, to the contrary
notwithstanding, Payee shall not in any event be entitled to receive or collect,
nor shall or may amounts received hereunder be credited, so that Payee shall be
paid, as interest, a sum greater than the maximum rate of interest permitted by
applicable law. If any construction of this Note, or any and all other papers,
agreements or commitments, indicates a different right given to Payee to ask
for, demand or receive any larger sum as interest, such is a mistake in
calculation or wording, which this clause shall override and control; it being
the intention of the parties that this Note and all other instruments relating
to this Note shall in all things comply with applicable law, and proper
adjustment shall automatically be made accordingly. In the event Payee ever
receives, collects or applies as interest, any sum in excess of the maximum rate
of interest permitted by applicable law, such excess amount shall be applied to
the reduction of the unpaid principal balance of this Note in the inverse order
of maturity, and if this Note is paid in full, any remaining excess shall be
paid to Maker. In determining whether or not the interest paid or payable, under
any specific contingency, exceeds the maximum rate of interest permitted by
applicable law, Maker and Payee shall, the maximum extent permitted under
applicable law (i) characterize any nonprincipal payment as an expense, fee or
premium rather than as interest, (ii) exclude voluntary prepayments and the
effects thereof, and (iii) "spread" the total amount of interest throughout the
entire term of this Note so that the interest rate is uniform throughout the
entire term hereof.

         12. Attorneys' Fees and Costs. In the event an Event of Default shall
occur, and in the event that thereafter this Note is placed in the hands of an
attorney for collection, or in the event this Note is collected in whole or in
part through legal proceedings of any nature, then and in any such case Maker
promises to pay all costs of collection, including, but not limited to,
reasonable attorneys' fees incurred by the holder hereof on account of such
collection, whether or not suit is filed.

         13. Headings. The headings of the sections of this Note are inserted
for convenience only and shall not be deemed to constitute a part hereof.

         EXECUTED as of the day and year first above written.

                                             PORTSMITH LLC

                                             By: /s/ HOLMES LUNDT
                                                -------------------------------
                                                    Holmes Lundt,
                                                    General Manager

                                       3<PAGE>   1
                                                                    EXHIBIT 4.15

                                   SUPPLEMENT
                              TO NOMINEE AGREEMENT

         This SUPPLEMENT TO NOMINEE AGREEMENT, executed and delivered this 27th
day of July, 2000 (this "Supplement"), by and between CENTEX CORPORATION, a
Nevada corporation ("Centex"), 3333 HOLDING CORPORATION, a Nevada corporation
("Holding"), CENTEX DEVELOPMENT COMPANY, L.P., a Delaware limited partnership
("CDC"), of which 3333 Development Corporation, a Nevada corporation that is a
wholly-owned subsidiary of Holding is the sole general partner, CHASEMELLON
SHAREHOLDER SERVICES L.L.C., a New Jersey limited liability company
("ChaseMellon"), and THE CHASE MANHATTAN BANK, a New York banking association
("Chase");

                                   WITNESSETH:

         WHEREAS, Centex, Holding, CDC and First RepublicBank Dallas, National
Association ("First RepublicBank") entered into a Nominee Agreement as of
November 30, 1987 (the "Nominee Agreement"), pursuant to which Centex effected a
distribution to First RepublicBank, for the benefit of those persons who are
from time to time holders of the common stock of Centex, of certain securities,
as more fully described therein; and

         WHEREAS, in accordance with the provisions of the Nominee Agreement,
Centex has appointed Chase as successor Nominee for all purposes under the
Nominee Agreement; and

         WHEREAS, in accordance with the provisions of the Nominee Agreement,
Centex has appointed ChaseMellon to act as successor Transfer Agent for all
purposes under the Nominee Agreement; and

         WHEREAS, the holders of a majority of the common stock of Centex voted
in July 1995 to approve the extension of the termination date of the Nominee
Agreement from November 30, 1997 to November 30, 2007;

         WHEREAS, Centex, Holding, CDC and ChaseMellon entered into a Service
Agreement for Transfer Agent Services in connection with the securities of
Centex and those securities traded in tandem therewith as of May 1, 1999 (the
"Service Agreement"); and

         WHEREAS, the parties hereto are executing this Supplement to formally
recognize the intervening appointments and other action in order to clarify the
rights and obligations of the parties hereto and the respective parties to the
Nominee Agreement;

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants set forth herein and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Centex, Holding, CDC,
ChaseMellon and Chase do hereby agree as follows:

         1. Definitions.

                  (a) Except as expressly defined otherwise in this Supplement,
         capitalized terms used and otherwise defined herein shall have the
         respective meanings assigned to them in the Nominee Agreement.

                  (b) The following defined terms in the Nominee Agreement are
         hereby amended:

         "Corporate Office" means (a) as to the Nominee, the corporate office of
         the Nominee, located at c/o ChaseMellon Shareholder Services L.L.C.,
         2323 Bryan Street, Suite 2300, Dallas, Texas 75201, and (b) as to the
         Transfer Agent, the principal corporate office of the Transfer Agent,
         which at the date hereof is located at 85 Challenger Road, Ridgefield
         Park, New Jersey 07660.

         2. Appointment of Successor Nominee. Centex has previously appointed
Chase as successor Nominee under the Nominee Agreement pursuant to Section 8.1
of the Nominee Agreement. Chase hereby accepts such appointment for all
purposes. Each reference in the Nominee Agreement to the Nominee or to the Bank
(each as defined in the Nominee Agreement), in its capacity as Nominee, shall be
deemed after the effective date of such appointment and acceptance to be a
reference to Chase, unless the context otherwise plainly requires.

                                     4.15-1
<PAGE>   2

         3. Appointment of Successor Transfer Agent. Centex has previously
appointed ChaseMellon as successor Transfer Agent under the Nominee Agreement
pursuant to Section 8.1 of the Nominee Agreement. ChaseMellon hereby accepts
such appointment for all purposes. Each reference in the Nominee Agreement to
the Transfer Agent or the Bank (each as defined in the Nominee Agreement), in
its capacity as Transfer Agent, shall be deemed after the effective date of such
appointment and acceptance to be a reference to ChaseMellon, unless the context
otherwise plainly requires.

         4. Extension of Detachment Date. By reason of the action of the holders
of common stock of Centex in July 1995, each reference in the Nominee Agreement
to "November 30, 1997", including such references in Section 3.2(b) (legend) and
Section 8.3(a) of the Nominee Agreement, shall be replaced by a reference to
"November 30, 2007".

         5. Addresses for Notices. The addresses listed in Section 9.4(a) of the
Nominee Agreement are hereby replaced with the following:

                  To Centex or Holding, at

                  2728 North Harwood Street
                  Dallas, Texas  75201
                  Attention:  President

                  To CDC, at:

                  Centex Development Company, L.P.
                  c/o 3333 Development Corporation
                  2728 North Harwood Street
                  Dallas, Texas  75201
                  Attention:  President

                  To the Nominee, at:

                  c/o ChaseMellon Shareholder Services L.L.C.
                  2323 Bryan Street, Suite 2300
                  Dallas, Texas 75201
                  Attention: Centex Relationship Manager

                  To the Transfer Agent, at:

                  ChaseMellon Shareholder Services L.L.C.
                  2323 Bryan Street, Suite 2300
                  Dallas, Texas 75201
                  Attention: Centex Relationship Manager

         6. Fees. Provided Centex pays all requested fees under the Service
Agreement, Centex, Holding or CDC will not be required to pay any additional
fees under the Service Agreement or the Nominee Agreement to ChaseMellon or
Chase.

         7. Other Changes. The following terms of the Nominee Agreement are
hereby amended:

                  (a) Section 5.2(b) of the Nominee Agreement is hereby deleted
         in its entirety;

                  (b) Section 6.5 of the Nominee Agreement is hereby amended by
         adding the following text at the end thereof:

                                     4.15-2
<PAGE>   3

         "In no event will the Nominee or the Transfer Agent be liable for
special, indirect, incidental or consequential loss or damages of any kind
whatsoever (including but not limited to lost profits), even if the Nominee or
the Transfer Agent has been advised of the possibility of such damages. Any
liability of the Nominee or the Transfer Agent will be limited to the amount of
fees paid by Centex under the Services Agreement.";

                  (c) Section 8.1(g) of the Nominee Agreement is hereby amended
         by deleting all references to the Transfer Agent therein.

         Except as otherwise expressly contemplated or amended by this
Supplement, the terms and provisions of the Nominee Agreement shall continue in
full force and effect.

         IN WITNESS WHEREOF, Centex, Holding, CDC, ChaseMellon and Chase have
duly executed this Supplement as of the day and year first set forth above.

                                   CENTEX CORPORATION

                                   By: /s/ Raymond G. Smerge
                                       -----------------------------------------
                                       Name:  Raymond G. Smerge
                                       Title: Executive Vice President

                                   3333 HOLDING CORPORATION

                                   By: /s/ Stephen M. Weinberg
                                       -----------------------------------------
                                       Name:  Stephen M. Weinberg
                                       Title: President

                                   CENTEX DEVELOPMENT COMPANY, L.P.

                                   By: 3333 DEVELOPMENT CORPORATION,
                                          General Partner

                                            By: /s/ Stephen M. Weinberg
                                                --------------------------------
                                                Name:  Stephen M. Weinberg
                                                Title: President

                                   CHASEMELLON SHAREHOLDER SERVICES L.L.C.

                                   By: /s/ David M. Cary
                                       -----------------------------------------
                                       Name:  David M. Cary
                                       Title: Assistant Vice President

                                   THE CHASE MANHATTAN BANK

                                   By: /s/ Eric R. Leason
                                       -----------------------------------------
                                       Name:  Eric R. Leason
                                       Title: Vice President

                                     4.15-3

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