Document:

<PAGE>

                                                                 EXHIBIT 10.14

                      Addendum to Co-Development Agreement
                      ------------------------------------

     This Addendum to the Co-Development Agreement (the "Addendum Agreement")
is dated and effective as of June 21, 1999 and is by and between Mitsubishi
Electric Corporation acting through its Communication Systems Business Division,
a corporation registered and organized under the laws of Japan, having its
office at 8-1-1, Tsukaguchi-Honmachi, Amagasaki City, Hyogo 661, Japan.
("Mitsubishi"), and Centillium Technology Corporation, a corporation organized
under the laws of the state of California. U.S.A., and having an address at
46531 Fremont Blvd, Fremont, California. 94538, U.S.A. ("Centillium").

The purpose of this Addendum Agreement is to be included as an addendum to the
Co-Development Agreement Between Centillium Technology Corporation and
Mitsubishi Electric Corporation signed on October 15th, 1997 (the "Co-
Development Agreement").

1. This Addendum Agreement reduces to writing in whole, all the verbal
   agreements made between both parties in connection with development and
   support of G.992.2 Annex C.

2. Centillium agrees to make available to Mitsubishi, the CopperLite Annex C
   product at prices to be agreed upon between both parties. Centillium's Annex
   C products for Central office are known by system part number of CT-
   L40DC04&CT-L40AC04 and its Annex C CPE products are known by system part
   number of CT-L40DT01 & CTL40AT01. Each system part number is comprised of an
   analog device, digital devices and two software components for Annex C. Both
   these software components are required for operations of annex C. A detail
   description is shown in table below.

<TABLE>
---------------------------------------------------------------------------
<S>                                                  <C>
Digital CPE Rev AC-4 ports                           CT-L41DT01-IL-AC
---------------------------------------------------------------------------
Analog CO Rev BB -4 ports                            CT-L41AC04-1M-BB
---------------------------------------------------------------------------
Annex A - Software                                         CT-L21FC04
---------------------------------------------------------------------------
Annex C - Software                                         CT-L41FC04
---------------------------------------------------------------------------
(C) CPE
---------------------------------------------------------------------------

---------------------------------------------------------------------------
Digital CO Rev AC -4ports                            CT-L41DC04-IG-AC
---------------------------------------------------------------------------
Analog CPE RevBB -4ports                             CT-L41AT01-IT-BB
---------------------------------------------------------------------------
Annex A - Software                                         CT-L21FT01
---------------------------------------------------------------------------
Annex C - Software                                         CT-L41FT01
---------------------------------------------------------------------------
</TABLE>
<PAGE>

3.  The expected availability of Annex C solution is as shown in table below:

    ----------------------------------------------------------------------------
    Item  Delivery Date  Deliverable               Remarks
    ----------------------------------------------------------------------------
    1     Upon signing   Data related to           Performance & Data Sheet
                         Chip Set (Rls.5)          (Annex A data Sheet.
                                                   Differences for Annex C)
    ----------------------------------------------------------------------------
    2     July/15        Firmware for Annex C
                         (Rls.5)
    ----------------------------------------------------------------------------
    3     TBD            E/S(CO,CPE) 10 sets/each  Mitsubishi requests delivery
                                                   on July 15.
    ----------------------------------------------------------------------------
    4     Sep/1          Rls 6 Firmware & Design
                         Reference
    ----------------------------------------------------------------------------

    Centillium shall make its best effort to meet the above schedule regarding
    availability of the products subject to G.992.Annex C, including completion
    of this development.
4.  For the services rendered and to partially cover the expenses incurred by
    Centillium in development of Annex C solution. Mitsubishi agrees to pay
    Centillium the sum of 200,000 U.S. Dollars as follows:
    1)  Upon signature of this addendum by both parties
           US$100,000
    2)  At the time of Centillium's submission of Rls 6 Firmware & Design
        Reference
           US$100,000
[_] This development fee shall cover the above deliverables item 1),2),3),4) and
     updated software for release six and onward.
5.  All other provisions of the aforementioned Co-Development is hereby
    incorporated into this agreement.
6.  This Addendum together with the Co-Development Between Centillium Technology
    Corporation and Mitsubishi Electric Corporation constitutes the
    understanding and agreement between the parties with regard to the subjects
    hereof and thereof. The full and entire understanding for sales and purchase
    of Annex A&C products shall be included in the business agreement for sales
    and purchase.

Mitsubishi Electric Corporation             Centillium Technology Corporation

/s/ Teruhiko Moriyama                       /s/ Faraj Aalaei
-------------------------------------       ------------------------------------
Name:  Teruhiko Moriyama                     Name:  Faraj Aalaei
Title: General Manager                       Title: Vice President
Date:  8-24-99                               Date:  8-17-99<PAGE>

                                                                 EXHIBIT 10.15

                         COOPERATION AGREEMENT BETWEEN
                       CENTILLIUM TECHNOLOGY CORPORATION
                                      AND
                        MITSUBISHI ELECTRIC CORPORATION

     This Cooperation Agreement (this "Agreement") is effective as of August 25,
1997 (the "Effective Date") by and between Centillium Technology Corporation, a
California corporation, having its principal place of business at 46824 Lakeview
Blvd., Fremont, Ca, 94538, United States of America ("Centillium"), and
Mitsubishi Electric Corporation, Communication Systems Business Division, a
Japan corporation, having its principal place of business at 8-1-1, Tsukaguchi-
Honmachi, Amagasaki City, Hyogo 661, Japan, ("MITSUBISHI ELECTRIC").

                                 - WITNESSETH -

     WHEREAS, Centillium has expertise in the area of telecommunications
integrated circuits, including those which utilize multilevel modulation
techniques to enable high speed communications over copper, also known as
Digital Subscriber Line (xDSL) Technology; and

     WHEREAS, MITSUBISHI ELECTRIC has expertise in the area of
telecommunications systems and associated technologies; and

     WHEREAS, the parties hereto believe that each will derive benefits from a
business relationship in which they will work cooperatively to define an
integrated circuit solution for the copper interface portion of transmission
systems (the "Interface Devices"); and

     WHEREAS, Centillium will be in a position to provide MITSUBISHI ELECTRIC
with early models, system support, and loop environment engineering support in
exchange for MITSUBISHI ELECTRIC's commitment to use Centillium as its preferred
vendor of the Interface Devices for its products; and

     WHEREAS, the parties hereto wish to set forth in this Agreement the
guidelines of their cooperation.

     NOW, THEREFORE, in consideration of the covenants herein contained, the
parties hereto agree as follows:

1.   Overall Objective of Cooperation

     (a) The objective of this cooperation is for both companies to work
together in developing the proper "Interface Device" technologies and systems
that can significantly develop the broadband telecommunications market. The
first phase of this activity includes product definition and joint marketing to
telco customers.

     (b) Each of the parties hereto will act in a commercially reasonable
fashion to provide the resources necessary to accomplish the objectives set
forth in this Agreement.
<PAGE>

Each of the parties hereto acknowledge and agrees that there are substantial
technical, financial and commercial risks associated with a development activity
of this type and, therefore, that neither of the parties hereto can guarantee
that any of the development activities described in this Agreement or any
subsequent activities to be described in any future agreements will be completed
successfully.

     (c) Both parties agree that the future phases of this Agreement, including
those described in section 2 below, may include a product development phase, and
production phase. Both parties will work together to in good faith to develop a
detailed agreement covering the relationship agreement between the two companies
for future phases.

2.   Scope of Cooperation

The parties' cooperation will be conducted in phases:

     (a) Phase 1: Product Definition Phase

          (i)       The parties hereto will jointly approve specifications for
               an integrated circuit solution for the Interface Devices.

          (ii)      Centillium provide assistance to MITSUBISHI ELECTRIC in
               marketing their DSL solution which utilizes Centillium's
               Interface Devices to telcos.

     (b)  Phase 2: Prototyping Phase

          (i)       MITSUBISHI ELECTRIC will evaluate Centillium's first
               prototype of the Interface Devices. Centillium will provide
               evaluation boards to the MITSUBISHI ELECTRIC for this evaluation.
               MITSUBISHI ELECTRIC will provide a written report on the
               evaluation. MITSUBISHI ELECTRIC agrees to provide additional
               evaluation reports throughout the term of this Agreement as
               subsequent iterations of the Interface Devices are prototyped and
               forwarded to MITSUBISHI ELECTRIC for evaluation. Prototypes of
               second and/or future iterations of the Interface Devices will be
               provided free of charge unless they are custom iterations for
               MITSUBISHI ELECTRIC in which case Centillium may charge a fee to
               be mutually agreed upon by the parties hereto.

     (c)       Phase 3: Pre-production and Production Phase

          (i)            MITSUBISHI ELECTRIC will supply written acknowledgment
                   that the Interface Devices meet all the specifications listed
                   in the Product Specification agreed to by the parties. Upon
                   acceptance of the Interface Devices and upon acceptance of
                   Centillium's pricing, MITSUBISHI ELECTRIC will provide
                   Centillium with a requirements forecast of its delivery
                   requirements covering the next six month period for the
                   Interface Devices to be purchased from Centillium.
<PAGE>

          (ii)           MITSUBISHI ELECTRIC will also assist Centillium in
                   establishing standards utilizing the its modulation schemes
                   and will support Centillium's documentation efforts for the
                   Interface Devices by reviewing and providing feedback to
                   Centillium. If Centillium produces devices based upon such
                   standards, Centillium will provide Mitsubishi Electric with
                   access to such devices (e.g., alpha and beta test site) prior
                   to the general commercial release of such devices.

          (iii)          MITSUBISHI ELECTRIC and Centillium agree that
                  reasonable standard commercial terms and conditions will apply
                  to the sale of all devices to MITSUBISHI ELECTRIC.

3.   Technology Ownership and Licenses

     (a) Nothing herein will be constructed as an express or implied license
(under patents or otherwise) of any kind.

     (b) Centillium will have exclusive right to design and market Interface
Devices and any improvements or modifications thereto resulting from the
activities of this Agreement.

     (c) Any information disclosed by MITSUBISHI ELECTRIC to Centillium under
this Agreement with respect to direct support of the Interface Devices (i.e.,
                                                                        ----
with respect to characterization and performance of the Interface Devices) may
be freely used by Centillium for the purpose of implementation of design and
implementation of manufacturing and supporting Interface Devices sold to
Centillium's customers, and other substantially similar activities; the
restrictions with regard to Confidential Information set forth in Section 4
hereof will not apply to the specific information set forth above but will apply
to all other MITSUBISHI ELECTRIC information.

4.   Confidential Information and Non-Disclosure

     Any technical information and other business information disclosed
hereunder by either party hereto to the other will be held in strict confidence
by the receiving party from the date of disclosure until three years after the
date this agreement expires or terminates, using the same degree of care as the
receiving party uses for its own information of a similar kind, and will not be
transferred or divulged to any third-party or any employee who does not have the
requisite need to know.

a)   For purposes of interpreting this Agreement, employees of MITSUBISHI
     ELECTRIC affiliates, divisions or subsidiaries do not have the requisite
     need to know to obtain access to Centillium Confidential Information
     disclosed to MITSUBISHI ELECTRIC hereunder. Such confidentiality obligation
     will not apply to portions of such technical information and other business
     information, if any, (a) which were previously known to the other party
     hereto free of any confidentiality obligation, (b) which are or become
     known to the public, provided that such public knowledge is not
     attributable to a breach of this Agreement by the other party hereto, (c)
     which the furnishing party explicitly agrees in writing need not be kept
<PAGE>

     confidential (either in this Agreement or separately), (d) which are
     received by the other party hereto rightfully from a third-party without a
     restriction on disclosure.

b)   Further, Centillium agrees not to disclose to any other party any of the
     specifics of MITSUBISHI ELECTRIC's system testing and results that might
     specifically indicate a problem related to such system. However, if the
     Interface Devices are modified by Centillium to remedy a system problem,
     Centillium may disclose such test information as may be necessary to
     support such modified Interface Devices, to the extent that such disclosure
     coincides with the availability of the modified Interface Devices.

5.   Term

     This Agreement will expire four (4) years after the Effective Date unless
extended in writing by mutual parties.

6.   Governing Law.

     The construction, interpretation and performance of this Agreement will be
governed by the laws of the State of California without reference to its
conflict of laws and principals. The United Nations Convention on Contracts for
the International Sale of Goods will not apply to the sale of Interface Devices
hereunder.

7.   Entire Agreement

     This Agreement sets forth the entire agreement and understanding between
the parties hereto with respect to its subject matter. It merges all discussions
between them and voids and replaces each and every other agreement or
understanding which may heretofore have existed between Centillium and
MITSUBISHI ELECTRIC regarding such subject matter. Notwithstanding the
foregoing, the Nondisclosure Agreement between the parties hereto, dated June,
1997, as amended, will be deemed incorporated herein by reference, provided that
each party hereto will be permitted to use any information disclosed thereunder
to the extent permitted in this Agreement.

8.   Publicity

     Neither party hereto will disclose the existence or content of this
Agreement without the other's prior written consent, which will not be
unreasonably withheld. Each party will submit to the other all proposed
publicity material relating to the disclosure of this Agreement or the
relationship of the parties hereto.

9.   Resolution of Disputes.

     All disputes, controversies or differences which may arise between the
parties hereto, out of or in relation to or in connection with this Agreement,
whether during or after its term, shall be finally resolved by arbitration in
accordance with the Rules of Conciliation and Arbitration of the International
Chamber of Commerce. The arbitration shall be held (a) in Tokyo, Japan in case
of an arbitration claim filed by Centillium and (b) in California, USA in case
of an arbitration claim filed by Melco. The arbitration panel shall consist of
three (3) arbitrators appointed in accordance with the said Rules. All
arbitration proceedings shall be conducted in English language.
<PAGE>

     IN WITNESS WHEREOF, each of CENTILLIUM TECHNOLOGY CORPORATION and
MITSUBISHI ELECTRIC Corporation have executed this Agreement, in duplicate
originals, by their respective officers hereunto duly authorize, the day and
year first above written.

CENTILLIUM TECHNOLOGY CORPORATION                 Communication
---------------------------------                 -------------
                                                  Business Division of
                                                  --------------------
                                                  MITSUBISHI ELECTRIC
                                                  --------------------
                                                  Corporation
                                                  -----------

By         /s/ Kamran Elahian             By       /s/ Teruhiko Moriyama
  ---------------------------------         ---------------------------------

Name      Kamaran Elahian                 Name       Teruhiko Moriyama
    -------------------------------           -------------------------------
                                              General Manager Telecom Network
Title    Chairman CEO                   Title     Systems Business Center
     ------------------------------           -------------------------------

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