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AN610
Using the 24LC21 Dual Mode Serial EEPROM
Author: Bruce Negley Memory Products Division
TRANSMIT ONLY MODE (DDC1)
The 24LC21 will always power-up in the transmit only mode. In this mode, the 24LC21 will output one bit of data at the SDA pin for every rising edge on the VCLK pin. The data will be transmitted in 8-bit words, with each word followed by a 9th null bit. This null bit will always be high. A timing diagram for transmit only mode is shown in Figure 1. As long as VCLK is present and no falling edges on SCL are received, the 24LC21 will repeatedly cycle through the entire memory array.
INTRODUCTION
The Microchip Technology Inc. 24LC21 is a 1K-bit (128 x 8) dual mode serial EEPROM that was developed primarily for use in computer monitors. This part was developed with inputs from several computer monitor manufacturers, in accordance with the VESA(R) (Video Electronics Standards Association) monitor committee. This committee has developed a serial communication protocol called Data Display Channel (DDCTM) which was created to eliminate the need to change dip switches when configuring a new system or adding a new monitor or video card. The 24LC21 device is used in the monitor to store and transmit the EDID (extended display ID) table which contains all set-up parameters needed by the video card to operate with a particular monitor. With this system, the user can now plug any compatible monitor into any compatible graphics board and the graphics board will automatically know what type of monitor is being used and configure itself accordingly. This automatic configuration is the cornerstone for Microsoft(R)'s `Plug and Play' capability being built into the new `Windows 95TM' release.
PACKAGE TYPE
NC NC NC VSS
1 2 3 4
5 6 7 8
VCC VCLK SCL SDA
DEVICE OPERATION
The 24LC21 can operate in two modes of operation. These two modes of operation are the transmit only mode and bi-directional mode. Upon power-up, the device will always be in the transmit-only mode. Transmit only mode is also referred to as DDC1 mode. The transmit only mode only allows the video card to read the contents of the 24LC21 in a sequential manner, one bit at a time. Writing to the device is not possible in transmit only mode. The device will automatically transition to the bi-directional mode whenever a falling edge is seen on the SCL pin. Bi-directional mode is also referred to as DDC2 mode, and is implemented as the standard I2CTM protocol. This allows a controller to read and write specific addresses in the device like a standard I2C Serial EEPROM device. Once the device has transitioned to the bi-directional mode, there is no way to return to transmit only mode other than to reset (power-down) the device.
Upon power-up, the device will not output valid data until it has been initialized. This initialization procedure (Figure 1) data will not be available until after the first 9 clocks are sent to the device. The exact memory location that the 24LC21 begins to transmit data is unknown at power-up, and the initialization procedure only initializes the device, not the starting address or bit location. In order to for a controller to determine what address is being read, a `framing' or `syncing' procedure must be executed by the video card.
(c) 1995 Microchip Technology Inc.
DS00610A-page 1
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AN610
A framing procedure involves looking for the header portion of the EDID table which is a byte of 00H followed by 6 bytes of FFH and another byte of 00H. A framing routine would continue to clock data from the 24LC21 until this unique header has been found. At this point, the current location in the EDID table has been determined and the controller has now synchronized itself with the device. Care must be taken while using the device in the transmit only mode to prevent noise on the SCL pin, as a falling edge seen on this pin will immediately send the part into the bi-directional mode. In a DDC1-only monitor, SCL is not connected to the VGA connector, but must still be terminated to VCC through a pullup resistor.
FIGURE 1:
TRANSMIT ONLY MODE
SCL
Bit8 (MSB)
SCL must remain high for transmit-only mode
SDA VCLK
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1 (LSB)
Null Bit
Bit8
Bit7
FIGURE 2:
DEVICE INITIALIZATION FOR TRANSMIT-ONLY MODE
VCC SCL
SDA at high impedance for 9 clock cycles Valid data begins on 10th clock
Bit8 Bit7 Bit6 Bit5 Bit4
SDA VCLK 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
DS00610A-page 2
(c) 1995 Microchip Technology Inc.
AN610
BI-DIRECTIONAL MODE (DDC2)
Bi-directional mode is essentially the standard I C protocol and allows the controller to read and write to the device. The 24LC21 supports byte and page writes and byte and sequential reads in the bi-directional mode. This mode will be used primarily before the monitor leaves the factory to load the EDID table into the device, but it also provides a means of updating the table if necessary. It is also used for faster (up to 100 kHz) data transmission, or transmission of only specific requested data in a DDC2 system. (The I2C protocol allows the host to request data from a specific portion of the EDID table rather than waiting for the entire table.) When writing to the device, the VCLK pin must be held high while the write command is being loaded or the write will be aborted and no data will be written. Note that this is the opposite of the 24LC01B, where the WP pin must be held low for the device to be written.
2
EDID TABLE
The EDID table is the Extended Display ID table, specified by VESA, that will be stored in the 24LC21 and contains information about what type of display it is and the capabilities of the display. The basic EDID table consists of 128 bytes of data. A breakdown of the table is shown below in Table 1. A complete description of the table can be found in the VESA DDC Specification.
TABLE 1:
Bytes 8 10 2 15 19 72 1 1
EDID TABLE DESCRIPTION
Description Header Vendor/Product Description EDID Version/Revision Basic Display Parameters/Features Established/Standard Timings Detailed Timing Descriptions (18 bytes each) Extension Flag Checksum
(c) 1995 Microchip Technology Inc.
DS00610A-page 3
AN610
USING THE 24LC21 IN A SYSTEM
In order to use the 24LC21 in a monitor system, it must be programmed with a proper EDID table and then properly connected to the signals coming from the video controller card. The VESA committee has specified that the connections for DDC transmission can be part of the standard 15-pin VGA connector. A table of pinouts for this connector are shown in Table 2. Signals that pertain to the use of the 24LC21 are highlighted. Programming of the 24LC21 can be accomplished via Microchip Technology's SEEVAL programming and evaluation system or by any final test system at the customer site which can communicate over the I2C bus.
TABLE 2:
Pin 1 2 3 4 5 6 7 8 9 10 11 12
VGA CONNECTOR DESCRIPTION
Standard VGA DDC1 Host Red Video Green Video Blue Video Monitor ID Bit2 Return Red Video Return Green Video Return Blue Video Return +5V Supply (optional) Sync Return Monitor ID Bit0 Data from Display (SDA) DDC2 Host Red Video Green Video Blue Video Monitor ID Bit2 Return Red Video Return Green Video Return Blue Video Return +5V Supply (optional) Sync Return Monitor ID Bit0 Bi-directional Data (SDA) DDC1.2 Display Red Video Green Video Blue Video Return Return Red Video Return Green Video Return Blue Video Return +5V Supply (optional) Sync Return Optional Bi-directional Data (SDA) Horizontal Sync Vertical Sync (VCLK) Data Clock (SCL)
Red Video Green Video Blue Video Monitor ID Bit2 Test (Ground) Red Video Return Green Video Return Blue Video Return No Connection Sync Return Monitor ID Bit0 Monitor ID Bit1
13 14 15
Horizontal Sync Vertical Sync Monitor ID Bit3
Horizontal Sync Vertical Sync (VCLK) Monitor ID Bit3
Horizontal Sync Vertical Sync Data Clock (SCL)
DS00610A-page 4
(c) 1995 Microchip Technology Inc.
AN610
SYSTEM CONFIGURATION
A typical system configuration is shown below. The DDC specification states that a 47 k pull-up resistor is required on the SDA line at the monitor end. It also states that a 15K pullup resistor is needed on both the SCL and SDA lines at the video controller end.
FIGURE 3:
USE OF 24LC21 IN VIDEO SYSTEM
Video Monitor System Video Controller Card 15K VGA Controller
VCC Pin 14 (VSYNC) 15K Pin 15 (SCL) Pin 12 (SDA) VCC 47K* 47K Monitor Controller Card
VCC SDA SCL 6
* 47 k resistor is recommended on SCL line at the monitor end although it is not required by the VESA specification.
8
7
VCLK
5
CRT 24LC21 1
2
3
4
GND
(c) 1995 Microchip Technology Inc.
DS00610A-page 5
AN610
POTENTIAL PROBLEMS CAUSED BY NOISE IN A VIDEO SYSTEM
Because the typical application for the 24LC21 is in a computer monitor where electronic noise is prevalent, some precautions may need to be made in order for this device (or any other CMOS device) to work properly. The diagram below (Figure 4) shows a filter circuit that can be used to reduce the amount of noise seen by the device on the SCL and VCLK pins.
FIGURE 4:
RECOMMENDED FILTER CIRCUIT FOR MONITOR APPLICATIONS
VCC
CBP
RS VCLK RPD CR1 CS 24LC21 RPU RS SCL CS SCL VSYNC
CBP = 1 F RS = 100 - 300 RPD = 4.7 k CS = 100 - 1000 pF RPU = 47 k
CBP RPD
Bypass capacitor Can be as a termination resistor on VGA cable. Also will discharge the series capacitor going to the MCU and horizontal/Vertical processor.
CS and RS on VCLK Acts as low pass filter to clean-up noise on VSYNC line CS and RS on SCL Acts as low pass filter to clean-up noise and dampen power transient spikes that may cause accidental mode switching from DDC1 to DDC2 RPU on SCL Keeps SCL pulled high, although a high enough value is used that the host will not power the 24LC21. Lets the 24LC21 reset when the monitor power is turned off Eliminates undershoot on VSYNC
CR1
DS00610A-page 6
(c) 1995 Microchip Technology Inc.
AN610
NOTES:
(c) 1995 Microchip Technology Inc.
DS00610A-page 7
Note the following details of the code protection feature on PICmicro(R) MCUs. * * * The PICmicro family meets the specifications contained in the Microchip Data Sheet. Microchip believes that its family of PICmicro microcontrollers is one of the most secure products of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the PICmicro microcontroller in a manner outside the operating specifications contained in the data sheet. The person doing so may be engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as "unbreakable". Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our product.
* * *
If you have any further questions about this matter, please contact the local sales office nearest to you.
Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip's products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights.
Trademarks The Microchip name and logo, the Microchip logo, FilterLab, KEELOQ, microID, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. (c) 2002, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company's quality system processes and procedures are QS-9000 compliant for its PICmicro(R) 8-bit MCUs, KEELOQ(R) code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip's quality system for the design and manufacture of development systems is ISO 9001 certified.
2002 Microchip Technology Inc.
M
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01/18/02
2002 Microchip Technology Inc.

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