label: PCB design,Printed-circuit-board
Abstract: There are two ways to identify a printed circuit board (PCB). The 'minimalist' approach is based on devices using ROM technology. The 'nameplate' approach requires user-programmable NV memory to store the PnP-related information. The most obvious way to implement Plug-and-Play capabilities is by means of a memory chip that holds the required information and does not lose it if power is switched off. 1-Wire chips for PCB identification are available in ROM, EEPROM, and EPROM (One-Time-Programmable) technology.
Introduction
Anyone who has ever tried to add a modem- or network-card to a PC knows that such an undertaking may produce unwanted results and can take more time than expected. To achieve cost savings from high volume production, the add-in board is designed to be logically compatible to a large variety of motherboards from various vendors. This flexibility makes it necessary to configure the board for its operating environment before it is ready to function. To perform the configuration one needs two pieces of information: sufficient knowledge of the system that the board is expected to reside in and knowledge of the board itself. Once the suitable settings are determined one needs to set jumpers or flip switches on the board. This completes the tricky part of the hardware installation.
As long as such upgrades or changes are made routinely and by trained personnel, they are time-efficient and do not involve much risk. Every time one has to deal with an unfamiliar product, a new learning cycle begins with studying the associated documentation (which may not be well-written) and a phase of trial and error, which may result in damage to the product or the system it is supposed to work in. In 1994, the concept of "Plug-and-Play" (PnP) was introduced to get out of this awkward situation, particularly with PCs. In order to function, each PnP device must have all of the following capabilities:
It must be uniquely identified.
It must state the services it provides and the resources it requires.
It must identify the software driver that supports it.
It must allow software to configure it.
Although these requirements were originally specified for PC products, all electronic systems that consist of more than one device (or circuit board) do benefit from Plug and Play capabilities.
The most obvious way to implement PnP capabilities is by means of a memory chip that holds the required information and does not lose it if power is switched off. In some cases, suitable memory is already on the board (e.g., inside a microcontroller or as a byte-wide memory chip or serial EEPROM). In other cases or if the entire memory is already assigned to other functions, an additional memory chip is needed. Due to their smaller physical size and minimal additional I/O requirements, serial memories are preferred over byte-wide memories. Of all serial memories, devices with 1-Wire interface are easiest to interface because they require only a single data line plus ground reference. Most 1-Wire devices do not even have a VCC pin; they draw the energy for operation right from the data line. All 1-Wire devices have a unique registration number, which allows for traceability and performs as an address if multiple 1-Wire devices (not just memories) are connected in parallel to form a 1-Wire net. This feature is not found with other serial devices. Parts of the registration number can be customized, which turns a 1-Wire device into a custom chip that authenticates the board and makes it impractical to clone.
Spare memory bytes not used for PnP functions are available to enable/disable some of the resources of the circuit board, opening the door to license management rather than changing hardware for upgrades. Depending on storage capacity and chip technology, the memory could hold manufacturing data for quality control, warranty information, and repair history. Having such information readily accessible reduces the time to repair.
Device Technologies
1-Wire chips for printed circuit board identification need to maintain stored data independent of any power supply. This limits the choice to devices that are manufactured in nonvolatile (NV) technologies such as ROM, EEPROM, and EPROM (Table 1). The lack of a window in the plastic package or the mounting technique of chip scale packages with the active side towards the circuit board converts EPROM chips to One-Time-Programmable (OTP) memories.
Circuit Board Identification Overview
As indicated in column APPL. TYPE of Table 1, there are two ways to identify a printed circuit board. The minimalist approach is based on devices using ROM technology. The nameplate approach requires user-programmable NV memory to store the PnP-related information. Table 2 shows the identification alternatives of both approaches. Reference numbers beginning with 1 indicate the minimalist approach. For the nameplate approach there are devices in EEPROM technology (reference numbers beginning with 2) and OTP EPROM (numbers beginning with 3). Within each of these groups, the reference letter A stands for generic, off-the-shelf parts. The letter B is used for parts that have a customized ROM. The ROM customization is explained in section Minimalist Approach. The letter C indicates protected EEPROM (2C). For more details on this see section Nameplate Approach.
With customized devices, the cloning protection is achieved by restricted supply. Clone manufacturers will not be able to buy the customized parts. To achieve protection with generic devices (excluding the DS2432 and DS28E01) it is necessary to manipulate the data, e.g., by means of encryption.
Common encryption methods (e.g., cipher block chaining) rely on a key ("secret") and an initialization vector. Embedding the unique ROM registration number of the memory chip in the initialization vector ensures that identical source data looks different for every individual device. This way, if data is copied from one chip to another, the copy will not work in the application. Details about encryption algorithms and software are available on the Internet.
