Many legacy data loggers, such as medical health monitor devices and 8/16 bit microcontroller based designs intended for sampling various sensors data, use UART/LPT for communicating to PC (Figure 1, below , with the PC running a specific application to analyze the data. The primary reason for this shift is data rate.
For a sensor data logging application, a slow interface rate and single bus line can result in hours of time to read data even a single day’s data over USB from storage.
|Figure 1: conventional UART based architecture|
Since the birth of USB, serial interfaces like PS/2 and UART are being phased out by USB. The rapid growth of the mobile and embedded device market demands new features and capabilities such as High-Speed USB connectivity, support for the latest mass storage devices, and access to higher storage capacity.
To implement High-Speed USB and mass storage access in products like data loggers and 8/16 microcontroller-based designs, there are two approaches: using a single-chip, high-end processor or keeping the existing design architecture intact and introducing USB and storage through a bridge controller.
The key factors to keep in mind while comparing these approaches are feasibility, performance, cost, design/development effort, and time to market.
Single chip approach
In many cases, system processor architectures cannot keep pace with the increasingly faster evolution of new technologies and standards. This means that any integrated interfaces are often at least one generation of any standards behind the technology curve.
Those processors which do support the latest standards typically require designers to scratch existing designs and begin anew. These processors which can support High-Speed USB must be able to talk to sensors as well (Figure 2, below ). They also need to support access to mass storage over multiple lines at high speed.
Once such a processor has been selected, there are the added costs of needing to develop new firmware from scratch and then test and debug it until it passes specifications.
|Figure 2: High-end, single-chip MCU-based architecture|
Generally, such high-end MCUs utilize development tools which are far more complex than is necessary for a simple data logging application (Figure 2, below ). Such complex increases time to market for such projects and increase total cost as well.
Implementing a bridging architecture such as a West Bridge gives developers flexibility in that they keep their existing design intact while allowing the existing MCU to communicate to various mass storage memories, cards, and High-Speed USB at faster data rates.
A West Bridge can act both as a bridge between USB and the MCU as well as between mass storage (i.e., SD/MMC/SDIO) and the MCU without blocking. Blocking is avoided through side loading. Side loading allows the USB host to access data from mass storage independent of the MCU.
This results in being able to maintain significantly faster data than is possible in a single-chip MCU approach where USB and mass storage compete for the same limited bandwidth.
To implement a bridge architecture requires only the addition of a single communication protocol layer between the MCU and bridge (Figure 3, below ).
This engineering effort is substantially less complex than creating entirely new drivers, therefore reducing time to market and development cost. Such bridges are relatively inexpensive cost, so the total cost of a bridge approach stays well below that of switching to a high-end processor.
|Figure 3: Bridge based architecture|
A bridge approach is also useful for introducing mass storage through the use of a dongle-like card reader connecting to mass storage such as fixed NAND Flash or a removable SD/MMC card.
Fixed storage can be partitioned (read only partition) to store documentation, PC applications, drivers, and links to website. It can even include functionality to automatically update information and firmware from a PC connected to the device and the Internet.
Using a West Bridge facilitates a straightforward way to integrate High-Speed USB and mass storage in existing designs while at the same time reducing development time, speeding time to market, and lowering overall cost.
Dhanraj Rajput is an application engineer at Cypress Semiconductor Corp. He can be contacted at firstname.lastname@example.org