Nuts and Volts - December 2008

THEDESIGNCYCLE ADVANCED TECHNIQUES FOR DESIGN ENGINEERS

■ BY FRED EADY

AVOIDING TUITION AT USB UNIVERSITY

I HAVE A LOVE/HATE RELATIONSHIP WITH USB. I love it because it is convenient and user friendly. I hate it because understanding the underlying processes of USB can be difficult. Think about this. You don’t have to know the down and dirty details of how a PIC ALU (Arithmetic Logic Unit) works to employ a PIC in a microcontroller-based application. So, why should we have to know so much about USB to put it to work for us?

I’m obviously not alone when it comes to wondering why deploying USB in an embedded application has to be so darned complicated. The war began when personal desktop computers and laptop computers dropped the RS-232 interface for multiple USB interfaces. Microcontroller manufacturers such as Microchip, Texas Instruments, and Atmel stepped in and offered lines of microcontrollers with built-in USB engines and embedded USB interface devices. Microchip is still on the front line of the embedded RS-232-to-USB revolution.

A USB firmware package called USB Framework is available for download from Microchip. The Microchip USB development package extends from the PIC18F eight bit microcontrollers up through the PIC24F 16 bit microcontrollers and out into 32 bit space with backward compatible USB API extensions designed for the 32 bit PIC32. Officially, the Microchip USB tools are packaged as the MCHPFUSB Framework for the eight bit and 16 bit PIC microcontrollers. The MCHPFUSB Framework opens up PIC18F and PIC24F projects, drivers, and PC-based resources. The MCHPFUSB USB stack is API compatible with its 32 bit counterpart, the USB Device and Embedded Host Stack for PIC32.

Other microcontroller manufacturers tackled the lack of PC RS-232 port access by introducing USB-to-UART bridge integrated circuits. For instance, FTDI (Future Technology Devices International) has a full line of RS-232 converter replacement integrated circuits. The FTDI RS-232/USB ICs do everything necessary to build a data link between a USB device and a microcontroller UART. Most of the FTDI parts also contain additional user-accessible I/O pins. The latest FTDI device — the FT232R USB UART IC — can internally generate a clock that is suitable for driving an external host microcontroller. The FT232R doesn’t leave any USB hardware loose ends. The clock generation is performed on-chip and no external crystal is required. All of the USB termination resistors are

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also fully integrated on-chip. The FTDI engineers thought of everything including on-chip power conversion and transmit and receive LED drive signals. USB likes to “describe things.” So, the external EEPROM that normally holds some of the USB descriptions has been incorporated into the bowels of the FT232R.

If you don’t need the extra I/O and only want to convert the interface from RS-232 to USB, Silicon Labs offers the CP2102 Single Chip USB-to-UART Bridge. Like the FTDI FT232R, the CP2102 pulls the USB clock, termination resistors, 1024 byte EEPROM, and power conversion down to the chip level.

All of the aforementioned USB conversion ICs are supported by free PC drivers that are supplied by each of the USB-to-RS-232 converter IC manufacturers. With the USB hardware distilled into a single IC and free PC driver software, the only USB deployment obstacle we may possibly see as hobbyists is soldering down the very small USB IC SMT packages. You can overcome the SMT soldering obstacle in your development phase by purchasing preassembled USB modules from FTDI and DLP Designs.

Thus far, we’ve only discussed the replacement of RS-232 converter ICs with spiffy USB interface ICs. Sure, easily getting a PIC’s UART to communicate directly with a PC’s USB port is a step forward. However, if we design in a dedicated USB-to-UART bridge IC, we’re only going to be able to communicate in the same manner (UART-to-PC terminal emulator) as the RS-232 circuit we replaced. That is not a bad thing if all we need to do is replace the RS-232 interface on our embedded device. What if we wanted to interface our embedded creation with another USB device other than a PC?