Jump On The Bus

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JUMP ON THE BUS

Applications Taking a Look at the USB
by Don Powrie

Start ı Speed Limit ı The Mechanics ı Destination ı Sources and PDF

THE MECHANICS

The FT8U245AM is available from FTDI in surface-mount form as a 32-pin MQFP, and samples of the FT8U245AM are available through Saelig. The evaluation board (DLP-USB1) can be seen in Photo 1.

Photo 1ıThe DLP-USB1 from DLP Design allows you to easily evaluate the FT8U245AM. Mounting pins on 0.1ı spacings makes easy work of interfacing to target electronics.

Interfacing to the FT8U245AM is made via eight data lines and four handshaking lines, as outlined in Figure 1 and Table 1. The FT8U245AMıs internal FIFO is comprised of two buffers, which can hold 128 bytes of received data coming from the host PC and 384 bytes of data to be transmitted to the host.

Figure 1ıThe FT8U245AM communicates with a target microcontroller via an 8-bit data bus and four handshaking lines.

Additional circuitry can be used to detect when the device enters Standby mode. This is usually nothing more than a quad NAND gate

Pin	Direction	Function
*RD	Input	When pulled low, RD takes the eight data lines from a high impedance state to the current byte in the FIFOıs receive buffer. Taking RD high returns the data pins to a high impedance state and prepares the next byte (if available) in the FIFO to be read.
WR	Input	When taken from a high state to a low state, *WR reads the eight data lines and writes the byte into the FIFOıs transmit buffer. Data written to the transmit buffer is immediately sent to the host PC and placed in the RS-232 buffer, which is opened by the application program.
*TXE	Output	When high, the FIFOıs 384-byte transmit buffer is full or busy storing the last byte written. Do not attempt to write data to the transmit buffer when *TXE is high.
*RXF	Output	When low, at least one byte is present in the FIFOıs 128-byte receive buffer and is ready to be read with RD. RXF goes high when the receive buffer is empty.
Table 1ıThe FT8U245AMıs four handshaking lines give the target microcontroller complete access to the FIFO buffer.

Power for your target electronics can be taken from the USB port, provided you are careful not to exceed the limits of 500 mA during normal operation and 500 ıA when in Standby mode. The FT8U245AM enters Standby mode after 3 ms of no USB activity (meaning there is no start of frame packets). Meeting this power specification will require that you shutdown your target electronics when the FT8U245AM enters Standby mode, most likely by adding a MOSFET power switch or switches.

The product ID (PID), vendor ID (VID), device description, and manufacturer name can be stored in an EEPROM device via a built-in interface on the FT8U245AM. The EEPROM can also store a unique serial number that is generated and written by the program 232PROG.EXE, which can be downloaded from FTDIıs web site. If you are planning to commercialize a product with a USB port, you must register your own PID and VID with the USB-IF.

USB analyzers are available from a number of companies and tend to be rather expensive, but they are worth the money if you are attempting to isolate an elusive software bug. The level of information that is provided by these analyzers requires that you be extremely familiar with the USB specification. At this level of understanding, you will probably want to write your own drivers.

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