Chapter 15
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The FT232R USB UART and FT245R USB FIFO are USB controllers that
can each appear as a virtual COM port on a USB host. Each chip has a
full-speed USB device port. The FT232R converts between USB and an asyn-
chronous serial interface, and the FT245R converts between USB and a parallel
interface. Both chips have a 128-byte transmit buffer and a 256-byte receive
buffer. The chips use bulk transfers to send and receive COM-port data.
Another controller from FTDI is the FT2232C Dual USB UART/FIFO. The
chip contains two controllers that each support several configurations, includ-
ing two virtual COM ports in one chip.!
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Figure 15-1 shows the pin functions for the FT232R USB UART. The
USBDM and USBDP pins interface to the USB data lines. TXD and RXD are
the asynchronous serial output and input. Six pins support status and control
signals that correspond to the signals defined in the RS-232 standard. TXD,
RTS#, and DTR# are outputs, and RXD, CTS#, DSR#, RI#, and DCD# are
inputs.
On receiving asynchronous serial data at RXD, the chip sends the data to the
USB host in bulk transfers. On receiving data in bulk transfers from the USB
host, the chip writes the data in asynchronous serial format to the TXD pin.
The asynchronous serial port can transfer up to 300 kbytes/s of data assuming
one Start bit and one Stop bit per byte.
Five additional pins form the configurable CBUS. The pins are programmable
via a utility provided by FTDI and can have these functions:- LED driver that pulses when transmitting or receiving via USB.
- Output that goes low when the device is in the USB Suspend state.
- General-purpose I/O.
- Output that goes high when data is transmitting on TXD. The output can
interface directly to the transmit-enable input of an RS-485 transceiver,
eliminating the need to enable the transmitter using firmware or additional
hardware. - Configurable clock output for driving an external CPU or microcontroller.