■ SCHEMATIC 1. This is an SP232 RS-232
converter IC on steroids. The active low
SUSPEND line is used to conserve power
when the CP2103 goes into SUSPEND mode.
standard EIA modem signals include:
RTS - Request To Send
CTS - Clear To Send
DSR - Data Set Ready
DTR - Data Terminal Ready
DCD - Data Carrier Detect
RI - Ring Indicate
The DTR and RTS modem signals are generated by
the DTE, or Data Terminal Equipment. Your PC qualifies as
a DTE device. Under normal circumstances, the modem —
or Data Communications Equipment (DCE) — responds to
a DTR signal with a DSR signal which indicates that the
modem is powered up and can communicate with the
DTE. The DTE device raises the RTS signal to the modem
when it wants to transmit. If the coast is clear, the modem
responds to the DTE device by raising the CTS signal.
When a remote modem is contacted and the carrier is
sensed, the DCD line is asserted, telling the DTE that a
link has been established. For switched (dial-up) links,
the modem raises RI when the phone line it is attached
to is ringing. The RI signal is used by the software for
unsophisticated modems that cannot auto answer.
Every conceivable baud rate from 300 to 921600 bps
is supported by the CP2103's UART interface, which can also
speak all of the parity, data bit, and stop bit combinations.
One of my concerns about moving to a USB interface
involves the customization of each USB equipped device.
Once again, the engineers at Silicon Laboratories have
provided an out in the customization maze. As long as we
stick with their USB ICs, we can use the free Product ID (PID)
provided by Silicon Laboratories in conjunction with their
Vendor ID (VID). Using Silicon’s PID and VID with a unique
serial number will assure the uniqueness of each USB device
sporting a CP2103. All of the customization we just discussed
is written to the CP2103's internal EEPROM. Yep, Silicon
Laboratories provides the stand-alone EEPROM programming utility we will need to perform the customization.
This drop-in-no-software-modification USB wonder IC
sounds too good to be true. However, before we commit
anything CP2103 to a Design Cycle project PCB, we'll
take a complete tour of the Eval Kit’s hardware and
software. If adding USB capability to a microcontroller
is as easy as the datasheet says it is, we'll exercise some
PIC test hardware along the way.
THE CP2103 EVALUATION KIT
The Evaluation Kit PCB is laid out in such a way as
to allow us to easily tap into the CP2103's resources. The
user-accessible tap points can be found under every jumper
block you see in Photo 1. The jumpers connect the CP2103
to the SP3243EU RS-232 converter IC and a gaggle of
indicator LEDs. As you can see in Schematic 1, a complete
RS-232 interface is built around the SP3243EU. The idea is
to connect a legacy RS-232 device to the nine-pin end of the
Kit which becomes a transparent bridge to a PC's USB port.