Using the CP2102N USB to UART bridge

• Introduction
• The power supply and USB side
• The charge control pins
• More interesting features
• A simple USB to TTL and USB to RS485 board
  • The schematic
    • Component list
    • The PCB
      • First test
  • Configuring the chip
    • Second test
• Tools used

Introduction

This blog entry is just a summary of the most basic configurations of the CP2102N USB to TTL chip as well as a description of a playground board design that exposes all pins of the CP2102N to the outside for evaluation purposes and in addition provides functionality of a half duplex RS485 bridge. The CP2102N is one of the most robust and simple to use (especially also on the software side) chip that I encountered up until now - and it provides all features one would require for simple devices. It’s a very versatile device that can be customized so the described usage only represents a small subset of what’s possible. The CP2102N supports USB 2.0 full-speed which is sufficient for microcontroller applications under most circumstances. The device requires no external clocking, supports data transfer at up to 3 MBaud, requires only small operating current and can be run either bus powered or device powered depending on the application. In addition the CP2102N also supports chargers with information about available bus current and presence to implement battery charging in a simple way.

In addition the manufacturer provides a customization tool that allows one to change the VID/PID instead of using the default one (the biggest drawback is then that you need to provide your own mappings in the devd or devfs configuration or even need to write your own signed device drivers on some more obscure operating systems (like MS Windows)). In addition to standard UART operation the device also supports triggering an external half duplex RS485 driver like the MAX485 by just setting a single bit so one can build a two component low cost RS485 interface using the CP2102. Contrary to other cheap solutions each device has a unique serial number that can also be customized using the manufacturer supplied tool which allows one to distinguish the devices when assigning device links. On top of all that the device - when using a custom driver - offers 7 freely usable GPIO pins; in the usual configuration they are used to drive TX/RD LEDs, the RS485 TX/RD switch and support the transmission of a wake up signal to a suspended host via. The other 3 GPIOs are unassigned by default. Note that the default configuration also does not enable the LEDs as well as the RS485 feature. You have to enable those via the configuration utility!

In contrast to some chips like the famous FTDI USB to serial adapters the CP2102 are easier to use - also from the software side since they are delivered as standard communication class device so there is no additional driver installation required. One can still distinguish the devices (and assign them specific alias in the devd.conf by identifying them by serial number which is unique for the CP2102).

The power supply and USB side

The first thing to consider when using the CP2102 is the way the device is powered. There are three main paths one can take:

• Power the device and the CP2102 via the USB bus and use the internal 3.3V regulator to supply up to 100 mA by the internal LDO to the application circuit’s 3.3V rail. In this case one has to connect the bus power to the regulator input and use VDD (with a decoupling network) for the application circuit.
• Power the CP2102 and the application circuit by some other means (external power supply, etc.). In this case the bus power supply only has to be attached to VBUS. REGIN and VDD have to be attached to the application circuits 3.3V rail.
  • In case one hasn’t got any 3.3V rail available one can also supply the LDO with 3.6 to 5.25V via the REGIN pin.

In any case the reset pin has to be pulled up with a $1k\Omega$ resistor to VDD. It’s also good practice to connect the bus voltage to VBUS via a voltage divider. Since VBUS is used only to detect presence of an USB host this still works but clamps the voltage seen by the CP2102 to stay within allowed absolute maximum limits even when the device itself is not powered by the application circuit. This is of course not required but not a problem anyways when powering the application directly from the LDO of the CP2102.

Often not mentioned one should add a proper TVS diode to each of the bus lines. A quad array like the TPD4E02B04DQA might be a good idea. One usually forgets those but this provides additional protection against voltage surges and static electricity. I’ve personally already lost 3 CP2021N by high voltage spikes in a lab when arcing occurred on another power line. Adding TVS diodes prevented any further faults. They don’t cost much and may safe your whole device. Use them.

All power lines (REGIN and VDD) have to be properly decoupled. For REGIN this could be a $1 \mu F$ and $100 nF$ capacitor, for VDD a $4.7 \mu F$ and $100 nF$ capacitor in parallel. One has to adjust this to ones own needs of course.

That’s already the most basic way of using the CP2102:

In this circuit the GPIO pins 0 and 1 have also been attached to LEDs that indicate transmission and reception of data. This might be nice especially in an experimental environment - but is entirely optional of course. One can also reconfigure the GPIOs for other purposes or just leave them unconnected.

The two solder jumpers allow to select how the device should be powered.

JP1 1-2	JP1 2-3	JP2	Power mode	CP2102 bus powered	Application bus powered
Open	Open	Open	Won’t work, device and regulator not powered
Open	Open	Closed	Won’t work, not specified since the regulator input is floating
Open	Closed	Open	Won’t work, regulator input and VDD don’t get power from anywhere
Open	Closed	Closed	The device is powered by the application circuit at 3.3V, bus power is not used	N	N
Closed	Open	Open	The device is bus powered, the application is not powered by the bus	Y	N
Closed	Open	Closed	The device is bus powered, the application draws its supply from the 3.3V LDO of the CP2102 (bus powered)	Y	Y
Closed	Closed	Open	Won’t work, bad idea
Closed	Closed	Closed	Won’t work, bad idea

The charge control pins

The CP2102 offers another nice feature. The pins CHREN, CHR0 and CHR1 are especially interesting when one wants to charge batteries via an external charge controller. After the CP2102N performed negotiation on the USB bus they signal if and how much current a charger may draw.

• In case only CHREN goes high one may draw at most 100 mA from the bus
• CHR0 signals capability of drawing up to 500 mA
• And when CHR1 goes high one can draw up to 1.5A

More interesting features

• One can use the CP2102 as clock source up to $24 MHz$
• When enabled the remote wake up pin allows the device to wake up any host that allows waking up by an USB device
• The RS485 pin can drive an half duplex drivers receive enable / transmit enable pin (for example on an MAX485) to build simple RS485 half duplex interfaces
• The suspend pins allow the host to perform power management on the application circuit. Whenever the device should enter suspend mode the CP2102 asserts both suspend lines (they should use a $10 k\Omega$ pull-down to prevent floating while powering on)

A simple USB to TTL and USB to RS485 board

As the most simple example lets take a look at a very simple USB to TTL and USB to half duplex RS485 board. Of course boards just offering USB to TTL serial conversion can be bought very cheap on common merchant platforms - but it’s a good learning exercise and supporting RS485 is less common on such boards. This board has been designed around the CP2102N as well as the MAX485E - contrary to the small boards sold on most platforms all I/O pins of the CP2102N have been exposed via a pin header so the board is rather bulky. This has been done to provide a playground for GPIOs, provide access to the flow control pins and allow access to the charge control pins during evaluation of the chips. It’s more of a playground than a practical implementation. The only assumptions that have been made are:

• The device is bus powered
• GPIO0 and GPIO1 are used for RX and TX LEDs
• GPIO2 toggles the MAX485E between receive and transmit states.

The schematic

All design files are provided in a GitHub repository. The schematic design is pretty simple:

• At the USB input side it uses an USB B socket (J1). On the board a USB 2.0 B socket has been used since it’s compatible with most existing USB installations and cabling out in the world as of today. Of course one could consider an USB 3.0 B connector for new designs though the speeds utilized by the CP2102N do not reach anywhere in the USB 3.0 regions.
• As recommended in the datasheet a TVS diode has been added. The used TPD4E02B04DQA has been used though this has been designed for USB-C devices. As one can see only three of the diodes are in use, the forth is just tied to ground. This is also the hardest part to solder due to the very small package size.
• The USB 5V rail is supplied to REGIN as for a bus powered device. Two decoupling capacitors ($1 \mu F$ and $100 nF$ are provided as close to the chip as possible).
• The 5V rail of the USB socket is also divided via a voltage divider before being fed into VBUS. As mentioned above this is not required for a bus powered device but keeps the device in absolute supported margins when providing power via the application circuit. Even though this is not possible on this board adding those resistors does not hurt (when one’s not designing in industry scale where every small saving in material cost helps).
• The reset pin is just pulled to the 3.3V rail via a $1 k\Omega$ (or on some of my built boards using a $10 k\Omega$) resistor.
• The output of the on-chip 3.3V regulator is decoupled again via two $1 \mu F$ and $100 nF$ capacitors.
• GPIO.0 and GPIO.1 are used as open collector outputs to drive the RX and TX LEDs from the 3.3V rail. Since the LEDs have a voltage drop of around 1.2V the remaining 2.1V determine the current - when using a $100 \Omega$ resistor this yields a current of around $21 mA$ for the LEDs. One can also use larger resistors (in my implementation I just used the $120 \Omega$ resistors since I had a bunch of them at hand which did not really change the current in any noticeable way - running at 17.5 mA doesn’t yield a visible noticeable effect for most cheap LEDs)
• The jumpers JP1 and JP2 allow one to select between RS485 and TTL operation.
  • When set to position 1 the RX and TX of the CP2102N is fed into the pins of the MAX485E. The output of the MAX485E is divided via a simple voltage divider to reduce it’s voltage from the 5V domain down into the 3.3V range. The voltage divider is sufficient for the speeds supported by the two devices anyways.
  • When set to position 2 the RX and TX pins are directly fed to the TTL screw terminal
• The MAX485E is fed directly by the 5V rail of the USB connector. In addition separate decoupling capacitors are used for the MAX485.
• To allow selection of bus termination for the RS485 network (which should usually only be done at it’s edges) one can use the jumper JP3 to add a $120 \Omega$ termination resistor between A and B line.
• Supply voltages are supplied also at the screw terminals.
• All other signals of the CP2102N are routed to a huge 16 pins pin header that is of course optional.

Component list

The following table contains a list of all components I’ve personally used to build such boards. Most of them (like resistors, capacitors and terminals) are freely interchangeable. Capacitors are all metal film capacitors, resistors thick film resistors - for them there had been no particular reason to choose them except for a roll of them having already been kept on stock.

Identifier	Mouser Nr.	Manufacturer number	Component Description
U1	595-TPD4E02B04DQAR	TPD4E02B04DQAR	ESD suppressor / TVS diode array
U3	700-MAX485ECSA	MAX485ECSA+	MAX485E RS485 interface (half duplex)
U2	634-CP2102NA02GQFN28	CP2102N-A02-GQFN28	CP2102N USB interface
R1, R6	603-AC0805FR-0724KL	AC0805FR-0724KL	$24 k\Omega$ resistor, 0805 package (any type will do)
R2, R7	603-AC0805FR-1047KL	AC0805FR-1047KL	$47 k\Omega$ resistor, 0805 package (any type will do)
R4, R5, R8	603-AC0805FR-07120RL	AC0805FR-07120RL	$100 \Omega$ resistor in schematic, used $120 \Omega$ due to them being on local stock; R8 has to be $120 \Omega$ (any type is ok)
C1, C3, C5	581-08055C105K4T4A	08055C105K4T4A	$1 \mu F$ capacitor, 0805 package (any type will do)
C2, C4, C6	581-08055C104K	08055C104KAT2A	$100 nF$ capacitor, 0805 package (any type will do)
D1, D2	593-LSM0805412V	LSM0805412V	Standard 0805 packaged LED. Any generic LED is usable.
J1	710-61400416121	61400416121	USB B connector by Wurth
J2			2.54mm pitch 2x8 pin pin header
J3, J4			2.54mm pitch 4 pin screw terminal
JP1, JP2			2.54mm pitch 3 pin pin header
JP3			2.54mm pitch 2 pin pin header

The PCB

The board layout is also rather simple.

• It has been decided to use simple 0805 parts for resistors and capacitors wherever possible. Those parts can be soldered by hand. I personally prefer using reflow paste since it’s used for other parts of the board anyways and yields nicer looking boards and usually better solder connections when not exposed to mechanical stress (like for connectors)
• The MAX485E has been used in a SOIC-8 package. Those packages can be soldered by hand using a traditional solder iron with some practice though I personally prefer using reflow for this kind of stuff.
• Connectors are through hole technology (THT) parts. Those should be soldered either with traditional soldering iron and solder wire (which is more likely for the hobbyist) or Gallium wave soldering (more likely in the professional environment). I’ve chosen THT over SMD connectors due to the fact that the boards are not automatically assembled via a pick and place machine and due to their way higher tolerance to mechanical stress when connecting and disconnecting (who hasn’t ripped off SMD connectors before?)
• The CP2102N comes in a QFN-28 package. This is a surface mounted package that has it’s pad completely hidden below the package. Though there are some tricks with massive amounts of flux and traditional solder that require much skill and investment of time. I’d personally say this is the point where one really wants to perform reflow soldering when not wanting to play around but achieving good results.
• The TPD4E02B04DQA TVS diode array is the most challenging part to solder on this board. The array is provided in a USON-10 DQA package. This has $500 \mu m$ pitch of pads hidden below the package of the device. Reflow soldering this part is very simple. Soldering in any other way very tricky, nearly impossible as well as error prone and time consuming. Of course the TVS diodes are optional and can be left out - but one will loose ESD protection in this case.

As for any more complex board routing directions for top and bottom layer have been chosen to be (nearly) orthogonal (horizontal routing on the back side, vertical routing on the top side). Again the design goal was not the most compact board but a working and simple to assembly playground.

The PCB has then been ordered at PCBWay. Note that this board does usually not qualify for the cheapest option since the package of the TVS diode array is not possible with $6/6 mil$ option. When leaving out the TVS array the board is simple and cheap though. Just use default options, choose a color for your silkscreen and upload the ZIP file provided in the GitHub repository or ZIP up all exported Gerber files from KiCad (make sure to follow the guidelines provided by PCBWay). PCBWay manufactures your PCBs in around 24 hours and ships them very fast.

As usual for this manufacturer PCBs arrived pretty fast:

The PCBs have then been first reflow soldered with a heat gun - first the capacitors and resistors, then the TVS diode array, the CP2102N and then the MAX485E. The last element that has been added on the surface had been the LED. After that the THT components have been added (first the screw clamps, then the pin headers and jumpers and in the end the bulky USB connector).

At the end the PCBs have been cleaned in an ultrasonic Isopropyl alcohol bath for 10-15 minutes to remove any residues of the reflow soldering paste. This is usually a good idea for electronics devices (as long as they’re not damaged by ultrasonic sound like quartz resonators and some plastic parts for example)

First test

As the absolute first test the PCB has been attached to the USB port and the dmesg output has been observed. Then one should see the attachment of an CP2102N device in the kernel log.

One can then directly use any terminal application or software to transmit and receive data with arbitrary Baud rates via the TTL port when jumpering correctly. Neither the LEDs nor the RS485 port will work (the latter one at least not bidirectionally - the MAX485E will permanently drive the A/B line pair) at this point in time. This is caused by missing configuration of the chip. To test the receive part one can simply connect the center pin of both mode selection jumpers. This will echo any written data back into the device.

Configuring the chip

To configure the chip one requires Simplicity Studio by Silicon Labs. The old standalone configurator doesn’t work for the N series of the CP2102. You have to register for an account and download a pretty huge archive. After that one has to log in into the application. The easiest way to download the correct SDK is by attaching a device and letting the application figure out which components to download. Then one can launch the new Xpress Configurator from the menu.

The first thing to configure are the basic device configurations:

One can assign any product and vendor ID - keep in mind that then you have to provide your own devd or devfs configuration or even own device driver (for MS Windows using masochists). One can also freely choose the manufacturer string and the serial as well as the announced required bus power. One might be tempted to write a custom serial - the easiest way to guarantee a unique serial is using the internal serial number though. In this case the serial exposed will be calculated from a unique 128 bit number unique to the given chip. For this board one should also announce that it’s bus powered and configure a proper maximum power usage which depends on the application circuit upstream. When using more power one should also use the charge control pins to trigger power supply to the application circuit.

The next configuration group are the port settings for all I/O ports of the CP2102N.:

Here one can decide on the modes as well as latching of the I/O pins. Usually defaults are what one needs when one doesn’t know better for a simple USB to TTL conversion board. The GPIO configuration though will require some tweaking to enable RX and TX LEDs as well as the driving of the RS485 half duplex transmission enable pin on the MAX485E:

The last configuration option configures the virtual serial port. Here one can configure the maximum allowed Baud rate, a set of supported Baud rates (when selecting User Programmable any Baud rate up to the maximum rate can be used independent of other selections)

Keep in mind that RS485 mode switching is only supported for Baud rates larger or equal to 300 bps so it’s a good idea to limit the supported Baud rates to default ones larger or equal to 300 bps. The supported set of data, stop and parity bits depends on the particular application. Application software is only capable of setting modes that are enabled on the device.

Now one can simply select Program To Device - that’s it for small series of devices. When building a larger number of devices one can use the SDK provided by Silicon Labs to write automatic configuration scripts that can program devices while being transferred through the production line.

Second test

The last initial test carried out on the device has been RS485 mode. Simply jumper the jumpers accordingly and start the data transfer. One will see that during transmission the device always enables the RS485 line driver. After all data has been transmitted the driver goes low again.

Tools used

Note that all links in this section are Amazon affiliate links. This pages author profits from qualified purchases

Which tools have been used in construction of this board? Since the board contains reflow solder parts it has been convenient to use:

• A cheap heat gun like the 858D where you can reduce the airflow and temperature low enough. Don’t try to use your hardware stores heat gun - they just provide way too much airflow to solder effectively.
• Reflow solder paste like the EO-FLP-005 which reflows at low temperatures (the EO-FLP-005 flows at around 138 Celsius).
• Some anti statics ESD pliers
• Some ultrasonic cleaning bath to remove any residue of flux or solder paste
• Isopropyl alcohol for cleaning in the ultrasonic bath. You might want to use Nitril gloves when handling alcohol especially when using larger amounts.

This article is tagged: Tutorial, Basics, Electronics, DIY, RS485