3V-5V Switchable I2C Real Time Clock Shield

This is a prototyped Arduino shield that is 5V and 3V switchable.  The real time clock (RTC) is a DS1307 running at 5V, with a backup battery to keep time while not plugged into another power source.  It connects with an Arduino or similar board via I2C, but using a voltage translation (level shifter) circuit using FETs (reference: Philips) to isolate the RTC chip that always runs at 5V from the Arduino CPU which can be running at either 5V or 3V.  This provides support for 3V Arduino format boards such as the Arduino Due which can be damaged if 5V is applied to its lines.  Jumpers are provided to select the voltage (red), the I2C lines used (D3/D2 or A5/A4), and whether 3.3K pull-ups are used on the microcontroller side of the circuit.  Note the RTC always has its pull-ups in place.

Note that due to USPS regulations, Lithium batteries cannot (without meeting specific conditions) be shipped via US airmail so my Digi-Key order that included the FETs and crystal was revised to eliminate the batteries.  These are available cheap in bulk from many distributors, but to save immediate costs including shipping I ended up purchasing a single one at the local drug store.  Next time I may use ground shipping for such items, but I also found a local electronics supplier that has a decent price.

 

The DS3107 chip was purchased overseas via eBay as it was available much cheaper than domestically.  Hint: look for free shipping.

Schematic

A different 3V RTC chip could also be used, such as a surface mount chip, and then the level shifting circuit (FET drain/source) needs to be reversed per the original circuit recommended by Philips Semiconductors because the low/high sides of the circuit are reversed when the RTC is running at 3V and the microcontroller is running at 5V.

For prototyping the FETs ordered are in package TO92-3.  I ordered two different FETs and ended up using the more expensive ones with a higher Vdss rating (200V).  A cheaper FET in SOT-23 packaging with a lower rating of 50V was also sourced but not tested.

Adafruit has a tutorial including an Arduino library for use with the DS1307.  I tested this library on the Arduino Leonardo and Duemillanove running at 5V.  For the 3V Due, I used simple raw I2C (Wire library) commands to talk to the DS1307.  Only SCL1/SDA1 worked for me on the Due (had to jumper these as the ProtoShield is pre-R3 and doesn't have dedicated SCL/SDA pins).

(I had meant to publish this article in February but due to some bad soldering on my part and challenges with the Due, this prototype wasn't working properly.  Took multiple re-visits to fix everything.  Now I am very glad it is working now.)

I2C EEPROM Shield and Platforms Roundup

 This I2C EEPROM shield was built to study I²C communications.  This two-wire bus involving bidirectional clock and data lines was invented by Philips (now NXP) to support a single master and multiple slaves.  A concise explanation and pseudo code is available on Wikipedia.   The shield is composed of a Microchip 24FC1025 which is a 1024 megabit (128 kilobyte) EEPROM, and supporting circuitry of jumpers, sockets, and resistors.  The jumpers provide options for the implementation to support a variety of platforms. 

 

Note: there is a strong advantage to using the I2C 24FC1025 over its SPI cousin the 25AA1024 that I highlighted in another EEPROM Shield article.  The I2C version has a simple API: write the address, then read/write bytes.  The SPI version has a complex API including erasing the whole chip, erasing a page, writing a page of memory, etc.  While the SPI device adheres to some JEDEC standards allowing for interoperability and has faster protocol speeds (20MHz) than the slower I2C device (100 kHZ, 400 kHZ, or 1MHz), using an I2C EEPROM is much faster to develop for time to market when engineering from scratch.

 

 

The form factor of the EEPROM shield supports the original Arduino such as the Duemilanove.  This also makes it compatible with a variety of other Arduino boards, and boards that are compatible with the Arduino shield form factor.  I have access to quite a few boards that are either directly compatible with this shield, or easily adapted.  Testing these boards with the EEPROM is summarized in the following table:

 

Board	Type	VCC	Pullups	SDA/SCL	Solution	Notes
Arduino Duemilanove	arduino	5V	internal	A4/A5	Wire.h
Arduino Leonardo	arduino	5V	external	D2/D3	Wire.h
chipKIT Max32	arduino	3.3V	external	D20/D21	Wire.h	1
chipKIT Uno32	arduino	3.3V	external	A4/A5	Wire.h	2
Netduino Mini	netmf	3.3V	on board	9/10	I2CDevice	3
Netduino Plus	netmf	3.3V	external	A4/A5	I2CDevice
Netduino Plus 2	netmf	3.3V	external	SDA/SCL	I2CDevice	4
Panda II	netmf	3.3V	on board	D2/D3	I2CDevice
FEZ Cerbuino Bee	netmf	3.3V	external	D2/D3	SoftwareI2CBus	5, 6
Netduino Go Shield Base (Standalone)	netmf	3.3V	internal	A4/A5	SoftwareI2C	7

 

Notes:

1. jumpered D20/D1 pins to D2/D3

2. JP6/JP8 jumpered to RG3/RG2

3. custom board used to adapt the Mini to Arduino form factor

4. jumpered SDA/SCL pins to D2/D3 or A4/A5

5. hardware I2C pins on Cerbuino not available to shields so using software method

6. WARNING: RESET/A1/A4 are not 5V tolerant on the Cerbuino.  I could have damaged the Cerbuino if I had used different SDA/SCL jumper settings on the EEPROM shield.

7. waiting on a fix from Secret Labs so can use I2CDevice

 

I was able to successfully access the EEPROM using all of these platforms.  The Arduino and chipKIT boards use Arduino and Arduino compatible IDEs.  The .NET Micro Framework (netmf) boards used either the included support for the I2C hardware lines, or in a couple cases had to use either included or added software drivers.  The upside of the software drivers is that the wiring is more flexible and can use CPU internal pullups, but the downside in this case is the software drivers are slower than hardware drivers.  It is recommended to use the hardware drivers if you can, but there are also software drivers available that work for all these platforms (Arduino and netmf).

 

The circuit was designed on the fly to quickly get up and running with I2C.  There are a few changes I would make to improve this circuit.

1. Remove the 3.3V pullup options.  These were not needed.
2. Add jumper for switching the entire circuit between 3.3V and 5V.  The EEPROM can run at lower voltages, and this would allow the shield to be compatible with non-5V tolerant platforms like the Arduino Due.
 

Schematic