Do you need to save pins in low pin count microcontrollers? UNI/O protocol is the answer.
The purpose of this article is to present a few attributes of serial EEPROMs featuring this new technology, also providing a short insight in how they should be used with a microcontroller. Their very existence is probably owned to the very low pin-count microcontrollers already present on the market. If one thinks that a PIC10 series device only has 6 pins (out of which one is VCC and another GND) it is obvious that using a one, two or three pin interface to connect to an external EEPROM is not unimportant, and this decision can affect the design from a very early stage.
For the hardware engineer, just having a comparative look at the pin functions of such an EEPROM will make him feel better, since the PCB layout can be significantly less complex with less tracks to route in a possibly very populated area of the board:
Not only the number of traces is reduced, but also the number of additional discrete components is lower, as the usual pull-ups required on the I2C lines for example are inexistent in a UNI/O based design:
And in order to make things even more attractive, Microchip provides this type of memories in a space-saving 3-pin SOT package which is significantly smaller than 8-pin packages. Additional facilities include: industrial and automotive qualified devices, 1.8V to 5.5V supply range, low current consumption (active current: 1mA) and 1 million guaranteed erase/write cycles.
Having a proprietary bus protocol (like UNI/O) in your design also brings you some advantages as being a tested and proven protocol. The UNI/O Bus is an asynchronous single I/O bus that uses the Manchester Encoding scheme to multiplex the data and clock on to a single I/O (SCIO). With data stream Manchester-encoded the UNI/O device synchronizes to the MCU data rate. The clock signal is extracted by the receiver to correctly decode the data value of each bit. The bit period is determined by the master and forms an integral part of the communication between the master and the receiver. It is with this bit period that clock and data can be extracted.
Of course, having to do so many things using a single communication data line might be fine from a hardware point of view, but it might not be the first choice of a software developer. As we must remember that the target for these special EEPROM devices is to be used with the smallest available microcontrollers which also happened to have quite small program and data memories. So although at a project level some reduced Bill of Material cost might be achieved, the software effort to develop the “drivers” for writing to these memories might overcome it. Sometimes, with extremely small program memory micros it is not even possible to squeeze in the code for such an elaborated protocol and the project might have to move on to a larger and more expensive device (which is exactly what the use of these memories is meant to preempt).
Indeed, the protocol is without doubt more complicated than on a regular I2C, SPI or Microwire bus. It is made up fast sequences of headers, addresses, data and acknowledgments from both the master (microcontroller) and the slave (EEPROM). The biggest disadvantage is that due to the existence of the bit period (TE) it is not possible to stop a transmission in the middle of one byte, when for instance, the microcontroller might have to service an urgent interrupt request. For older busses, this feature is generally ensured using a HOLD pin, or by using microcontrollers with dedicated internal SPI or I2C modules. However, the low pin-count devices meant to be used with UNI/O bus protocol must not be expected to provide such hardware features. Low cost means not only few pins, but also means more software developing effort.
As such, after a brief encounter with the new technology by which Microchip surely intends to conquer yet another share of the market, one can easily spot at a glance the pros and cons of using these EEPROM memories. Of course they will find their place in some applications (the bus would not have been introduced without a thorough market research) but their real success will only be measured by the degree of which other major EEPROM/microcontroller manufacturers will be wiling to invest in similar devices (of course with the higher cost of paying the right tribute to the bus proprietor).
Further useful information about this type of products may be found on the microchip website: Microchip
And if you are really in a hurry and want to start taking advantage of the new available features, just download a datasheet, for the 11AAxxx memory family:1K-16K UNI/O™ Serial EEPROM Family Data Sheet