About E.H.A. Serial mode

The most important thing about LCD Boosterpack V2 is the propietary E.H.A. serial protocol.
E.H.A. Serial is the result of thinking about how to improve timing on LCD comunication and to accelerate distinctive stream sequences

LCD displays, such as the ones used in LCD BP V2, have LCD chip to drive TFT grid pixels, in this case SSD1289 chip. This chip links each pixel with his MCU and provide Serial or parallel control to any external MCU or FPGA/CPLD.

Serial control has two issues:

-Common to both serial and parallel control, is that communication has many repetitive actions as drawing filled polygons on screen (it needs to write same color pixel a few times).
parallel to serial design, because compatibility issues or lack of wit, only uses the final state of the bit, not the edge, so it is needed one clock pulse to indicate each action, included control actions like starting or ending communication or selecting read or write actions. 
With integrated design based on a CPLD (ex: 74xx series) and some code modifications has been achieved overcome the drawbacks. You get faster speeds and noise supression because integrated design.

Now, I will show the timming diagram of paralell and serial comunication.


Parallel comunication


Serial communication

As can be seen in the diagrams, parallel communication seems to be (and in fact is) more efficient, but with the disadvantage of taking 16bits of data plus three bits of control, versus the 4 total bits in series mode, what is enough in some applications while video timming is not critical and mcu doesn't have too many free pins.

In both modes, serial and parallel, has been programmed a new mode to improve performance: writedatasec (write sequence of data). This mode can be used because CPLD (in series comunication) and MCU (in parallel comunication) keeps data in his output buffer (to SSD1289), so is not needed to update it, saving time and MCU load.

Some other news about serial mode are dual bit data and edge indications: Because CPLD design can parallelize process without losing time, so it uses two separated circuits to obtain data at the same time and later feeds register buffer. Edge indicates almost all actions of the integrated CPLD circuit to save time.

In short, E.H.A. serial mode is the fastest mode possible with serial communication and allows low end MCU to drive LCD at good speed and high end LCD to save pins with speed of parallel.
In fact, using writedatasecwith any MCU (comparec with the same clock speed),parallel and serial mode spend the same ammount of time.

LCD BP V2 Shop item .

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LCDBPV2 por Daniel Rebollo Lorenzo se encuentra bajo una Licencia Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 Internacional.