Expanding the Pervasive Displays e-Paper EPD Extension Kit Gen 3
Compared to previous generations, the Pervasive Displays e-Paper EPD Extension Kit Gen 3 provisions pads for an 4-wire SPI memory IC.
This allows to use external memory for pre-recorded pages and fonts, for the frame-buffer of larger screens, or for data retention.
Using initial Flash
The memory soldered on U1 is an SPI 8 Mb or 1 M x8 Flash on SOIC-8 package.
It requires a 4-wire SPI bus, with MOSI and MISO, plus a chip select signal.
The exact reference is MX25V8006E from Macronix.
Using the free slot
The slot numbered U2 provisions all the pads for an additional memory with SOIC-8 package.
The distance between pins is 0.05millis or 1.28mm, which remains doable even when soldering by hand.
The corresponding chip select signal is exposed by connector number 20, bottom black colour.
The SPI bus requires one single active device at a time. Thus,
if the slot U2 is populated and even if the memory is not used, all the chip select signals shall be HIGH except for the active device where the signal is LOW.
Another point to take care of is the speed of the SPI bus. The Pervasive Displays panels top at 5 MHz, so taking the same speed at 4 MHz for all the devices, panels and memories, is a safe choice.
Using additional Flash
The first option is to use another Flash memory on the U2 pad, for example the same MX25V8006E from Macronix.
This allows to store fons, icons and even recorded pages for later display.
Using additional SRAM
Large screens require large memory for the frame-buffer. for example, the iTC 12.2" BWR needs 180 kB.
Using an external SRAM for the frame-buffer allows greater flexibility. I have tested two SRAM references:
The 1 Mb capacity brings enough space for screens up to 9.7" BWR, while the iTC 12.2" BWR screen requires 2 Mb.
Why not combine Flash and SRAM together? Here comes the FRAM technology, with similar large capacities albeit costlier.
I have explored a 1 Mb or 128 k x8 FRAM with reference FM25V10-PG from Cypress Semiconductors on a separate board.
The main benefits are low-power and data retention, perfect for device with a very limited power budget or prone to power outage.
On the example shown on the right, a MSP430F5529 with only 8 kB of internal RAM drives an iTC 7.4" BWR screen thanks to the external SPI SRAM. The 800x480 iTC 7.4" BWR screen requires 94 kB for the frame-buffer.
With a free slot ready for extra memory, the Pervasive Displays e-Paper EPD Extension Kit Gen 3 brings a nice option for expandability and flexibility.