EPD

EPD SRAM MEMORY

As explained earlier, the EPD controller's RAM architecture is organized very differently from a TFT framebuffer, and this is the part most likely to trip up code ported from TFT experience.

If the target MCU doesn't have enough internal RAM to hold a full frame buffer — common on smaller 8-bit PICs when driving larger panels —

external SRAM can be used instead. Provided the external SRAM is large enough to hold the frame buffer, you can declare which plane it should be used for:

$define

EpdFrameBuffer

SRAM_1

SRAM_1 - By default used for the B&W plane.

$define

EpdFrameBuffer

SRAM_2

SRAM_2 - By default used for the R/Y plane.

$define

EpdFrameBuffer

SRAM_A

SRAM_A - Automatic mode: the library manages both planes' buffering for you, based on the declared panel type - ($define EpdType BW, $define EpdType BWR, or $define EpdType BWY).
Recommended unless your application has a specific reason to manage buffer placement manually.

The library supports a maximum of two virtual buffer regions within a single external SRAM chip, each sized (EPD width × EPD height) / 8 bytes.
By default, buffer 1 holds the B&W plane and buffer 2 holds the R/Y plane — but this is only a default assignment, not a fixed rule: either buffer can be assigned to hold either plane, and reassigning a buffer or plane at runtime simply redirects subsequent frame buffer operations to a different region within the chip - it does not move or copy any data.

This flexibility is useful in situations such as:

• BW-only content on a BWR panel. If your panel supports red/yellow but a given application or screen doesn't use it, the R/Y buffer would otherwise sit unused. Instead, assign both buffers to the B&W plane and use them as two independent frame slots — for example, to prepare a second screen in advance while the first is still on display, without needing extra external memory.
• Preserving one plane's data while rebuilding another. On a BWR panel using both buffers as intended (one B&W, one R/Y), you may need to modify and resend the B&W plane while the R/Y plane's data in the controller's GRAM stays unchanged and must not be disturbed. Since building a new B&W frame in buffer 1 would overwrite the B&W data currently backing what's on screen, instead: set buffer 2 (normally R/Y) as the working buffer, set B&W as the working plane, and build the new frame there instead. Transfer that to the EPD's B&W GRAM and display it, then switch back to buffer 1 as the working buffer for whatever comes next — modifying it or not as needed — and return to buffer 2 for R/Y work when required. This avoids needing a third buffer just to protect data you don't want touched.

Select the current working buffer:

$define EPD_Buffer_A        0

$define EPD_Buffer_1        1

$define EPD_Buffer_2        2

Global Dim bWorkingBuffer As Byte Shared = EPD_Buffer_A

Proc EpdSetBuffer(bWorkingBuffer As bWorkingBuffer)

'Nothing

EndProc

Select the current working plane

$define EPD_Plane_BW        BLACK

$define EPD_Plane_OLD       BLACK

$define EPD_Plane_RY        RED

$define EPD_Plane_NEW       RED

Global Dim wWorkingPlane As Word Shared = EPD_Plane_BW

Proc EpdSetPlane(wWorkingPlane As wWorkingPlane)

'Nothing

EndProc

EpdSetBuffer selects which of the two virtual buffer regions within the external SRAM chip subsequent frame buffer operations act on; EpdSetPlane selects which logical plane, within that region, those operations write to.

Call these before a drawing operation to redirect it to a different buffer/plane combination than the current default.

Note: EPD_Plane_BW/EPD_Plane_OLD and EPD_Plane_RY/EPD_Plane_NEW are the same underlying values (BLACK and RED respectively) under different names, provided so your code can use whichever naming reads more clearly for the operation at hand — EPD_Plane_BW/RY when thinking in terms of display planes, EPD_Plane_OLD/NEW when thinking in terms of a previous vs. incoming frame.

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