add a section describing limitations

Author: rorosaurus

README.md

@@ -12,7 +12,6 @@
 * The PCB connects all 16 pins needed to drive HUB75 panels using [`ESP32_FORUM_PINOUT`](https://github.com/pixelmatix/SmartMatrix/blob/teensylc/src/MatrixHardware_ESP32_V0.h#L37) from SmartMatrix library.
   * Compatible with 1/8, 1/16, and 1/32 scan type HUB75 panels.
   * Supports multiple HUB75 panels daisy-chained in series! (Verified working with 5 panels!)
-  * There is no logic level shifter, so this is only compatible with 3.3V tolerant LED panels. (I haven't found a panel that doesn't work yet!)
 * [Two different ways to connect to your LED panel](https://github.com/rorosaurus/esp32-hub75-driver/blob/master/ASSEMBLY.md#step-1-mount-output-connector-onto-the-pcb):
   * Output Mode 0: directly plug this PCB into your LED panel using 2x8 female pin headers.
   * Output Mode 1: connect this PCB to your LED panel via 16 pin IDC socket (ribbon cable not included).
@@ -24,6 +23,14 @@
 * Optional pads for one through-hole Electrolytic Capacitor (I use 1000uF) to smooth the `5V` power.
 * Optional pads for one 1206 SMD Ceramic Capacitor (I use 10uF) to enable the automatic bootloader. No need to press buttons to upload new firmware!
 
+## Limitations
+* There is **no logic level shifter** on my PCB, so this is only compatible with 3.3V tolerant LED panels. Personally, I haven't found a panel that doesn't work yet!
+* There is **no external latch IC**, which means compared to other solutions my PCB will use a few more pins and require more memory. In practice, I haven't found this to be particularly problematic unless working with large displays.
+* The resolution of displays you can do is primarily limited by the amount of DMA-capable memory on the ESP32.
+  * I would recommend using 64x32px (2,048px total) or smaller displays if you want to avoid most issues.
+  * Using 64x64px (4,096px total) is possible, but you will start needing to optimize your .gifs with smaller color palletes and fewer frames of animation. With this amount of pixels (or more) stored in DMA-capable memory, you might also have issues getting Wifi to work, since that also requires some DMA-capable memory.
+  * 128x64px (8,192px total) is technically possible, but you'll be limited to a small .gif color pallete and < 20 frames of animation, otherwise you will experience lag/stutter artifacts in the output visuals. To get Wifi working with this size of panel, I [hacked SmartMatrix to remove the double buffer](https://github.com/rorosaurus/SmartMatrix/commit/c46fe8d7be686caaaa3b7198bc4b7b24c6114df8). This results in some screen tears, so I don't recommend this for everyone.
+
 ## Example Projects
 * [FurretTotem](https://ravefurret.com/): HE WALK! This cute little guy just keeps on walking.
 * [Project Mc2 LED Purse](https://github.com/rorosaurus/project-mc2-led-purse): Cheap children's toy you can harvest a good beginner panel from!