This is fully self-contained development board, ATtiny45 was pre-programmed with SD Card bootloader (using 3 pins and SD mode, not SPI mode!). This bootloader was compiled with AVR Basic Compiler, and did fit into 1KByte, leaving 3KB available for the application.
PCB is single layer, uses v-scoring panelization. Was produced in Taiwan. If I recall correctly the self cost was below 2 USD even if produced in relativly small batches.
The bootloader uses NoFAT(tm) file system, so it can use real files on any filesystem, without the need to actually recognize or parse the filesystem.
is there any source codes or details? Thanks