SD Memory Card Reader Interface
This device lets you 'talk' to SD cards through your serial port. Communication is in ASCII so you just need a terminal program to access it.
The PIC16F819 was chosen for its hardware support for SPI. (SD cards will usually work through an SPI interface) Running at 20Mhz, the SPI port clocks data at 5Mhz, and the serial communication is able to run at 115200 baud in software.
A quick look at the schematic shows one way to interface the 5 volt PIC to the 3.3 volt SD card. A red LED drops the 5 volt supply by about 1.8 volts to feed about 3.2 volts into the card. This is within the operating range.
Signals from the PIC go through a 1k/2k voltage divider to feed signals to the card. Signals back from the card feeding SDI input are a problem because the PIC uses schmit-trigger inputs in SPI mode requiring 3.5 volts for the high level. The circuit provides a 0.6 volt shift so the output of the card back to the PIC ranges from 0.6 to 3.8 volts. Cheesy, but it works.