Schematics and PCB for an STM32F4-board with external SRAM and micro-SD card.

The board is designed around the STM32F40X ARM Cortex M4 microcontroller in
LQFP-144 package. This package has full support for the built-in external
memory controller (FSMC).

Provision is provided for one SRAM module.

The SRAM chip used is Cypress CY621X7. This chip exists in three different
pin-compatible sizes, which allows flexibility between available memory and
component costs:

  CY62157EV30    0.5M x 16  (1 MByte / IC)   Approx. cost $9
  CY62167EV30      1M x 16  (2 MByte / IC)   Approx. cost $18
  CY62177EV30      2M x 16  (4 MByte / IC)   Approx. cost $47

Thus, up to 4MByte of external SRAM is supported.

A micro-SD card slot is also provided, connected to allow 4-bit 25MHz
operation (total of 100MBit data transfer max).

A micro-USB is connected. Power can come either from USB or from a standard
power jack, selectable by a jumper (an on-board 500mA 3.3V LDO linear voltage
regulator is on the board).

All GPIO are exposed on 4 16x2 dual pin headers. 7 of the 8 16-pin rows are
dedicated to PA0-15, PB0-15, ..., PG0-15. The final row is dedicated to
exposed GND/3.3V/5V pins and 6 pins for ST-LINK V2 / SWD programming (for
programming eg. with STM32F4 Discovery board in external programmer mode).

The board is layed out as four-sided, for ordering @ SeeedStudio. The
gerbers submitted to Seeed are in pcb_sram_rev1.zip.