Basic electrical info: GPIOs can sink or source up to ±8 mA, and sink or source up to ±20 mA (with a relaxed VOL/VOH)
except PC13, PC14 and PC15 which can sink or source up to ±3mA.
When using the PC13 to PC15 GPIOs in output mode, the speed should not exceed 2 MHz with a maximum load of 30 pF.
For more I/O static and AC characteristics refer to the product spec pdf page 109.
原文：http://www.cnblogs.com/shangdawei/p/4793301.html