I just bought an MCP23017 port expander and hooked it up to my an Arduino card sample code . I used the to turn on four LEDs when the program launches. Works great, except that after several minutes all the LEDs switch off and won't come back on until I actually power down the entire card and plug it back in (hitting reset won't cut it). I've checked and rechecked and everything else looks fine. Is it possible I've got a bad I2C chip? Then how to test mcp23017?