Question:
Is there a reason that the MAX7221 can lockup at power on, with all outputs on? All LED segments are illuminated, and the device does not respond to serial interface commands. The only way to "reset" the IC is to remove power and then restore it. This startup behavior is intermittent but repeatable. What can cause this?
Answer:
There are two things to consider. One is the power supply rise time--a slow power supply slew rate is not good, faster is better. Power supply rise times of around 1ms are good. The other consideration is the SPI interface lines at power-up--they should not be left floating or high impedance during the MAX7221 power-up. The MAX7221 can latch up on power-up if the digital inputs are left floating or unconnected; this occurs if the uC used with this part has the I/O lines not configured at power-up. Connect a 10kΩ resistor to ground at DIN, CS, and CLK to ensure the MAX7221 powers up with the interface pins held low.