PCA9685 16-Channel 12-Bit PWM Driver

Fig. 1. PCA9685 16-Channel 12-Bit PWM Driver, TSSOP-28. Hover or tap a signal to light up every pin that carries it.

• OE (pin 23) is active LOW. Ground it for normal operation. It is a hardware kill for all sixteen outputs at once, which is what you want wired to a panic button rather than left floating.
• EXTCLK (pin 25) must be grounded when you are not feeding it a clock. It is not a pin you can leave open.
• There are no internal pull-ups on A0 to A5. Every address pin has to be driven high or low. Six pins give 64 addresses, of which 62 are usable: Software Reset and LED All Call are both on at power-up and cannot be used as device addresses.
• The PWM frequency is global. All sixteen channels share one prescaler, so you cannot run servos at 50 Hz and dim LEDs at 1 kHz from the same chip.
• PRE_SCALE cannot be written unless the SLEEP bit in MODE1 is set. Writes are silently blocked otherwise, which is why so much servo code mysteriously runs at the 200 Hz default. Sleep, set the prescaler, wake, then wait 500 us before clearing RESTART.
• Each channel has a 12-bit ON time and a 12-bit OFF time, not just a duty cycle. Staggering the ON points spreads the current draw across the cycle instead of switching all sixteen at once.
• Outputs are totem pole by default, sinking 25 mA and sourcing 10 mA at 5 V, and can be switched to open drain in software. That is enough for an LED or a servo signal wire, not for a servo's power. On breakout boards the V+ screw terminal is a separate supply for the loads and is not a pin on the chip.
• Frequency range is about 24 Hz to 1526 Hz from the internal 25 MHz oscillator. An external clock up to 50 MHz on EXTCLK raises that and lets several chips share one timebase.

Table 2. Pin assignments.