Hardware¶

What to connect between your MCU and a PCA9685, with which bus settings, and the board-level traps to avoid.

All values here come from the PCA9685 datasheet (Rev. 4, 16 April 2015); page numbers refer to it. Pin numbers are for the TSSOP28 package. When they disagree with a breakout board's schematic, the schematic wins.

Wiring of one PCA9685 with I2C pull-ups and 16 PWM outputs, and two chips sharing one I2C bus at addresses 0x40
and 0x41

Pins¶

Pin	Signal	Function
26	SCL	In: I2C clock; pull-up to VDD
27	SDA	In/out: I2C data; pull-up to VDD
1-5, 24	A0-A5	In: address bits; no pull-ups, tie each high or low
23	OE	In: output enable, active low; no pull-up
25	EXTCLK	In: external clock up to 50 MHz; ground it when unused
6-13	LED0-LED7	Out: PWM outputs
15-22	LED8-LED15	Out: PWM outputs
28	VDD	Supply, 2.3 V to 5.5 V
14	VSS	Ground

HVQFN28 pin numbers

SCL 23, SDA 24, A0 to A4 26, 27, 28, 1, 2, A5 21, OE 20, EXTCLK 22, LED0 to LED7 3 to 10, LED8 to LED15 12 to 19, VDD 25, VSS 11. The exposed pad is also ground and must be soldered.

Source: Table 3, p. 6.

Address¶

The 7-bit address is 1 A5 A4 A3 A2 A1 A0 (Figure 4, p. 8): 0x40 with every address pin low, up to 0x7F with every pin high. Pass it to the constructor: PCA9685<MyI2c> pwm(&bus, 0x41).

Address	Use it?	Why
`0x40` to `0x6F`	Yes	Regular addresses
`0x70`	No	LED All Call address, enabled at power-up: every PCA9685 on the bus answers it
`0x71` to `0x77`	With care	`0x71`, `0x72` and `0x74` are the Sub Call defaults (disabled at power-up)
`0x78` to `0x7F`	No, on a shared bus	Reserved by the I2C specification (10-bit addressing, future use)

The datasheet counts 62 usable addresses (section 7.1, p. 7). The general-call Software Reset (0x00, data byte 0x06) resets every PCA9685 on the bus; the driver does not send it.

Bus settings¶

Setting	Value	Source
Speed	Standard mode 100 kHz, Fast mode 400 kHz, Fast-mode Plus 1 MHz	Table 15, p. 40
Write	START, address + W, register, data bytes, STOP	Figure 20, p. 32
Read	START, address + W, register, repeated START, address + R, data, STOP	Figure 22, p. 33
Multi-byte access	The register pointer advances only when MODE1.AI = 1	Table 5, p. 14
Output update	On STOP (default, MODE2.OCH = 0) or on ACK	Table 6, p. 16
Bus time-out	The interface resets if SCL or SDA is held low for at least 25 ms	Table 15, note 1
SDA drive	30 mA sink, for buses up to 4000 pF	Sections 1 and 2, pp. 1-2

The driver writes a channel as one four-byte transaction starting at LEDn_ON_L and reads single registers. Your port turns each call into exactly one transaction (Porting).

sequenceDiagram
    participant H as Host (driver)
    participant C as PCA9685
    Note over H,C: SetPwmFreq(50)
    H->>C: read MODE1
    C-->>H: MODE1 (SLEEP = 0)
    H->>C: write MODE1 = old | SLEEP (RESTART bit cleared)
    Note over C: oscillator off, PRE_SCALE writable
    H->>C: write PRE_SCALE = 121
    H->>C: write MODE1 = old (SLEEP = 0)
    Note over C: oscillator restarts, up to 500 us
    Note over H: DelayUs(500)
    H->>C: write MODE1 = old | RESTART
    Note over C: running channels resume

Output stage¶

The 16 outputs share one configuration in MODE2 (Table 6, p. 16).

MODE2 bit	Default	Effect	Driver call
`OUTDRV` (bit 2)	1: totem pole	0: open drain	`SetOutputDriverMode(bool totem_pole)`
`INVRT` (bit 4)	0: not inverted	1: outputs inverted while OE is low	`SetOutputInvert(bool invert)`
`OCH` (bit 3)	0: change on STOP	1: change on ACK	none
`OUTNE[1:0]` (bits 1:0)	00: LEDn = 0 while OE is high	01: 1 (totem pole) or high-Z; 1x: high-Z	none

Totem pole sinks 25 mA and sources 10 mA at 5 V; open drain sinks 25 mA. LEDs with built-in Zener diodes must be driven in open-drain mode (Table 6, note 1). For a servo signal or a logic-level gate driver input use totem pole, INVRT = 0.

OE is active low (section 7.4, p. 27). Low: outputs follow their registers. High: outputs take the OUTNE state, by default low. Tie it low, or wire it to a GPIO and drive it with SetOutputEnable() through your port's GpioSet() (Porting).

Power¶

Parameter	Min	Typ	Max	Unit	Source
Supply voltage VDD	2.3		5.5	V	Table 14, p. 38
Supply current, operating, no load, f_SCL = 1 MHz		6	10	mA	Table 14
Standby current (SLEEP, f_SCL = 0)		2.2	15.5	µA	Table 14
Power-on reset threshold V_POR		1.70	2.0	V	Table 14
Output current per LEDn pin			25	mA	Table 13, p. 38
Total ground current I_SS			400	mA	Table 13
Operating temperature	−40		+85	°C	Table 13

To repeat a power-on reset, VDD must fall below 0.2 V (section 7.5, p. 27). Place a 100 nF capacitor between VDD and VSS close to the chip. Loads (servos, LED strings, motors) take their own supply; only the signal and the ground are shared with the PCA9685.

Timing that affects the host¶

Event	Time	Driver behaviour
Oscillator start after MODE1.SLEEP is cleared	500 µs max	`Reset()`, `Wake()` and `SetPwmFreq()` wait 500 µs before they return (Table 5, note 2)
SLEEP low before writing RESTART	500 µs min	`Wake()` and `SetPwmFreq()` write RESTART only after the 500 µs wait (section 7.3.1.1, p. 15)
OE to output	40 ns to 60 ns	n/a (Table 15)

Breakout boards¶

The ESP32 examples run on an ESP32-S3 with a PCA9685 at 0x40: SDA on GPIO4, SCL on GPIO5, 100 kHz, OE not driven. The pins are in esp32_pca9685_test_config.hpp. Common 16-channel servo breakouts fit the I2C pull-ups, a pull-down on OE and solder jumpers for A0 to A5; check the board's schematic.

Reference documents¶

Document	What it is for
PCA9685 datasheet	Pins, registers, I2C protocol, electrical limits
NXP PCA9685 product page	Latest datasheet revision, application notes