3.2.2.8. Enhanced capture (eCAP)

3.2.2.8.1. Introduction

eCAP is a dual-mode hardware module:

Auxiliary pulse-width modulator (APWM) Mode: Generates pulse-width modulated (PWM) signals with configurable period and duty cycle
Capture Mode: Records timestamp when input signal transitions occur, used for pulse measurement and event timing

3.2.2.8.2. Hardware overview

eCAP provides a 32-bit free-running counter with up to 4 capture event registers. Key features include:

Single PWM output channel (APWM mode)
Up to 4 capture events per instance
Counter overflow detection
Configurable edge polarity (rising/falling)
Programmable interrupt generation

3.2.2.8.3. Device instances

AM62x family provides 3 eCAP instances:

ecap0 @ 0x23100000
ecap1 @ 0x23110000
ecap2 @ 0x23120000

Each instance is independently configurable for APWM or Capture mode.

3.2.2.8.4. Linux drivers

eCAP hardware module supports dual-mode operation. The DT compatible string determines which driver loads for the eCAP instance. PWM compatible exposes the eCAP instance through the PWM subsystem while capture compatible exposes the eCAP instance through the counter subsystem. Both drivers are enabled by default in TI Linux kernel.

APWM Mode:

Subsystem: PWM
DT Compatible: ti,am3352-ecap
Driver: drivers/pwm/pwm-tiecap.c

Menuconfig driver selection:

Device Drivers --->
   <*> Pulse-Width Modulation (PWM) Support --->
      <*> ECAP PWM support

Capture Mode:

Subsystem: Counter
DT Compatible: ti,am62-ecap-capture
Driver: drivers/counter/ti-ecap-capture.c

Menuconfig driver selection:

Device Drivers --->
   <M> Counter support --->
      <M> TI eCAP capture driver

3.2.2.8.5. Device tree node

APWM Mode:

ecap2: pwm@23120000 {
   compatible = "ti,am3352-ecap";
   #pwm-cells = <3>;
   reg = <0x00 0x23120000 0x00 0x100>;
   power-domains = <&k3_pds 53 TI_SCI_PD_EXCLUSIVE>;
   clocks = <&k3_clks 53 0>;
   clock-names = "fck";
   pinctrl-names = "default";
   pinctrl-0 = <&main_ecap2_pwm_pins_default>;
   status = "okay";
};

Capture Mode:

ecap2: pwm@23120000 {
   compatible = "ti,am62-ecap-capture";
   #pwm-cells = <3>;
   reg = <0x00 0x23120000 0x00 0x100>;
   power-domains = <&k3_pds 53 TI_SCI_PD_EXCLUSIVE>;
   clocks = <&k3_clks 53 0>;
   clock-names = "fck";
   interrupt-parent = <&gic500>;
   interrupts = <GIC_SPI 115 IRQ_TYPE_EDGE_RISING>;
   pinctrl-names = "default";
   pinctrl-0 = <&main_ecap2_capture_pins_default>;
   status = "okay";
};

The pinctrl node is not included here. Refer to the TI device tree overlay for your platform for an example of how to set pincltrl node for eCAP in APWM or Capture mode.

3.2.2.8.6. Pin configuration

TI Overlays set the following pin configuration for an eCAP instance in APWM or Capture mode:

APWM Mode:

PWM overlay: k3-am62x-sk-pwm.dtso

eCAP0: J3 header : pin 26

eCAP1: J3 header : pin 38

eCAP2: J3 header : pin 11

Capture Mode:

Capture overlay: k3-am62x-ecap-capture.dtso

eCAP2: J3 header : pin 11

Warning

Enabling eCAP pins may require disabling peripherals (UART, audio) that use the same pins as eCAP.

3.2.2.8.7. APWM mode

eCAP uses the standard PWM sysfs interface. See PWM guide for sysfs usage (export, period, duty cycle, polarity, enable).

eCAP differences:

Single channel per instance (compared to 2 channels for ePWM)

3.2.2.8.8. Capture mode

Capture mode records input signal transitions. The module timestamps each edge and optionally generates interrupts.

eCAP capture driver exposes counters through the generic counter sysfs interface at /sys/bus/counter/devices/.

Verify counter device availability:

$ ls /sys/bus/counter/devices/
counter0  counter1  ...

Identify eCAP device:

$ cat /sys/bus/counter/devices/counter0/name
ecap
$ cat /sys/bus/counter/devices/counter0/uevent
MAJOR=237
MINOR=0
DEVNAME=counter0
DEVTYPE=counter_device
OF_NAME=pwm
OF_FULLNAME=/bus@f0000/pwm@23120000
OF_COMPATIBLE_0=ti,am62-ecap-capture
OF_COMPATIBLE_N=1

Check counter0 attributes:

$ ls /sys/bus/counter/devices/counter0/count0/
capture0           function           signal0_action
capture1           num_overflows      signal1_action
capture2           ceiling            enable
capture3           count              function_available

Check function and signal actions:

$ cat /sys/bus/counter/devices/counter0/count0/function
increase
$ cat /sys/bus/counter/devices/counter0/signal0/name
Clock Signal
$ cat /sys/bus/counter/devices/counter0/count0/signal0_action
rising edge
$ cat /sys/bus/counter/devices/counter0/signal1/name
Input Signal
$ cat /sys/bus/counter/devices/counter0/signal1/polarity0
positive

function: Counter increments on clock signal
signal0: Clock signal driving counter with rising edge action
signal1: Action configurable input signal driving capture events
polarity0: Sets edge polarity for capture event on input signal

When the Input Signal signal1 edge detection occurs, eCAP automatically captures the current count (TSCNT) value to the next available capture register. Configure capture event edge type through polarity0-3.

Check counter0 ceiling:

$ cat /sys/bus/counter/devices/counter0/count0/ceiling
4294967295

Note that 32 bit ceiling is the maximum value of the counter.

Enable capture:

$ echo 1 > /sys/bus/counter/devices/counter0/count0/enable

Read count:

$ cat /sys/bus/counter/devices/counter0/count0/count
1313052346

Note that count (TSCNT) is a free-running timestamp counter that increments from module clock according to clock signal action. Every rising increments the counter by one.

Read captured timestamps:

$ cat /sys/bus/counter/devices/counter0/count0/capture[0-3]
3008643764
3008591266
3008612306
3008612768

On each edge, eCAP captures current counter value to circular buffer (4 registers):

capture0 (CEVT1): 1st edge timestamp

capture1 (CEVT2): 2nd edge timestamp

capture2 (CEVT3): 3rd edge timestamp

capture3 (CEVT4): 4th edge timestamp

On 5th edge, capture0 is overwritten with a new timestamp. On 6th edge, capture1 is overwritten, and so on.

Read overflow count:

$ cat /sys/bus/counter/devices/counter0/count0/num_overflows
2

Attribute num_overflows increments when counter wraps around (0xFFFFFFFF -> 0x00000000).

Disable capture:

$ echo 0 > /sys/bus/counter/devices/counter0/count0/enable

3.2.2.8.9. Troubleshooting eCAP

Verify PWM functionality:

See troubleshooting in PWM guide and monitor eCAP PWM through /sys/kernel/debug/pwm.

Verify Capture mode enumeration:

$ cat /sys/bus/counter/devices/counterX/name

If not present, verify the overlay applied and check for pin conflicts reported in dmesg log.

Verify capture interrupts are firing:

$ cat /proc/interrupts | grep pwm
409:      36506          0          0          0    GICv3 147 Edge      23120000.pwm

Interrupt count will increment on each capture event or overflow. A high count on the active signal indicates interrupts are working correctly.