Cannot sync accurate time

[Pansamic]

Detailed Description

I use rmw_uros_sync_session() to sync time with Micro-ROS-Agent. But when I use rmw_uros_epoch_millis() and rmw_uros_epoch_nanos() to get time, I got seconds starting from 0, not 170xxxxxx (Linux timestamp seconds) as expected.

Thinking

This result was no surprise because I cannot find any API that can change local time of MCU, while there is an API to get time called clock_gettime() in micro_ros_stm32cubemx_utils/extra_sources/microros_time.c.

I want Micro-ROS to set local time that high accuracy hardware timer ( DWT, TIM or RTC ) holds, rather than FreeRTOS ticks.

it's said that rmw_uros_sync_session() uses NTP to sync time so it must has high accurate and less latency, but it cannot function properly, pity.

Trials

My clock_gettime() is now modified to set RTC time. Micro-ROS subscribes /time_reference topic from Micro-ROS-Agent and transform timestamp to RTC Binary format. I use the following code to get current timestamp and it works well (superficially). This function seems to call clock_gettime() and get time from RTC. Actually this method introduces huge latency.

rcutils_time_point_value_t current_time;
rcutils_ret_t ret;
ret = rcutils_system_time_now(&current_time);
if(ret != RCUTILS_RET_OK)
    printf("[ERROR]read systime failed!\r\n");
msg->header.stamp.sec = RCUTILS_NS_TO_S(current_time);
msg->header.stamp.nanosec = (uint32_t)current_time;

Question

1. How does Micro-ROS set time?
2. Can maintainers expose some API to set local time?

[pablogs9]

How does Micro-ROS set time?

rmw_uros_sync_session will not change your local time it just stores a time offset of the XRCE session that is used when rmw_uros_epoch_nanos and rmw_uros_epoch_millis are called.

Can maintainers expose some API to set local time?

Out of micro-ROS scope.

[bnbhat]

Hi @Pansamic,

Were you able to find a solution for the time synchronization issue? I'm facing a similar challenge, as I need to timestamp all the messages published by a micro-ROS node running on an MCU.

@pablogs9, could you please advise on the recommended approach for timestamping messages within a micro-ROS node?

Best Regards

[bnbhat]

If anyone stumbles upon this issue looking for a way to timestamp messages in micro-ROS, here's a solution that worked for me:

1. Synchronize the Time with the Agent

#include <rmw_microros/rmw_microros.h>

// Sync timeout
const int timeout_ms = 1000;

// Synchronize time with the agent
rmw_uros_sync_session(timeout_ms);

2. Get the Synchronized Time

#include <rmw_microros/rmw_microros.h>

// Get corrected epoch time 
int64_t time_ns = rmw_uros_epoch_nanos();

// Convert to required format
msg.header.stamp.sec = (int32_t)(time_ns / 1000000000);
msg.header.stamp.nanosec = (uint32_t)(time_ns % 1000000000);

[l4es]

Hi @bnbhat,

Does one need to call rmw_uros_sync_session() everytime prior rmw_uros_epoch_nanos() for filling the timestamp of the topic being published ?

In the following example, rmw_uros_sync_session() is called once when the agent is available : https://github.com/adityakamath/akros2_firmware/blob/akros2_humble/akros2_firmware.ino#L331

And the time offset is calculated for being used later (filling the timestamp of published topics) : https://github.com/adityakamath/akros2_firmware/blob/akros2_humble/akros2_firmware.ino#L554

[bnbhat]

Hi @l4es

You don't need to call rmw_uros_sync_session() every time as it stores the offset. But you can call it once in a while to sync time if you want to.

When you call rmw_uros_epoch_nanos(), it adds the stored offset to the system time and gives you the synced time.

[flux0-0]

hi @bnbhat , If I pio run my code same you about rmw_uros_sync_session(timeout_ms) and int64_t time_ns = rmw_uros_epoch_nanos();, I have error "collect2: error: ld returned 1 exit status" . Have you get similar problem before?

[bnbhat]

Hi @l4es , could you share your code? or more details on your setup?

have you synced the time before calling rmw_uros_epoch_nanos() ?

[flux0-0]

This is my code " https://github.com/flux0-0/test_microros_time/blob/main/main.cpp " . I tried "pio run " in platform IO and I have " collect2: error: ld returned 1 exit status
*** [.pio/build/esp32doit-devkit-v1/firmware.elf] Error 1"