Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

[daten]

I'm experiencing a freeze - at least from user's perspective - when using a slightly modified "usbd_cdc_acm"-example and applying some human generated load (meaning: hitting the keyboard for a couple of seconds, incl. '\n' every now and then in order to reset the buffer / break out of the loop (see code below)).
Sooner or later it's going to freeze at the point right after the `break`.

Hardware is a nRF52840 USB dongle, SDK nRF5 SDK v16.0.0, SoftDevice s140_nrf52_7.0.1

The following diff to mentioned example code will render the nrf52840 unresponsive, meaning, it's stuck and doesn't loop through the main loop anymore.

diff --git a/examples/peripheral/usbd_cdc_acm/main.c b/examples/peripheral/usbd_cdc_acm/main.c
index c78f24cc..1ebc20bf 100644
--- a/examples/peripheral/usbd_cdc_acm/main.c
+++ b/examples/peripheral/usbd_cdc_acm/main.c
@@ -133,7 +133,8 @@ APP_USBD_CDC_ACM_GLOBAL_DEF(m_app_cdc_acm,

#define READ_SIZE 1

-static char m_rx_buffer[READ_SIZE];
+static uint8_t m_rx_buffer_pos = 0;
+static char m_rx_buffer[512];
 static char m_tx_buffer[NRF_DRV_USBD_EPSIZE];
 static bool m_send_flag = 0;

@@ -172,12 +173,21 @@ static void cdc_acm_user_ev_handler(app_usbd_class_inst_t const * p_inst,
 {
 /*Get amount of data transfered*/
 size_t size = app_usbd_cdc_acm_rx_size(p_cdc_acm);
- NRF_LOG_INFO("RX: size: %lu char: %c", size, m_rx_buffer[0]);
+ NRF_LOG_INFO("RX: size: %lu - char: %d - pos: %d", size, m_rx_buffer[m_rx_buffer_pos], m_rx_buffer_pos);

/* Fetch data until internal buffer is empty */
 ret = app_usbd_cdc_acm_read(&m_app_cdc_acm,
- m_rx_buffer,
+ &m_rx_buffer[++m_rx_buffer_pos],
 READ_SIZE);
+
+ if(m_rx_buffer[m_rx_buffer_pos-1] == '\r')
+ {
+ m_rx_buffer[++m_rx_buffer_pos] = '\0';
+ NRF_LOG_INFO("Transmission finished - resetting buffer");
+ m_rx_buffer_pos = 0;
+ break;
+ }
+
 } while (ret == NRF_SUCCESS);

bsp_board_led_invert(LED_CDC_ACM_RX);
@@ -296,6 +306,7 @@ int main(void)

ret = NRF_LOG_INIT(NULL);
 APP_ERROR_CHECK(ret);
+ NRF_LOG_DEFAULT_BACKENDS_INIT();

ret = nrf_drv_clock_init();
 APP_ERROR_CHECK(ret);

According to my debugger it appears to be stuck at __WFE(), however I'm not sure how much I can trust it in that state, also given I've seen it showing me various locations when hitting ^C at the moment of freeze:

Program received signal SIGINT, Interrupt.
0x000311b4 in __WFE () at ../../../../../../components/toolchain/cmsis/include/cmsis_gcc.h:396
396 __ASM volatile ("wfe");

Last line of output from above modified example after hitting the keyboard like a monkey:

<info> app: RX: size: 1 - char: 116 - pos: 1
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 13 - pos: 2
<info> app: Transmission finished - resetting buffer
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 32 - pos: 0
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 32 - pos: 1
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 13 - pos: 2
<info> app: Transmission finished - resetting buffer
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 32 - pos: 0
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 104 - pos: 1
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 13 - pos: 2
<info> app: Transmission finished - resetting buffer
<info> app: Bytes waiting: 0
<info> app: RX: size: 1 - char: 32 - pos: 0
<info> app: Bytes waiting: 1
<info> app: RX: size: 1 - char: 13 - pos: 1
<info> app: Transmission finished - resetting buffer

I'm chasing the root cause for a while now and herewith tried to break and narrow it down to a minimal example triggering the issue, hoping somebody else might be able to spot the flaw.

[hmolesworth]

I am having issues with something similar, and pushed it to one side. In this case the example only reads a singe byte at a time (what a strange example ..) and I see you modified that to attempt to read more, however this is now an issue:

ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[++m_rx_buffer_pos], READ_SIZE);

I would suggest trying:

m_rx_buffer_pos += READ_SIZE;
ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[m_rx_buffer_pos], READ_SIZE);

or:

ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[m_rx_buffer_pos], READ_SIZE);
m_rx_buffer_pos += READ_SIZE;

Assuming each invocation reads READ_SIZE bytes from the USN buffer

[daten]

Hello and thanks for your reply. I could change the code accordingly (and adjust READ_SIZE to sth. like 8 I suppose), however that rather sounds like the issue occurring "less likely" due to less loop cycles, as in hiding it instead of solving the issue.

The freeze is a state I don't know how to recover from and I can't risk it happening - even if it's "less likely" due to reading >1byte per loop cycle.

So I'd really like to know what's happening and how to get rid of the freeze for good.

[daten]

After rethinking the issue, its potential causes and occurrences, I don't think increasing READ_SIZE would help much, if at all:

a) Looking at the log output I posted the freeze in this very case already occurred after 2 loop cycles, which really isn't a lot, so even if I heavily increase READ_SIZE I most likely will still have more than 1 loop cycle.

b) The freeze seems not to happen within the loop, but when / after leaving it. Potentially related to re-setting the buffer array index to 0.