LPC5528 RTC wake-up from deep-sleep on cold start issue

[agario]

Hello

Currently having issues while trying to wake-up the part with RTC wake-up timer from deep sleep.

If this occurs first after cold start (i.e. entering sleep first time after power on), the delay before wakeup is abnormally long (around 1.5-2s) regardless of time set to wakeup (in my case i try to set wakeup after 100mS). However if i try to enter sleep first time after brief reset (reset pin held for ~0.5s), without disabling VCC, wakeup occurs at relatively expected time. But if reset pin is held for longer times - sleep time also skews and becomes longer.

When issue occurs it's regardless of RTC clock source - neither XTAL32K, nor FRO32K make any difference and issue can be reproduced on custom board as well as on LPCXpresso55S28 rev A2.

Using SDK 2.11.0

power_manager_lpc.c example only uses RTC alarm which i assume only allows 1 second precision for wakeup, while i need subsecond precision for wakeup. OS Event timer wakeup has similar issue where sleep time occurs the same way. Trying to monitor this issue via debugger only makes sleep work as intended so it's difficult to point out at which stage of POWER_EnterDeepSleep occurs.

In my test project i set up one pin (TEST) to be set high on start, set low before entering sleep and set high after exiting sleep. Other pin is set as CLKOUT and outputs expected 1MHz (FRO12/12) during active mode. 

Am i missing something obvious?

[Daniel-Aguirre]

Hi,

The value of the capacitance on the XTAL is important and needs to be designed as specified by the XTAL supplier (or even by analysis). This will impact the performance of the XTAL. Below will be a blog explaining the importance of this capacitance, and the impact of lowering or increasing the value.

8051 - why capacitors are used with crystal oscillator? - Electrical Engineering Stack Exchange

It seems that the capacitance is indeed affecting your XTAL32K, if using FRO32K gives the expected result.

Please, let us know if this helps you or not.

[agario]

I have re-checked if clocking RTC from FRO32K work and it indeed does work fine - delays work as intended in both cold start and after reset.

I have also looked at capbank API - seems like calling (i have not calculated correct arguments yet) it as it is proposed in application note and makes difference in how long cold start sleep delay will be.

Does capbank trimming impact XTAL32K that much so it can affect initial 32k clocking setup?