Power control and the watchdog

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by rfk on Wed Aug 07 06:06:14 MST 2013

currently I am working with an LPC1778 based board and want to implement power control. There are 2 SDRAM chips connected to the MCU. On the LPC2214 predecessor board I was using Power-down mode so I wanted to port that functionality to the new board. Unfortunately it seems like the LPC1778 only enters Sleep mode, no matter what's configured in the PCON and SCR registers. My assumption is based on the following facts:

  [*] Depending on the MCU frequency (CCLK) the current consumption of the board only changes relatively, e.g. the Deep Power-down current is higher when powering down from 120 MHz rather than from 30 MHz.
  [*] The standby current consumption of the different modes is always the same, it doesn't matter whether the SCR/PCON registers are configured to Sleep, Deep Sleep, Power-down or even Deep Power-down.
  [*] After wake-up (caused by an interrupt) it is not neccessary to reinitialize the PLL, also the timers remain running all the time.

Any thoughts on that?

Another issue is that a configured watchdog resets the system during "standby". I could send the ARM7-MCU into Power-down mode for several minutes with an active watchdog (configured to a few seconds). After wake-up the watchdog was immediately fed and the system continued to run. Maybe this issue is related to the problem above but according to the user manual the watchdog timer continues to run even in Power-down mode. Do I really have to permanently wake up before the timer expires, feed the watchdog and then power-down again?

Thanks in advance.