Simple question about the peripheral reset code

Question

Hi, 
 
 For a peripheral reset, I was recommended to use the code below. 
 I was just wondering about what the middle line does. Is this a kind of delay to make the reset valid?

*( volatile uint32_t *) 0x40002FFC = 0 ; 
 *(volatile uint32_t*)0x40002FFC; 
 *( volatile uint32_t *) 0x40002FFC = 1 ;

Thank you.

hmolesworth · Accepted Answer

That is the UART power control which is accessed by the AHB->APB bus, which takes lower priority than executing instructions on the AHB bus. The intention of the middle line is to ensure the first instruction has actually completed before the third instruction is issued by performing an intermediate read-back of the power-control register. Does this work as intended? Who knows, some compilers might optimise away the middle line even though the register is volatile. 
 A better solution is to enforce a wait - minuscule in time - until the instruction has completed thus: 
 // In app_uart_close() peripheral is disabled by writing 0 to the NRF_UARTE0->ENABLE
 // register. On 2840 this seems to not stop the HF clk and close the DMA bus, which could be a bug. Power
 // cycle the peripheral after calling app_uart_close() the current consumption goes down.
 *(volatile uint32_t*)0x40002FFC = 0;
 // Data Synchronization Barrier: completes when all explicit memory accesses before this instruction complete
 __DSB();
 *(volatile uint32_t*)0x40002FFC = 1;
 // Data Synchronization Barrier: completes when all explicit memory accesses before this instruction complete
 __DSB(); 
 However also note that the 3-line work-around advice you are using has been superceded as the original high-consumption issue was later found to be a bug in the Nordic driver, if I recall correctly. 
 So, is it still required? Depends on SDK version and probably best to perform a test on the application; high current consumption can be corrected with this work-around even if the work-around is slightly askew.

Simple question about the peripheral reset code

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