TWIM EasyDMA Delay Between Bytes

[snoopy20]

Hi,

I'm writing a library for the common Hitachi_HD44780 (LCD2004 etc) but need it to be non-blocking. I'll open source it once done. I'm using EasyDMA to do the transfers but the aging HD44780 needs a 50us delay in-between commands. My idea so far is;

1. set a timer with a period of 50us and a counter equal to the amount of bytes with a PPI to halt the timer and fire an interrupt to indicate "done"
2. timer period event fires EasyDMA which transfers one byte.

My question then is around EasyDMA List mode. Can I clarify that what it's doing is adding MAXCNT to the PTR object after each send?

In other words if I set MAXCNT = 1 and use list mode I should get the spaces inbetween the bytes without needing to interrupt to PTR++?

[snoopy20]

I've determined that no interrupt handler is hit. I thought these were picked up by ses_startup_nrf52.c and can't see any other reason why they're being ignored.

[snoopy20]

Moving back, I have an issue that the PPI isn't stopping the first timer. I'm wondering if by having a short on the compare (to stop and reset) that prevents the event reaching the PPI?

[haakonsh]

snoopy20 said:

I'm wondering if by having a short on the compare (to stop and reset) that prevents the event reaching the PPI?

 Not that I'm aware of, you should still get a COMPARE event event though you shorted it to the STOP and CLEAR tasks. You can try to disable one of the shorts and see if that changes anything.

[snoopy20]

Yupp, just done that and still no-go. What's odd is the interrupt on the same compare is firing, so that side is good, it's just not making it's way through PPI.

NRF_PPI->CH[CFG_PPI_LCD_CHANNEL_COUNTER_RESET].EEP = (uint32_t) &CFG_TIMER_LCD_COUNTER->EVENTS_COMPARE[0]; // wire timer compare
NRF_PPI->CH[CFG_PPI_LCD_CHANNEL_COUNTER_RESET].TEP = (uint32_t) &CFG_TIMER_LCD_SPACE->TASKS_STOP;
NRF_PPI->FORK[CFG_PPI_LCD_CHANNEL_COUNTER_RESET].TEP = (uint32_t) &CFG_TIMER_LCD_SPACE->TASKS_CLEAR;

To confirm when in COUNTER mode there's no means to stop the timer (TASK_START) doesn't need to be set? This potentially means the compare is always active and if so I''ll need to disable the interrupt (could be why it's refiring).

[haakonsh]

Can you read out the registers of the PPI channel you're using? 

Which timer instance are you using?

[snoopy20]

TIMER4 in counter mode. TIMER3 for the spacing.

The interrupt fires and continues to fire, I'm unable to shut it down. 

Even when in the interrupt I do

  CFG_TIMER_LCD_SPACE->TASKS_STOP = 1;
  CFG_TIMER_LCD_COUNTER->TASKS_STOP = 1;
  CFG_TIMER_LCD_COUNTER->TASKS_CLEAR = 1;
  CFG_TIMER_LCD_COUNTER->CC[0] = 900;

The PPI should have already stopped TIMER3 and cleared TIMER4 but the above would ensure it. Weirdly the interrupt continues!

[snoopy20]

Oh, when EVENTS_COMPARE[X] fires it must be cleared in software for the interrupt, but for the PPI it's a single pulse?

[haakonsh]

Yes!

If you don't need the interrupt you can disable it, the event will still function.

[snoopy20]

Yup that's got it! Right, cooking with gas now. Thanks!