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take care: inverted DEPolarity at LTDC!!

Question asked by Tamas Novak on Dec 26, 2014
In LTDC_GCR register of LTDC at STM32F429   HSYNC, VSYNC and DE signal polarity may be chosen:

Bit 31 HSPOL: Horizontal Synchronization Polarity
This bit is set and cleared by software.
0: Horizontal Synchronization polarity is active low
1: Horizontal Synchronization polarity is active high
Bit 30 VSPOL: Vertical Synchronization Polarity
This bit is set and cleared by software.
0: Vertical Synchronization is active low
1: Vertical Synchronization is active high
Bit 29 DEPOL: Data Enable Polarity
This bit is set and cleared by software.
0: Data Enable polarity is active low
1: Data Enable polarity is active high

If you set all 3 bits to 0, then VSYNC an HSYNC signals are "0", but DE signal will be "1".
It's a question of representation: I think the DE signal is "active" while sending valuable pixels, and "not active" at the time of sync and back/frontporch. ST's representation is different: think of this signal as "Data Not Valid".  So

hltdc.Init.DEPolarity = LTDC_DEPOLARITY_AL;  //active low

means that DE signal is 1 while valuable pixels are being sent, and 0 when front+backporch and sync signals active.

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