Posted on December 26, 2014 at 13:07In 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.
