OV5640 HREF/VSYNC polarity vs STM32H747 DCMI polarity mismatch — why does it still work?
I'm using the STM32H747 DCMI peripheral to capture video from an OV5640 camera, and I've encountered a confusing situation regarding synchronization signal polarity.
Development Environment
Board: STM32H747I-DISCO
Camera Module: B-CAMS-OMV (OV5640)
Firmware: Official example project
STM32CubeH7/Projects/STM32H747I-DISCO/Examples/DCMI/DCMI_CaptureMode/CM7/Src/main.c
OV5640 Configuration
In the OV5640 register 0x4740 (POLARITY CTRL00), I configure:
{OV5640_POLARITY_CTRL, 0x22}According to the datasheet, this means:
Bit[5] = 1 → PCLK polarity: Active high
Bit[1] = 1 → HREF polarity: Active high
Bit[0] = 0 → VSYNC polarity: Active low
So the camera outputs:
HREF high during valid line data
VSYNC low during valid frame data
PCLK rising edge for pixel sampling
STM32 DCMI Configuration
In the STM32H747 DCMI_CR register, I configure:
DCMI->CR |= (1 << 7) | (1 << 6); // VSPOL = 1, HSPOL = 1
According to the STM32 reference manual:
VSPOL = 1 → VSYNC high = invalid frame
HSPOL = 1 → HSYNC high = invalid line
This implies:
VSYNC low = valid frame → matches OV5640 :white_heavy_check_mark:
HSYNC low = valid line → contradicts OV5640 HREF high
:question_mark: The Confusion
I connect OV5640's HREF signal to STM32's DCMI_HSYNC pin. According to the polarity settings:
OV5640 outputs HREF high during valid line
STM32 expects HSYNC low during valid line (HSPOL = 1)
This seems mismatched. Yet surprisingly:
Image capture works perfectly with this configuration
If I change HSPOL to 0 (i.e. HSYNC low = invalid), capture fails
What I Need Help With
Why does this polarity mismatch still result in correct image capture?
Are there known quirks or undocumented behaviors in DCMI signal handling?