Camera modules and basic concepts

This section provides a summarized description of camera modules and their main components. It also highlights the external interface focusing on parallel camera modules.

Basic imaging concepts

This section introduces the imaging field, and gives an overview of the basic concepts and fundamentals, such as pixel, resolution, color depth, and blanking.

Pixel: each point of an image represents a color for color images, or a gray scale for black-and-white photos
A digital approximation is reconstructed to be the final image. This digital image is a two-dimensional array composed of physical points. Each point is called a pixel (invented from picture elements). In other words, a pixel is the smallest controllable element of a picture. Each pixel is addressable. Figure 1 Original versus digital image illustrates the difference between the original image and the digital approximation.
Figure 1. Original versus digital image
Resolution: number of pixels in the image.
The more the pixel size increases, the more the image size increases. For the same image size, the higher the number of pixels is, the more details the image contains.
Color depth (bit depth): the number of bits used to indicate the color of a pixel (is also given in bits per pixel, bpp)
Examples:
- For a bitonal image, each pixel comprises one bit. Each pixel is either black or white (0 or 1).
- For a gray scale, the image is most of the time composed of 2 bpp (each pixel can have one of four gray levels) to 8 bpp (each pixel can have one of 256 gray levels).
- For color images, the number of bits per pixel varies from 8 to 24 (each pixel can have up to 16777216 possible colors).
Frame rate (for video): number of frames (or images) transferred each second, expressed in frame per second (FPS).
Horizontal blanking: ignored rows between the end of one line and the beginning of the next one.
Figure 2. Horizontal blanking illustration
Vertical blanking: ignored lines between the end of the last line of a frame and the beginning of the first line in the next frame.
Figure 3. Vertical blanking illustration
Progressive scan: a manner of dealing with moving images
The lines can be drawn one after the other in a sequence without separating the odd lines from the even ones, as for interlaced scan. To construct the image:
- In progressive scan, the first line is drawn, then the second, and, finally, the third
- In interlaced scan, each frame is divided into two fields (odd and even lines), which are displayed alternately

Camera module

A camera module consists of four parts: image sensor, lens, printed circuit board (PCB), and interface. Figure 4 Camera module examples shows some common camera modules examples.

Camera module components

The four components of a camera module are described below.

Image sensor

It is an analog device used to convert the received light into electronic signals. These signals convey the information that constitutes the digital image.

There are two types of sensors that can be used in digital cameras:

CCD (charge-coupled device) sensors
CMOS (complementary metal-oxide semiconductor) sensors

Both types convert the light into electronic signals, but each has its own conversion method. As their performance continually evolves and their cost decreases, CMOS imagers now dominate the digital photography landscape.

Lens

It is an optic, which allows the reproduction of the real image captured rigorously on the image sensor. Picking the proper lens is part of the user creativity, and affects considerably the image quality.

Printed circuit board (PCB)

It is a board that comprises electronic components to ensure the good polarization and the protection of the image sensor. The PCB also supports all the other parts of the camera module.

Camera module interconnect

The camera interface is a kind of bridge that allows the image sensor to connect to an embedded system, and to send/receive signals. The following signals are transferred between a camera and an embedded system:

Control signals
Image data signals
Power supply signals
Camera configuration signals

The camera interfaces are divided into two types: parallel and serial interfaces, depending on the method to transfer data signals.

Camera module interconnect (parallel interface)

As mentioned above, a camera module requires four main types of signals to transmit image data properly: control signals, image data signals, power supply signals, and camera configuration signals. Figure 5 Interfacing a camera module with an STM32 MCU illustrates a typical block diagram of a CMOS sensor, and the interconnection with an MCU.

Figure 5. Interfacing a camera module with an STM32 MCU

Control signals: used for clock generation and data transfer synchronization
The camera clock must be provided according to the camera specification. The camera also provides two data synchronization signals: HSYNC (for horizontal/line synchronization) and VSYNC (for vertical/frame synchronization).
Image data signals: each of them transmits a bit of the image data.
Their width represents the number of bits to be transferred at each pixel clock. This number depends on the parallel interface of the camera module, and on the embedded system interface.
Power supply signals:
As any embedded electronic system, the camera module needs to have a power supply. The operating voltage of the camera module is specified in its datasheet.
Configuration signals: used for the following:
- To configure the appropriate image features such as resolution, format, and frame rate
- To configure the contrast and the brightness
- To select the type of interface. (A camera module can support more than one interface: a parallel and a serial interface. The user must then choose the most convenient one for the application.)

Most camera modules are parametrized through an I²C communication bus.

STM32 DCMI overview

This section gives a general preview of the DCMI availability across the various STM32 devices, and gives an easy-to-understand explanation on the DCMI integration in the STM32 MCUs architecture.

The DCMI is a synchronous parallel data bus, which is used for an easy integration and easy adaptation to specific application requirements. The DCMI connects with 8-, 10-, 12-, and 14-bit CMOS camera modules, and supports a multitude of data formats.

DCMI availability and features across STM32 MCUs

Table 2 Availability of DCMI and related resources summarizes the STM32 devices embedding the DCMI, and highlights the availability of other hardware resources that facilitate the DCMI operation, or that can be used with the DCMI in the same application.

The DCMI applications need a frame buffer to store the captured images. It is then necessary to use a memory destination that varies depending on the image size and the transfer speed.

In some applications, it is necessary to interface with external memories that offer large sizes for data storage. Quad-SPI can be used in this case. For more details, refer to the application note Quad-SPI interface on STM32 microcontrollers and microprocessors (AN4760).

The DMA2D (Chrom-ART Accelerator controller) is useful for color space transformation (such as RGB565 to ARGB8888), or for data transfer from one memory to another.

The JPEG codec allows data compression (JPEG encoding) or decompression (JPEG decoding).

Table 2. Availability of DCMI and related resources
STM32 line	Max flash memory size	On-chip SRAM (Kbytes)	QUADSPI	OCTOSPI	HSPI	XSPI	Max FMC^¹ SRAM and SDRAM frequency (MHz)	Max DCMI pixel clock input (MHz)^²	JPEG codec	DMA2D	LCD_ TFT controller ^³	LCD parallel interface	MIPI DSI host^⁵	Max AHB frequency (MHz)
STM32F2x7	1 Mbyte	128	No	No	No	No	60	48	No	No	No	No	No	120
STM32F407 STM32F417	1 Mbyte	192	No	No	No	No	60	54	No	No	No	No	No	168
STM32F427 STM32F437	2 Mbytes	256	No	No	No	No	90	54	No	Yes	No	No	No	180
STM32F429 STM32F439	2 Mbytes	256	No	No	No	No	90	54	No	Yes	Yes	No	No	180
STM32F446	512 Kbytes	128	Yes	No	No	No	90	54	No	No	No	No	No	180
STM32F469 STM32F479	2 Mbytes	384	Yes	No	No	No	90	54	No	Yes	Yes	No	Yes	180
STM32F7x0	64 Kbytes	320	Yes	No	No	No	100	54	No	Yes	Yes	No	No	216
STM32F7x5	2 Mbytes	512	Yes	No	No	No	100	54	No	Yes	No	No	No	216
STM32F7x6	1 Mbyte	320	Yes	No	No	No	100	54	No	Yes	Yes	No	No	216
STM32F7x7	2 Mbytes	512	Yes	No	No	No	100	54	Yes	Yes	Yes	No	No	216
STM32F7x8 STM32F7x9	2 Mbytes	512	Yes	No	No	No	100	54	Yes	Yes	Yes	No	Yes	216
STM32H723/733	1 Mbytes	564	No	Yes	No	No	137	110	No	Yes	Yes	No	No	240
STM32H725 STM32H735	1 Mbyte	564	No	Yes	No	No	137	110	No	Yes	Yes	No	No	275
STM32H742 STM32H743 STM32H753	2 Mbytes	864	Yes	No	No	No	125	80	Yes	Yes	Yes	No	No	240
STM32H745 STM32H755	2 Mbytes	864	Yes	No	No	No	125	80	Yes	Yes	Yes	No	Yes	240
STM32H747 STM32H757	2 Mbytes	864	Yes	No	No	No	110	80	Yes	Yes	Yes	No	Yes	240
STM32H730	128 Kbytes	564	No	Yes	No	No	137	110	No	Yes	Yes	No	No	275
STM32H750	128 Kbytes	864	Yes	No	No	No	100	80	Yes	Yes	Yes	No	No	240
STM32H7A3 STM32H7B3	2 Mbytes	1180	No	Yes	No	No	100	80	Yes	Yes	Yes	No	No	280
STM32L4x6	1 Mbyte	320	Yes	No	No	No	40	32	No	Yes	No	No	No	80
STM32L4R9 STM32L4S9 STM32L4R7 STM32L4S7 STM32L4R5 STM32L4S5	2 Mbytes	640	No	Yes	No	No	60	48	No	Yes	Yes	No	Yes	120
STM32L4P5 STM32L4Q5	1 Mbyte	320	No	Yes	No	No	60	48	No	Yes	Yes	No	No	120
STM32U575/585	2 Mbytes	786	No	Yes	No	No	80	64	No	Yes	No	Yes	No	160
STM32U535/545	512 Kbytes	274	No	Yes	No	No	No	64	No	Yes	No	No	No	160
STM32U595/5A5 STM32U599/5A9	4 Mbytes	2514	No	Yes	Yes	No	80	64	No	Yes	Yes	Yes	Yes	160
STM32U5F7/5G7 STM32U5F9/5G9	4 Mbytes	3026	No	Yes	Yes	No	80	64	Yes	Yes	Yes	Yes	Yes	160
STM32H533/523	512 Kbytes	272	No	Yes	No	No	100	100	No	No	No	Yes	No	250
STM32H563/573 STM32H562	2 Mbytes	640	No	Yes	No	No	100	100	No	No	No	Yes	No	250
STM32N647/657	1 Kbyte	4200	No	No	No	Yes	166	100	Yes	Yes	Yes	No	Yes	200

FSMC for STM32F2x7, STM32F407/417, STM32L4+, and STM32U5 devices.

Refer to the datasheet for the pixel clock frequency (DCMI_PIXCLK).

See the application note AN4861 for more details on the STM32 LTDC peripheral.

⁴ LCD parallel interface via the FMC parallel interface.

Refer to the application note AN4860 for more details on the STM32 MIPI-DSI host.

Camera modules and basic concepts