Raspberry-Pi Pico OV7670 Camera Library

This library can be used to capture images from an OV7670 camera on the Raspberry Pi Pico, using PIO and DMA.

It's design for use with a 74165 shift register, to convert the 8-bit parallel data to a 2-bit serial interface, but directly connecting the parallel data bus directly to the Pico would also be possible with a slight modification to the PIO code.

The PIO portion uses 2-4 state machines from a single PIO instance, depending on the number of planes in the pixel format. The PIO program(s) use 26 of the 32 bytes of PIO program memory.

SM0

State machine 0 handles the "frame"-level timing. It receives the number of rows and columns via the FIFO, then waits for VSYNC (start of frame). Then, for each line it waits for HSYNC (start of line) and triggers the "pixel" state-machine(s) for each byte of data.

SM1-3

The other state machines handle each byte of data. They will wait for the pixel to get latched into the 74165 shift register (based on PXCLK), then shift the data into the PIO ISR, to be pushed into the output FIFO.

A different state machine is used for each "plane" of image data.

A different number of bits gets stored in the output FIFO depending on the pixel format. For example, for the single-plane YUYV/RGB565 formats, 4 bytes at a time are transferred (representing 2 pixels). For 3-plane YUV422, on the 'Y' state machine, each transfer is 16 bits (two 'Y' samples), wheras for the 'U' and 'V' state machines, a single byte is transferred at a time.

DMA

The DMA is configured to read from each state machine's input FIFO and transfer the data to the destination buffer.

OV7670 Driver

The OV7670 sensor is quite poorly documented - there are datasheets out there and an integration guide, but it leaves out lots of details and there's a huge number of undocumented (or even reserved) registers which need to be configured to get a picture out of the camera.

This library is using Adafruit's OV7670 driver from https://github.com/adafruit/Adafruit_OV7670.git

I'm aware that there's actually a Pico implementation in that repo, but I had already written most of my code before I found it. Also, the "arch" they have doesn't quite fit with how I want to integrate the code.

So, I've just taken the low-level bits (largely, the reference register settings), and hacked it up/wrapped it with my camera code.

This could definitely be done better, but I'm in a hurry and this is working :-).

Circuit

The code in this repo assumes a specific wiring.

The base_pin and xclk_pin can be specified in camera_platform_config, other pins are relative to base_pin.

xclk_pin must be one of the GPOUT clock sources (21, 23, 24, 25).

The i2c bus can be connected however you like - just pass in appropriate functions in camera_platform_config (it's done this way so that the i2c bus can be shared amongst different devices).

PCLK is used to latch the shift register, so it's connected to both the 74165 and the Pico.

OV7670 Module	74165	Pico	Pico (example)
SIOC	-	User decision	1
SIOD	-	User decision	0
VSYNC	-	base_pin + 2	18
HREF	-	base_pin + 3	19
PCLK	SH/nLD	base_pin + 4	20
XCLK	-	User decision	21
D7	A	-
D6	B	-
D5	C	-
D4	D	-
D3	E	-
D2	F	-
D1	G	-
D0	H	-
-	CLK	base_pin + 1	17
-	Qh	base_pin	16
-	CLK_INH	GND

Shortcomings

I've written this for a very specific use case (my Pi Wars robot), so I haven't put much effort into making it very flexible. I'm publishing it as a separate library because I think it could be a good base to work from.

Many of the shortcomings below could be quite easily fixed/improved.

• Resolution is fixed to 80x60
  • The Adafruit code can support a few different resolutions, it would be easy to add support for those. Totally arbitrary resolutions would be harder.
• Only includes PIO code for the shift-register implementation
  • A new "byte" program which uses a parallel input would be easy to add. I've tried that setup during my development, but the code changed a lot since then.
• Fudged timings/delay cycles.
  • I've tweaked some delay numbers to make it work on my set-up with a quite long cable between the camera and the Pico
  • My logic analyser isn't fast enough, and my oscilloscope is too dated to look at the signals and make some better judgement here.
• No control of frame-rate/PLL frequency
  • The register settings just set the OV7670 frequency to 1x the XCLK frequency.
• Could use more efficient transfer sizes
  • Could always do 4 byte DMA transfers as long as the buffer sizes are properly aligned to give multiples of 4 samples.
• Pretty lax error checking/handling.
• camera_term not implemented.

Anyway, if you just want 80x60 RGB565, YUYV or YUV422 images, this should work great :-)

Example

The example directory has a simple example which captures a frame then writes an ASCII representation of the Luma (Y) plane to stdio_usb. The character map is designed to look best using light text on a dark background.

Here's an example picture of a cat ornament:

+++++************************************************+++++++<
+*******************************************************++++<
********************************************************++++<
*******************************************************+++++<
+************************************************+++++++++++<
++++++++****************************************************<
*******************=--+*************************************<
******************=::::-+***********************************<
*****************+:::::::+**********************+++*********<
*****************=.::::..:=*******************+-:::+*****+++<
*****************-..::....:-=+===--==++*****+-:....-+***++++<
****************+:..::....:::::::::::::-=++-:......:++++++++<
++**************+:........::.::::::--::::::::......:+**+++++<
+***************=.........:..::::.:---:::::::......:+***++++<
****************=...........:...:.:---:.:::::......:+***++++<
****************=........:........:---:.:::........:+****+++<
****************=................:----::....::.....:+*****++<
****************=................:-----:.....:::...-*******+<
****************+................:-----:......::...=*******+<
****************+.::.............:-----:........:..=********<
****************+:::.............:----:.........:.:+********<
****************+::..............:--=-:.........::-*********<
****************+:...............:-===:..........:-*********<
****************+::.......:......:-===:.:........:=*********<
****************+::.......:......:-===-.:.........-*********<
****************-::.......::....:-==++=::...:.....:+********<
****************-::........:::::-======-::::::....:+********<
****************-:.......:::::-===-----==::::.....:+********<
****************=:....:::----======----===-:.......=********<
****************=::...::-===========--=====-:......=********<
****************=::..::======++=============-:.....=********<
****************+:..::-====++++++===-=======--:...:+********<
*****************-..:--====++++++===-=======--:...-*********<
*****************+:.:-====++==========-=====---:..-*********<
******************-::-=================--------:.:+*********<
*******************:::-=================-------:.=**********<
*******************+:.:::-------=======-------:::+**********<
*******************=....::----------=-------:::.:-+*********<
*******************:...::-------------------::...::-=*******<
******************+:...:--------------------::....:::-+*****<
******************+::::-=======---===-------=-:::.::::-+****<
******************=::--=========-====----======-:::::::-+***<
******************+--============---=---=========::::.::-**+<
******************+===============--=====++===+===:.....:=+=<
******************+==+============-=====+++++++===-...::::--<
******************++++++==========-=====+++++++====:::----::<
*********++++*******++++=========-==--===+++++=====-::---==:<
*******++++++++++***++++========--=--==============:::-====-<
*****+++++++++++++**++++=======---=================---======<
*****+++#%*****+++****+++======---=+=============++++++++++=<
****+++*#*****##++****+++======--=++=-==========++++++++++==<
****+++********##+****+++++=-===-=+++====--=====+++++==+++==<
****+++**********#*****+++*+====++++++==--=====+++++====+===<
****+++****************++++++++++++++++=======+++++=========<
****+++*****************++++++++++++++++++++++++++==========<
****+++********++++*****++++++++++=:-++++++++++++===========<
*****++********++++*****++++++++-:-.:=-=++++++++======++====<
*****+++*******++++*****++++++++-.:-:-.:++++++=========+====<
******++*******++=+******+++++++-:-==-.:=+++================<
******+++******++==+*****++++++-::=-:--=++=================-<

License

My code is BSD-3 Clause to match the Pico SDK.

Adafruit's OV7670 driver is MIT.