{"id":99,"date":"2017-07-09T18:11:03","date_gmt":"2017-07-09T16:11:03","guid":{"rendered":"https:\/\/next-hack.com\/?p=99"},"modified":"2020-06-05T20:17:35","modified_gmt":"2020-06-05T18:17:35","slug":"how-to-play-a-20-fps-video-on-arduino-26-enabling-high-speed-sd-transfers-on-a-1-8-tftsd-module","status":"publish","type":"post","link":"https:\/\/next-hack.com\/index.php\/2017\/07\/09\/how-to-play-a-20-fps-video-on-arduino-26-enabling-high-speed-sd-transfers-on-a-1-8-tftsd-module\/","title":{"rendered":"How to play a 20 fps video on Arduino 2\/6: Enabling high-speed SD transfers on a 1.8\u201d TFT+SD module."},"content":{"rendered":"

Introduction<\/span><\/h1>\n

Hi there!<\/span><\/p>\n

This is the second step of our \u201cHow to play a video on Arduino\u201d tutorial!<\/span><\/p>\n

For video playback we will need high transfer rates. High transfer rates are also required or desired in many other applications, so this hack might be useful even if you are not going to play a video! The Arduino\u2019s SPI can reach a clock speed of 8MHz. However, such a high speed is not supported by many shields or add-on cards. <\/span><\/p>\n

In this hack episode we will show you how to modify the popular 1.8\u201d TFT module with SD card, to enable high speed read\/write transfers.<\/span><\/p>\n

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\"TFT\"TFT<\/p>\n

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This module is very cheap, and it also features an SD socket, which is handy, because we are going to store our video on the SD card. The SD signals are routed on the left, and you can directly connect them to an Arduino. However, you\u2019ll find that you cannot use a clock frequency higher than 1MHz.<\/span><\/p>\n

This is due to the 10-kOhm resistors (R3 through R5) that are in series to SD_CLK, SD_MOSI, SD_CS, to protect the SD Card when you connect the module directly to a 5-V system, such as the Arduino Uno. (see the schematics below. You can find the full schematics QDtech_1.8<\/a><\/span>). These resistors make a low-pass filter together with the parasitic stray and SD capacitance. <\/span><\/p>\n

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\"Protection<\/p>\n

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The solution is to short the 10-kOhm resistors or, at least, change their values to 330 Ohm or less. Although it might still work, we do not recommend using higher values than 330 Ohms, as the signal could be too much degraded.<\/span><\/p>\n

To test our system we created a simple sketch using the Arduino IDE (you can find the complete source in the links below). We used the FAT Fs of ELM-ChaN ( <\/span>http:\/\/elm-chan.org\/fsw\/ff\/00index_e.html<\/span><\/a><\/u><\/span> ), which we adapted for Arduino. The test program performs a read test at 500 kHz, and a read test at 8MHz. The red LED stays on for the whole duration of the tests, and it is turned off once the read operations have been successfully completed. The green LED flashes during transfers, and stays on if the program ended successfully. The blue LED indicates if we are running at high speed (LED on) or low speed (LED off). The program advances to the high-speed phase only if the low-speed phase completes successfully. <\/span><\/p>\n

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\"Test<\/p>\n

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The following images show the connections required for the test program. <\/span>Important! Do not use this connection after you modified the display module!<\/b><\/span><\/span><\/p>\n

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\"Breadboard<\/p>\n

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The breadboard image was created with Fritzing (http:\/\/fritzing.org<\/a>).<\/span><\/p>\n

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\"Test<\/p>\n

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As you can see, at 500kHz the SD card works (the blue LED is on, indicating that the system is testing the SD card in high-speed mode, as the low-speed test was successful).<\/span><\/p>\n

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\"High<\/p>\n

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However, at 8MHz it doesn\u2019t. Playing with the frequency settings we found that at 1MHz works too (depending on the card, even at 1MHz it might not work). At 2 MHz, it doesn\u2019t. <\/span><\/p>\n

To better appreciate how the signal is degraded at the various speed, we soldered a wire close to the SD clock input (see image below) and we hooked it up to the oscilloscope. We also wrote a simple program <\/span>(download sd-test<\/a><\/span>) that outputs one character (0xFF) to the SPI at different clock frequencies. At 1MHz the signal is already very degraded, and it\u2019s a miracle if the card still works. At 2MHz we aren\u2019t surprised that the SD does not work. <\/span><\/p>\n

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\"Clock<\/p>\n

\"Degraded<\/p>\n

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(please note that even though due to the resolution of the scope screen, it seems that the CPU creates a bad square wave at high frequencies, it actually does not: the square wave is good even at 8MHz).<\/span><\/p>\n

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\"Degraded<\/p>\n

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So, let\u2019s modify the module and check if the hack works!<\/span><\/p>\n

Requirements for this Hack<\/span><\/h2>\n

Hardware required<\/span><\/h3>\n