Cypress Semiconductor CY8C21534 Dokumentacja Pobierz pdf (Strona 33)

MISO, MOSI, SCLK, and SS were not taken directly from the SD card to the MCU as

shown in the figure but rather switched through the HEF4066 bilateral switch

integrated circuit [15]. Only during the time of reading the file for display will the

enable signal to the HEF4066 and the power to the digital photo frame be turned on.

This allows the photo frame, upon power up, to read that there is a new file on the SD

card, as well as saving power while the unit is not in use. The circuit board for these

connections to the Communications MCU were fabricated using PADS Logic and PADS

Layout in the same fashion as the Control MCU circuit board was designed.

Using PSoC designer the SPI connection was setup to communicate over the pins to

which the SD card was connected. The clock of the SPI communications block was set

up to run from an 8 bit Counter Block. This gives the ability to change the speed of the

SPI connection in real time by changing the values of the counter within the program

code. The counter clock is taken from VC1 having a divider of 2. The main clock signal

is 24 MHz which is divided by the VC1 value of 2, then divided by the period value for

the counter which is initialized to 60. When the SPI connection block receives the clock

signal it divides this again by two to find its clock rate. This gives a calculated clock

signal of 100 KHz which is used for the initialization of the SD card. Once the

initialization is finished, the Counter Block is programmed to switch to a period of eight

for a clock speed of 750 KHz. This is the speed that is used for data transfer.

This speed can be taken up to several MHz if needed after the card initialization is

accomplished below a speed of 400 KHz. For stability issues the speed was kept

relatively low for this project. Additionally, the UART baud rate limits the speed of the

data flow, so higher rates on the SPI connection are not necessary.

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Cypress Semiconductor CY8C21534 Dokumentacja Strona 33