224
/* Disable USI */
USICTL0 |= USISWRST;
/*
* USI Control Register 0
*
* ~USIPE7 -- USI function disabled
* USIPE6 -- USI function enabled
* USIPE5 -- USI function enabled
* ~USILSB -- MSB first
* USIMST -- Master mode
* ~USIGE -- Output latch enable depends on shift clock
* USIOE -- Output enabled
* USISWRST -- USI logic held in reset state
*
* Note: ~<BIT> indicates that <BIT> has value zero
*/
USICTL0 = USIPE6 | USIPE5 | USIMST | USIOE | USISWRST;
/*
* USI Clock Control Register
*
* USIDIV_3 -- Divide by 8
* USISSEL_2 -- SMCLK
* USICKPL -- Inactive state is high
* ~USISWCLK -- Input clock is low
*
* Note: ~USISWCLK indicates that USISWCLK has value zero
*/
USICKCTL = USIDIV_3 | USISSEL_2 | USICKPL;
/* Enable USI */
USICTL0 &= ~USISWRST;
The function below does the actual SPI data transfer. It both transmits and receives data over SPI bus.
Data to be transmitted is loaded in USISRL register. USI bit counter is loaded with the number of bits
to transfer, here 8. In the hardware end, this counter is decremented with data being shifted and clock
signal being generated on each decrement. Since SPI bus is a circular bus, reading USISRL register back
returns received data. Note slave select pin is not used here as it is used in MAX7219 source file.
unsigned char SPI_transfer(unsigned char data_out)
{
unsigned char data_in = 0;
USISRL = data_out; // Load shift register with data byte to be TXed
USICNT = 8; // Load bit-counter to send/receive data byte
while (!(USIIFG & USICTL1)); // Loop until data byte transmitted
data_in = USISRL; // Read out the received data
return data_in;
}
The rest of the code is the implementation of MAX7219 driver and how to use it to create patterns in
the dot-matrix display. When started, the display scrolls the letters of the word “MICROARENA” - the
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