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// PRU program to communicate to the ADS7883 family of SPI ADC ICs. To use this
// program as is, use the following wiring configuration:
// Chip Select (CS): P9_27 pr1_pru0_pru_r30_5 r30.t5
// MISO : P9_28 pr1_pru0_pru_r31_3 r31.t3
// CLK : P9_30 pr1_pru0_pru_r30_2 r30.t2
// Sample Clock : P8_46 pr1_pru1_pru_r30_1 -- for testing only
// This program was writen by Derek Molloy to align with the content of the book
// Exploring BeagleBone. See
.setcallreg r29.w2 // set a non-default CALL/RET register
.origin 0 // start of program in PRU memory
.entrypoint START // program entry point (for a debugger)
#define PRU0_R31_VEC_VALID 32 // allows notification of program completion
#define PRU_EVTOUT_0 3 // the event number that is sent back
// Constants from the MCP3004/3008 datasheet
#define TIME_CLOCK 1 // T_hi and t_lo = 125ns = 25 instructions (min)
// Enable the OCP master port -- allows transfer of data to Linux userspace
LBCO r0, C4, 4, 4 // load SYSCFG reg into r0 (use c4 const addr)
CLR r0, r0, 4 // clear bit 4 (STANDBY_INIT)
SBCO r0, C4, 4, 4 // store the modified r0 back at the load addr
MOV r1, 0x00000000 // load the base address into r1
// PRU memory 0x00 stores the SPI command - e.g., 0x01 0x80 0x00
// the SGL/DIFF and D2, D1, D0 are the four LSBs of byte 1 - e.g. 0x80
MOV r7, 0x00000FFF // the bit mask to use on the returned data (i.e., keep 10 LSBs only)
LBBO r8, r1, 4, 4 // load the Linux address that is passed into r8 -- to store sample values
LBBO r9, r1, 8, 4 // load the size that is passed into r9 -- the number of samples to take
MOV r3, 0x00000000 // clear r3 to receive the response from the MCP3XXX
CLR r30.t2 // set the clock low
// Need to wait at this point until it is ready to take a sample - i.e., 0x00010000
// store the address in r5
MOV r5, 0x00010000 // LSB of value at this address is the clock flag
SAMPLE_WAIT_HIGH: // wait until the PRU1 sample clock goes high
LBBO r6, r5, 0, 4 // load the value at address r5 into r6
QBNE SAMPLE_WAIT_HIGH, r6, 1 // wait until the sample clock goes high
CLR r30.t5 // set the CS line low (active low)
MOV r4, 16 // going to write/read 16 bits (2 bytes)
SPICLK_BIT: // loop for each of the 16 bits
SUB r4, r4, 1 // count down through the bits
CALL SPICLK // repeat call the SPICLK procedure until all 16 bits written/read
QBNE SPICLK_BIT, r4, 0 // have we performed 16 cycles?
LSR r3, r3, 2 // SPICLK shifts left too many times left, shift right once
AND r3, r3, r7 // AND the data with mask to give only the 10 LSBs
SET r30.t5 // pull the CS line high (end of sample)
STORE_DATA: // store the sample value in memory
SUB r9, r9, 2 // reducing the number of samples - 2 bytes per sample
SBBO r3.w0, r8, 0, 2 // store the value r3 in memory
ADD r8, r8, 2 // shifting by 2 bytes - 2 bytes per sample
QBEQ END, r9, 0 // have taken the full set of samples
SAMPLE_WAIT_LOW: // need to wait here if the sample clock has not gone low
LBBO r6, r5, 0, 4 // load the value in PRU1 sample clock address r5 into r6
QBNE SAMPLE_WAIT_LOW, r6, 0 // wait until the sample clock goes low (just in case)
HALT // End of program -- below are the "procedures"
// This procedure applies an SPI clock cycle to the SPI clock and on the rising edge of the clock
// it writes the current MSB bit in r2 (i.e. r31) to the MOSI pin. On the falling edge, it reads
// the input from MISO and stores it in the LSB of r3.
// The clock cycle is determined by the datasheet of the product where TIME_CLOCK is the
// time that the clock must remain low and the time it must remain high (assuming 50% duty cycle)
// The input and output data is shifted left on each clock cycle
LSL r3, r3, 1 // shift the captured data left by one position
// Clock goes low for a time period
MOV r0, TIME_CLOCK // time for clock low -- assuming clock low before cycle
SUB r0, r0, 1 // decrement the counter by 1 and loop (next line)
QBNE CLKLOW, r0, 0 // check if the count is still low
SET r30.t2 // set the clock high
QBBC DATAINLOW, r31.t3 // check if the bit that is read in is low? jump
OR r3, r3, 0x00000001 // set the stored bit LSB to 1 otherwise
// Clock goes high for a time period
MOV r0, TIME_CLOCK // time for clock high
SUB r0, r0, 1 // decrement the counter by 1 and loop (next line)
QBNE CLKHIGH, r0, 0 // check the count
CLR r30.t2 // set the clock low