void init_gpio(void)
{
  /* PRU GPIOs */
  GPIO1_CTRL = 0x00000001; /* enable, full clock speed */
  GPIO1_OE &= ~(LED0_BIT | LED1_BIT | LED2_BIT | LED3_BIT);
  GPIO1_OE &= ~(0x80000000); /* PRU1 connector P8 pin 20 */

  /* pin mux pin A13 mcasp0_aclkx (GPIO3_14) -> pru0_r31_0 (mode6) */
  /* 28 bit shift mode is set up in PRU0 */
  GPIO3_CTRL = 0x00000001; /* enable, full clock speed */
  GPIO3_OE            |= 1 << 14;
  GPIO3_DEBOUNCENABLE |= 1 << 14;
  CONF_MCASP0_ACLKX    = 0x06;

  /* pin mux pin R1 lcd_data0 (GPIO2_6) -> pru1_r31_0 (mode6) */
  /* 28 bit shift mode is set up in PRU1 */
  GPIO2_CTRL = 0x00000001; /* enable, full clock speed */
  GPIO2_OE            |= 1 << 6;
  GPIO2_DEBOUNCENABLE |= 1 << 6;
  CONF_LCD_DATA0       = 0x06;
}


START:
    // constant 4: PRU-ICSS CFG, register SYSCFG
    LBCO r0, C4, 0x04, 4
    CLR  r0, r0, 4    // clear b4: Enable OCP master ports
    SBCO r0, C4, 0x04, 4

    // PRU0

    // constant 4: PRU-ICSS CFG, register GPCFG0
    // set up serial 28 bit shift mode
    // at 200 MHz

    MOV  r20, 0x00000002
    SBCO r20, C4, 0x08, 4

    // r20 as write shadow of GPCFG0
    SET  r20, r20, 13

    // PRU1

    // constant 4: PRU-ICSS CFG, register GPCFG1
    // set up serial 28 bit shift mode
    // at 200 MHz

    MOV  r21, 0x00000002
    SBCO r21, C4, 0x0c, 4

    // r21 as write shadow of GPCFG1
    SET  r21, r21, 13

start_capture28:
    CLR  r31, 29
    SBCO r20, C4, 0x08, 4
    SBCO r21, C4, 0x0c, 4


poll_capture28_done:
//    // r22 as read shadow of GPCFG0
//    LBCO r22, C4, 0x08, 4
//    QBBS capture28_done, r22, 13
//    // r23 as read shadow of GPCFG0
//    LBCO r23, C4, 0x08, 4
//    QBBC poll_capture28_done, r23, 13

    QBBC poll_capture28_done, r31, 29

capture28_done:

    // raise ARM interrupt
    MOV R31.b0, PRU_VECTOR_VALID + PRU_VECTOR + PRU_NR

    JMP start_capture28