libpruio  0.6.6
Fast and easy Digital/Analog Input/Output for Beaglebones
Memory Organisation

libpruio uses three blocks of memory to organize its data

  • PruIo::DInit : A memory block of variable size in user space memory, allocated by libpruio, containing the data arrays PruIo::Init and PruIo::Conf.
  • PruIo::DRam : A memory block of 2 x 8 kB on the PRUSS, mapped by the kernel driver uio_pruss for ARM access.
  • PruIo::ERam : A contingous memory block of variable size (up to 8 MB) in kernel space memory, allocated by the kernel driver uio_pruss.

These blocks are available after the constructor call PruIo::PruIo() and get destroyed in the destructor PruIo::~PruIo(). The constructor reads the initial configuration of the CPU subsystems and stores the values in PruIo::Init. Those values are used to restore the subsystems in the destructor. PruIo::Conf contains the current configurations, adapted by the user code.


The PruIo::DInit data block holds the register context of the subsystems. It gets allocated by the constructor PruIo::PruIo(). Its size is twice the size of all register sets. This is

The pasm_init.p instructions prepares these structures for each subsystem and sets the member variables by the values read from the registers. All structures are packed in a single data block that gets stored in DRam. The constructor allocates memory and makes one copy of the block starting at adress PruIo::DInit and a second copy starting at adress PruIo:DConf.

The first copy is used by the destructor PruIo::~PruIo() to restore the original configuration. The second block is used for customized configurations and gets transfered back to the subsystem registers in the call to function PruIo::config(). The initialize member function of the libpruio subsystems code (like GpioUdt::initialize() or PwmssUdt::initialize() ) prepare further pointers for direct access to the data block sections:

Pointer Description
PruIo::DInit Start of initial block (= AdcUdt::Init)
AdcUdt::Init Initial ADC configuration
GpioUdt::Init (0) Initial GPIO-0 configuration
GpioUdt::Init (1) Initial GPIO-1 configuration
GpioUdt::Init (2) Initial GPIO-2 configuration
GpioUdt::Init (3) Initial GPIO-3 configuration
PruIo::BallInit Initial CM-pad configuration (pinmuxing)
PwmssUdt::Init (0) Initial PWMSS-0 configuration
PwmssUdt::Init (1) Initial PWMSS-1 configuration
PwmssUdt::Init (2) Initial PWMSS-2 configuration
TimerUdt::Init (0) Initial TIMER4 configuration
TimerUdt::Init (1) Initial TIMER5 configuration
TimerUdt::Init (2) Initial TIMER6 configuration
TimerUdt::Init (3) Initial TIMER7 configuration
PruIo::DConf Start of custom block (= AdcUdt::Conf)
AdcUdt::Conf Custom ADC configuration
GpioUdt::Conf (0) Custom GPIO-0 configuration
GpioUdt::Conf (1) Custom GPIO-1 configuration
GpioUdt::Conf (2) Custom GPIO-2 configuration
GpioUdt::Conf (3) Custom GPIO-3 configuration
PruIo::BallConf Custom CM-pad configuration (pinmuxing)
PwmssUdt::Conf (0) Custom PWMSS-0 configuration
PwmssUdt::Conf (1) Custom PWMSS-1 configuration
PwmssUdt::Conf (2) Custom PWMSS-2 configuration
TimerUdt::Conf (0) Custom TIMER4 configuration
TimerUdt::Conf (1) Custom TIMER5 configuration
TimerUdt::Conf (2) Custom TIMER6 configuration
TimerUdt::Conf (3) Custom TIMER7 configuration

In order to minimize memory consumption libpruio may generate uncomplete subsystem sets. In such a set only the first four member variables are valid (DeAd, ClAd, ClVa, [REVISION, IDVER]). Such reduced sets get generated

  • when a subsystem should get ignored, that is its activate bit isn't set in the parameter Act in the constructor call (member variable ClAd = 0), or
  • when a subsystem didn't wake up due to a hardware failure (member variable [REVISION | IDVER] = 0).

After the constructor call also the member variable ClVa in the Conf section is set to 0 (zero) for uncomplete sets. When you intent to access the subsystems sets directly, you must check if the set is complete, first. This is the case when member variable ClVa = 2.

It's forbidden to write to variables in an uncomplete subsystem set. This can destroy the following set in the data block and may result in non-revisible hardware damages.


The DRam area (data ram) is a fixed sized block of 2 * 8 kB, allocated by the PRUSS kernel driver. The host (ARM) can access only the first 8 KB block, but the PRU software may use both blocks in MM mode in case of a great number of pre-trigger samples.

The PruIo::DRam pointer (type UInt32) gets initialized by the constructor PruIo::PruIo() and points to the DRam of the used PRUSS. This is DRam-0 when the PRU bit of parameter Act is cleared and DRam-1 when the bit is set (the default).

The first value (PruIo::DRam [0] is always used to report the state of the PRUSS software. It contains either one of the message codes defined in file pruio.hp or it contains the index of the last stored sample (in RB or MM mode).

The rest of the DRam area is used for exchanging parameters between the host (ARM) and the PRU software. The context changes, depending on the current operational state of libpruio.


Before the constructor loads and executes the pasm_init.p instructions, it calls the constructors of the subsystem structures AdcUdt, GpioUdt and PwmssUdt. These initialize the DRam area with the adresses for the subsystems and their Control Module (CM) clock register. So before that instructions the DRam area contains

Value Description
DRam[1] Start of the data block
DRam[2] Base Adress of ADC subsystem registers
DRam[3] Adress of the ADC clock register
DRam[4] Base Adress of GPIO-0 subsystem registers
DRam[5] Adress of the GPIO-0 clock register
DRam[6] Base Adress of GPIO-1 subsystem registers
DRam[7] Adress of the GPIO-1 clock register
DRam[8] Base Adress of GPIO-2 subsystem registers
DRam[9] Adress of the GPIO-2 clock register
DRam[10] Base Adress of GPIO-3 subsystem registers
DRam[11] Adress of the GPIO-3 clock register
DRam[12] Base Adress of CM-pad registers
DRam[13] Base Adress of PWMSS-0 subsystem registers
DRam[14] Adress of the PWMSS-0 clock register
DRam[15] Base Adress of PWMSS-1 subsystem registers
DRam[16] Adress of the PWMSS-1 clock register
DRam[17] Base Adress of PWMSS-2 subsystem registers
DRam[18] Adress of the PWMSS-2 clock register
DRam[19] Base Adress of TIMER4 subsystem registers
DRam[20] Adress of the TIMER4 clock register
DRam[21] Base Adress of TIMER5 subsystem registers
DRam[22] Adress of the TIMER5 clock register
DRam[23] Base Adress of TIMER6 subsystem registers
DRam[24] Adress of the TIMER6 clock register
DRam[25] Base Adress of TIMER7 subsystem registers
DRam[26] Adress of the TIMER7 clock register

In case of an inactive subsystem (configuration bit in parameter Act cleared) the clock adress is set to 0 (zero) so that the PRU can still read the subsystems identifier (REVISION, IDVER).

Otherwise the PRU reads the complete register context of the subsystem and stores the context in the data block. When a block of subsystems (ADC, 4xGPIO, CM, 3xPWMSS) is finished, the offset of the next block gets written to the last parameter. So after the pasm_init.p instructions, the DRam area contains the following information:

Value Description
DRam[1] Offset of ADC register block
DRam[3] Offset of GPIO register block
DRam[11] Offset of CM-pad register block
DRam[12] Offset of PWMSS register block
DRam[18] Length of complete data block
DRam[128] Data block context

The constructor PruIo::PruIo() generates two copies of the data block in host memory (at PruIo::DInit and PruIo::DConf) and calls the initialize() member functions of the subsystems (ie. AdcUdt::initialize(), GpioSet::initialize() ), where the pointers (ie. AdcUdt::Init and AdcUdt::Conf) to the individual register sets (ie. AdcSet, GpioSet) get initialized.

This method allows to extend libpruio by new subsystem code with minimal adaption in the existing source.


The function PruIo::config() uploads the customized configuration to the subsystems and starts operation in the declared run mode. Before the pasm_run.p instructions get executed, the DRam area contains

Value Description
DRam[1] Start of the data block
DRam[2] Number of samples AdcUdt::Samples
DRam[3] Step mask AdcSet::STEPENABLE
DRam[4] Bit encoding mode AdcUdt::LslMode
DRam[5] Timer value AdcUdt::TimerVal
DRam[128-] Conf data block context

The PRU software reads these parameters and writes the configuration to the subsystem registers. Then it prepares the DRam area as follows

Value Description
DRam[0] PRUIO_MSG_xxx
DRam[1-15] var. parameters
DRam[16] 4 x GpioArr (4*16 bytes)
DRam[64] 3 x PwmssArr (3*32 bytes)
DRam[72] AdcSet::DeAd (4 bytes)
DRam[73] 17 x AdcUdt::Value data (34 bytes)
DRam[89] 4 x TimerArr (4*16 bytes)

The type of PRUIO_MSG_xxx depends on the required run mode. It's either PRUIO_MSG_IO_OK in case of IO mode (parameter Samp = 1) or PRUIO_MSG_MM_WAIT in case of RB or MM mode (parameter Samp > 1).

The arrays GpioArr and PwmssArr are used to transfer data from the digital subsystems to the host. In IO and RB mode the PRU software reads some registers and writes the context in to these arrays, where the host (ARM) software can read them.

The field Adc data contains the subsystem adress of the ADC subsystem (if enabled) and 17 UInt16 values for samples (the first is unused).

  • In IO mode it contains the sampled data (AdcUdt::Value).
  • In RB mode it's not used.
  • In MM mode it contains the ring buffer for pre-trigger samples and its size vary. Depending on the number of pre-trigger samples the DRam areas of both PRUSS may be used.

Each array entry starts with the subsystem adress. In case of a disabled subsystem the adress entry is 0 (zero) and the PRU software skips this subsystem.

Furthermore the beginning of the DRam area is used to pass parameters to the running PRU software

  • Function PruIo::rb_start()
    Value Description
    DRam[1] Number of samples AdcUdt::Samples
    DRam[2] Adress of ERam PruIo::ERam
    DRam[3] Pre-trigger samples (= 0)
    DRam[4] Trigger specification (= RB mode)
    When running in RB mode the PRU software reports its last write index
    Value Description
    DRam[0] Last write index 0 <= i < AdcUdt::Samples * AdcUdt::ChAz
  • Function PruIo::mm_start()
    Value Description
    DRam[1] Number of samples AdcUdt::Samples
    DRam[2] Adress of ERam PruIo::ERam
    DRam[3] Pre-trigger samples
    DRam[4] Trigger 1 specification
    DRam[5] Trigger 2 specification
    DRam[6] Trigger 3 specification
    DRam[7] Trigger 4 specification
    After performing a maesurement in MM mode the PRU software reports success
    Value Description
  • PRU command PRUIO_COM_GPIO_CONF (set GPIO direction and value)
    Value Description
    DRam[2] GPIO subsystem adress (+ &h100)
    DRam[3] Value for GpioSet::CLEARDATAOUT register
    DRam[4] Value for GpioSet::SETDATAOUT register
    DRam[5] Value for GpioSet::OE register
  • PRU command PRUIO_COM_PWM (set PWM frequency and duty cycle)
    Value Description
    DRam[1] PRUIO_COM_PWM SHL 24
    DRam[2] PWMSS subsystem adress (+ &h200)
    DRam[3] Value for PwmssSet::CMPA & PwmssSet::CMPB registers
    DRam[4] Value for PwmssSet::AQCTLA & PwmssSet::AQCTLB registers
    DRam[5] Value for PwmssSet::TBCNT & PwmssSet::TBPRD registers
  • PRU command PRUIO_COM_CAP_PWM (switch PWMSS-CAP module in PWM mode and set frequency and duty cycle)
    Value Description
    DRam[1] PRUIO_COM_CAP_PWM SHL 24 + PwmssSet::ECCTL2 register value
    DRam[2] PWMSS subsystem adress (+ &h100)
    DRam[3] Value for PwmssSet::CAP3 register (counter shadow)
    DRam[4] Value for PwmssSet::CAP4 register (period shadow)
  • PRU command PRUIO_COM_CAP (switch PWMSS-CAP module in CAP mode)
    Value Description
    DRam[1] PRUIO_COM_CAP SHL 24 + PwmssSet::ECCTL2 register value
    DRam[2] PWMSS subsystem adress (+ &h100)
  • PRU command PRUIO_COM_QEP (switch PWMSS-QEP module in QEP mode)
    Value Description
    DRam[1] PRUIO_COM_QEP SHL 24
    DRam[2] PWMSS subsystem adress (+ &h100)
    DRam[3] Value for PwmssSet::QPOSMAX register
    DRam[4] Value for PwmssSet::QUPRD register
    DRam[5] Value for PwmssSet::.QDECCTL OR .QEPCTL SHL 16 registers
    DRam[6] Value for PwmssSet::QCAPCTL register
  • PRU command PRUIO_COM_TIM_PWM (switch TIMERSS in PWM mode)
    Value Description
    DRam[1] PRUIO_COM_TIM_PWM SHL 24 + TimerSet::TCLR register value
    DRam[2] TIMER subsystem adress
    DRam[3] timer load register value
    DRam[4] timer match register value
    DRam[5] timer counter register value
  • PRU command PRUIO_COM_ADC (set a new step mask in IO mode)
    Value Description
    DRam[1] PRUIO_COM_ADC SHL 24 + AdcSet::STEPENABLE register value
    PRU commands are valid in IO and RB mode. After a PRU command execution the DRam[1] value gets set to 0 (zero). You must wait for that zero value before you set a new command or write new parameters.


The destructor PruIo::~PruIo() restores the subsystems before the PruIo structure gets destroyed. It load the initial configuration data block to the DRam and executes the configuration instruction on the PRU. Therefor it prepares the DRam area as in the following table:

Value Description
DRam[1] Start of the data block
DRam[2] Number of samples (= 0)
DRam[128] Init data block context

When the PRU finished successfully the DRam area contains

Value Description


The ERam area (external memory) is the biggest memory block in use. It's allocated by the kernel driver when loaded. The default size is 256 kB (= 128 kSamples). libpruio uses the external memory in RB and MM mode to store the ADC samples.

In both cases the number of total samples (= AdcUdt::Samples x AdcUdt::ChAz) must not be greater than the available number of samples in the external memory. The block size is available in member variable PruIo::ESize. The pointer AdcUdt::Samples gets set to the start of the ERam block.

The kernel driver allows to customize the size of the external memory. This must be done at driver loading time. Ie. execute (with admin privileges) modprobe uio_pruss extram_pool_sz=0x800000 to set the maximum size of 8 MB (= 0x800000).

The kernel driver also gets loaded by a device tree overlay that enables the PRUSS. Make sure that the above command gets executed before the device tree overlay loads. (Or unload the kernel driver by modprobe -r uio_pruss and reload again using the above command.)