IO Processors: Writing applications


The previous section described the software architecture and the software build process. This section will cover how to write the IOP application and also the corresponding Python interface.

The section assumes that the hardware platform and the BSPs have already been generated as detailed in the previous section.

IOP header files and libraries

A library is provided for the IOPs which includes an API for local peripherals (IIC, SPI, Timer, Uart, GPIO), the configurable switch, links to the peripheral addresses, and mappings for the mailbox used in the existing IOP peripheral applications provided with Pynq. This library can be used to write custom IOP applications.

The only IP that is specific to each IOP is the configurable switch. There is a pmod_io_switch and an arduino_io_switch. The header files for the IOPs are associated with the corresponding configurable switch, and can be found here

<GitHub Repository>/Pynq-Z1/vivado/ip/pmod_io_switch_1.0/  \
   <GitHub Repository>/Pynq-Z1/vivado/ip/arduino_io_switch_1.0/  \

The corresponding C code, pmod.c and arduino.c can also be found in this directory.

Configurable switch header files

There is a separate header file that corresponds to each configurable switch. These files include the API for the configuration switch and predefined constants that can be used to connect to the physical interface on the board.

Pmod Configurable Switch header

You can find the header file for the Pmod IOP switch here:

<GitHub Repository>/Pynq-Z1/vivado/ip/pmod_io_switch_1.0/  \

This code is automatically compiled into the Board Support Package (BSP).


The corresponding files for the Arduino IOP switch can be found here:

<GitHub Repository>/Pynq-Z1/vivado/ip/arduino_io_switch_1.0/  \

Files to include

To use these files in an IOP application, include the header file(s):

For a Pmod IOP:

#include "pmod.h"
#include "pmod_io_switch.h"

or for an Arduino IOP:

#include "arduino.h"
#include "arduino_io_switch.h"

Pmod applications should call pmod_init() at the beginning of the application, and Arduino applications, arduino_init(). This will initialize all the IOP peripherals in the subsystem.

Controlling the Pmod IOP Switch

The IOP switch needs to be configured by the IOP application before any peripherals can be used. This can be done statically from within the application, or the application can allow Python to write a switch configuration to shared memory, which can be used to configure the switch. This functionality must be implemented by the user, but existing IOP applications can be used as a guide. For example, the arduino_lcd18 IOP project shows and example of reading the switch configuration from the mailbox, and using this to configure the switch.

There are 8 data pins on a Pmod port, that can be connected to any of 16 internal peripheral pins (8x GPIO, 2x SPI, 4x IIC, 2x Timer). This means the configuration switch for the Pmod has 8 connections to make to the data pins.

Each pin can be configured by writing a 4 bit value to the corresponding place in the IOP Switch configuration register. The first nibble (4-bits) configures the first pin, the second nibble the second pin and so on.

The following function, part of the provided pmod_io_switch_v1_0 driver (pmod.h) can be used to configure the switch from an IOP application.

void config_pmod_switch(char pin0, char pin1, char pin2, char pin3, char pin4, \
    char pin5, char pin6, char pin7);

While each parameter is a “char” only the lower 4-bits are used to configure each pin.

Switch mappings used for IOP Switch configuration:

Pin Value
GPIO_0 0x0
GPIO_1 0x1
GPIO_2 0x2
GPIO_3 0x3
GPIO_4 0x4
GPIO_5 0x5
GPIO_6 0x6
GPIO_7 0x7
SCL 0x8
SDA 0x9
MISO 0xb
MOSI 0xc
SS 0xd
PWM 0xe



This would connect a SPI interface:

  • Pin 0: SS
  • Pin 1: MOSI
  • Pin 2: GPIO_2
  • Pin 3: SPICLK
  • Pin 4: GPIO_4
  • Pin 5: GPIO_5
  • Pin 6: GPIO_6
  • Pin 7: GPIO_7

Note that if two or more pins are connected to the same signal, the pins are OR’d together internally.


This is not recommended and should not be done unintentionally.

Controlling the Arduino IOP Switch

Switch mappings used for IO switch configuration:

Pin A/D IO A_INT Interrupt UART PWM Timer SPI IIC Input-Capture
A0 A_GPIO A_INT              
A1 A_GPIO A_INT              
A2 A_GPIO A_INT              
A3 A_GPIO A_INT              
A4 A_GPIO A_INT           IIC  
A5 A_GPIO A_INT           IIC  
D0 D_GPIO   D_INT D_UART          
D1 D_GPIO   D_INT D_UART          
D2 D_GPIO   D_INT            
D3 D_GPIO   D_INT   D_PWM0 D_TIMER Timer0     IC Timer0
D4 D_GPIO   D_INT     D_TIMER Timer0_6      
D5 D_GPIO   D_INT   D_PWM1 D_TIMER Timer1     IC Timer1
D6 D_GPIO   D_INT   D_PWM2 D_TIMER Timer2     IC Timer2
D7 D_GPIO   D_INT            
D8 D_GPIO   D_INT     D_TIMER Timer1_7     Input Capture
D9 D_GPIO   D_INT   D_PWM3 D_TIMER Timer3     IC Timer3
D10 D_GPIO   D_INT   D_PWM4 D_TIMER Timer4 D_SS   IC Timer4
D11 D_GPIO   D_INT   D_PWM5 D_TIMER Timer5 D_MOSI   IC Timer5
D12 D_GPIO   D_INT       D_MISO    
D13 D_GPIO   D_INT       D_SPICLK    

For example, to connect the UART to D0 and D1, write D_UART to the configuration register for D0 and D1.

config_arduino_switch(A_GPIO, A_GPIO, A_GPIO, A_GPIO, A_GPIO, A_GPIO,
                      D_UART, D_UART, D_GPIO, D_GPIO, D_GPIO,
                      D_GPIO, D_GPIO, D_GPIO, D_GPIO,
                      D_GPIO, D_GPIO, D_GPIO, D_GPIO);

IOP Application Example

Taking Pmod ALS as an example IOP driver (used to control the PMOD light sensor):

<GitHub Repository>/Pynq-Z1/sdk/pmod_als/src/pmod_als.c

First note that the pmod.h header file is included.

#include "pmod.h"

Some COMMANDS are defined. These values can be chosen to be any value. The corresponding Python code will send the appropriate command values to control the IOP application.

By convention, 0x0 is reserved for no command/idle/acknowledge, and IOP commands can be any non-zero value.

#define READ_AND_LOG      0x7
// Log constants
#define LOG_ITEM_SIZE sizeof(u32)

The ALS peripheral has as SPI interface. The user defined function get_sample() calls an SPI function spi_transfer(), defined in pmod.h, to read data from the device.

u32 get_sample(){
   ALS data is 8-bit in the middle of 16-bit stream.
   Two bytes need to be read, and data extracted.
   u8 raw_data[2];
   spi_transfer(SPI_BASEADDR, 2, raw_data, NULL);
   //  return ( ((raw_data[0] & 0xf0) >> 4) + ((raw_data[1] & 0x0f) << 4) );
   return ( ((raw_data[1] & 0xf0) >> 4) + ((raw_data[0] & 0x0f) << 4) );

In main() notice config_pmod_switch() is called to initialize the switch with a static configuration. This application does not allow the switch configuration to be modified from Python. This means that if you want to use this code with a different pin configuration, the C code must be modified and recompiled.

int main(void)
   int cmd;
   u16 als_data;
   u32 delay;

   config_pmod_switch(SS, GPIO_1, MISO, SPICLK, \
                      GPIO_4, GPIO_5, GPIO_6, GPIO_7);
   // to initialize the device

Next, the while(1) loop continually checks the MAILBOX_CMD_ADDR for a non-zero command. Once a command is received from Python, the command is decoded, and executed.

// Run application

   // wait and store valid command
   while((MAILBOX_CMD_ADDR & 0x01)==0);

Taking the first case, reading a single value; get_sample() is called and a value returned to the first position (0) of the MAILBOX_DATA.

MAILBOX_CMD_ADDR is reset to zero to acknowledge to the ARM processor that the operation is complete and data is available in the mailbox.

   // write out reading, reset mailbox
   MAILBOX_DATA(0) = get_sample();

Remaining code:

         case READ_AND_LOG:
         // initialize logging variables, reset cmd
         cb_init(&pmod_log, LOG_BASE_ADDRESS, LOG_CAPACITY, LOG_ITEM_SIZE);
         delay = MAILBOX_DATA(1);
         MAILBOX_CMD_ADDR = 0x0;

               als_data = get_sample();
               cb_push_back(&pmod_log, &als_data);
            } while((MAILBOX_CMD_ADDR & 0x1)== 0);


            // reset command
            MAILBOX_CMD_ADDR = 0x0;

Examining the Python Code

With the IOP Driver written, the Python class can be built that will communicate with that IOP.

<GitHub Repository>/python/pynq/iop/

First the MMIO, request_iop, iop_const, PMODA and PMODB are imported.

import time
from pynq import MMIO
from pynq.iop import request_iop
from pynq.iop import iop_const
from pynq.iop import PMODA
from pynq.iop import PMODB

ALS_PROGRAM = "pmod_als.bin"

The MicroBlaze binary for the IOP is also declared. This is the application executable, and will be loaded into the IOP instruction memory.

The ALS class and an initialization method are defined:

class Pmod_ALS(object):
   def __init__(self, if_id):

The initialization function for the module requires an IOP index. For Grove peripherals and the StickIt connector, the StickIt port number can also be used for initialization. The __init__ is called when a module is instantiated. e.g. from Python:

from pynq.pmods import Pmod_ALS
als = Pmod_ALS(PMODB)

Looking further into the initialization method, the _iop.request_iop() call instantiates an instance of an IOP on the specified pmod_id and loads the MicroBlaze executable (ALS_PROGRAM) into the instruction memory of the appropriate MicroBlaze.

self.iop = request_iop(if_id, PMOD_ALS_PROGRAM)

An MMIO class is also instantiated to enable read and write to the shared memory.

self.mmio = self.iop.mmio

Finally, the iop.start() call pulls the IOP out of reset. After this, the IOP will be running the als.bin executable.


Example of Python Class Runtime Methods

The read method in the Pmod_ALS class will simply read an ALS sample and return that value to the caller. The following steps demonstrate a Python to MicroBlaze read transaction specific to the ALS class.

def read(self):

First, the command is written to the MicroBlaze shared memory using mmio.write(). In this case the value 0x3 represents a read command. This value is user defined in the Python code, and must match the value the C program running on the IOP expects for the same function.

                    iop_const.MAILBOX_PY2IOP_CMD_OFFSET, 3)

When the IOP is finished, it will write 0x0 to the command area. The Python code now uses to check if the command is still pending (in this case, when the 0x3 value is still present at the CMD_OFFSET). While the command is pending, the Python class blocks.

while (
                            iop_const.MAILBOX_PY2IOP_CMD_OFFSET) == 3):

Once the command is no longer 0x3, i.e. the acknowledge has been received, the result is read from the DATA area of the shared memory MAILBOX_OFFSET using


Notice the iop_const values are used in these function calls, values that are predefined in