Open Memory Window Wishbone Interface

[andkae]

Hello Stephan,

at the moment i try to interface a component connected via Wishbone. My Example code looks as follows:

/* Processor */
#include <neorv32.h>

/**
 *  main
 */
int main()
{
    /** Variables **/

    /* init UART */
    neorv32_uart0_setup(115200, PARITY_NONE, FLOW_CONTROL_NONE);

    /* entry message */
    neorv32_uart0_print("I2C\n");

    /* read from iicmb */
    neorv32_uart0_printf("%x;", *((volatile uint8_t*) (0xffffffb0)));

    /* write to iicmb */
    (*(volatile uint32_t*) 0xffffffb0) = 0xaaaa5555;

    /* stop programm counter */
    while ( 1 ) { }
    return 0;
}

The data value for (*(volatile uint32_t*) 0xffffffb0) = 0xaaaa5555; occurs of the wishbone bus, but the stb isn't set. How can I tell the processor system that this address should mapped to the wishbone interface? The Signal iicm_wb_adr_en marks that the address is driven on the bus.

Thanks for your Help!

[stnolting]

How can I tell the processor system that this address should mapped to the wishbone interface?

Short answer: you can't.

The address space from 0xFFFFFE00 to 0xFFFFFFFF is reserved for the processor-internal IO space. See data sheet section Address Space and especially sub-section External Memories:

Any CPU access (data or instructions), which does not fulfill at least one of the following conditions, is forwarded via the processor’s bus interface to external components:

• access to the processor-internal IMEM and processor-internal IMEM is implemented
• access to the processor-internal DMEM and processor-internal DMEM is implemented
• access to the bootloader ROM and beyond → addresses >= BOOTROM_BASE (default 0xFFFF0000) will never be forwarded to the external memory interface

Here is an overview from the Address Space section:

[stnolting]

oh, i see there is space in between, f.e. ((volatile uint32_t) 0xfffffdfc) = 0xaaaa5555; could work?

No, this is also inside the reserved address space. In summary, no address from the processor's IO/OCD/Bootrom space can be used for external accesses (0xFFFF0000 .. 0xFFFFFFFF).

• access to the bootloader ROM and beyond → addresses >= BOOTROM_BASE (default 0xFFFF0000) will never be forwarded to the external memory interface

[andkae]

and which address would forwarded? as initial example

[stnolting]

That depends on your memory configuration 😉
If the address does not "hit" the IMEM, not the DMEM and not the area mentioned above, it will be redirected to the external bus interface.

For example you could use address 0x90000000 if your internal DMEM is less than 256 MB (and if this base address is not used by the XIP module).

[andkae]

Got it thanks. For (*(volatile uint32_t*) 0xEFFFFFFB) = 0xaaaa5555; it is asserting the bus.

Thanks for your help! At the moment is no device there, this leads to the stuck.

[stnolting]

For developing modules connected to the external memory interface I recommend some points:

Make sure the external memory access timeout feature is enabled (via processor top generic):

  MEM_EXT_TIMEOUT => 255, -- cycles after a pending bus access auto-terminates (0 = disabled)

Enable the RTE in your C code to get a UART0 message if an external access times out:

  neorv32_rte_setup();

If you want to do "manual inspection" of the address space you can use the sw/example/bus_explorer program, which allows to trigger memory operations (read, write, dump, pretty hex view, ...) by hand (via an interactive UART console).