Nios cannot open/access EPSC

Custom Cyclone IV based board. Uses EPCS16 (ST 25P16) for AS configuration of FPGA. Board successfully configures from the EPCS and will then boot Nios, also from the EPCS.

However, Nios cannot subsequently open the EPCS, as tested with sample code lifted directly from the 'Nios II Software Developer's Handbook'. This permanently reports it cannot open the device.

Function called:
alt_flash_open_dev( EPCS_0_NAME );
Always returns NULL.

All relevant Dual-Purpose Pins (DCLK, Data[0], ASDO, nCSO) are defined as 'Use as regular I/O' after programming.

Relevant section of system.h :-


/*
* epcs_0 configuration
*
*/


#define ALT_MODULE_CLASS_epcs_0 altera_avalon_epcs_flash_controller
#define EPCS_0_BASE 0x9000
#define EPCS_0_IRQ 0
#define EPCS_0_IRQ_INTERRUPT_CONTROLLER_ID 0
#define EPCS_0_NAME "/dev/epcs_0"
#define EPCS_0_REGISTER_OFFSET 1024
#define EPCS_0_SPAN 2048
#define EPCS_0_TYPE "altera_avalon_epcs_flash_controller"

Hardware does not have any external RAM to put to use, only RAM internal to the FPGA. So, testing the EPCS using Altera's 'Memory Test' software template is not possible as its code size is too big. 'Memory Test Small' will run, but doesn't test any flash.

Running QuartusII 64-bit 12.1 SP1