External Bus Interfaces integrate circuitry that interfaces to NAND Flash devices.
To ensure that the processor preserves transaction order and thus the correct NAND Flash behavior, the NAND Flash address space is to be declared in the Memory Protection Unit (MPU) as “Device” or “Strongly-ordered” memory. Refer to the ARM Cortex-M7 Technical Reference Manual (ARM DDI 0489) available on www.arm.com.
External Bus Interface
The NAND Flash Chip Select (NANDCS) is driven by the Static Memory Controller on the NCS0, NCS1, NCS2 or NCS3 address space depending on value of SMC_SMCSx bits. For example, programming the SMC_NFC3 field in the CCFG_SMCNFCS Register in the Chip Configuration User Interface to the appropriate value enables the NAND Flash logic. For details on this register, refer to Bus Matrix (MATRIX). Access to an external NAND Flash device is then made by accessing the address space reserved to NCS3 (i.e., between 0x6300 0000 and 0x6FFF FFFF).
The NAND Flash logic drives the read and write command signals of the SMC on the NANDOE and NANDWE signals when the required SMC_NFCSx signal is active. NANDOE and NANDWE are invalidated as soon as the transfer address fails to lie in the selected NCSx address space. For details on these waveforms, refer to Static Memory Controller (SMC).
NAND Flash Signals
The address latch enable and command latch enable signals on the NAND Flash device are driven by address bits A22 and A21 of the EBI address bus. The command, address or data words on the data bus of the NAND Flash device are distinguished by using their address within the NCSx address space. The chip enable (CE) signal of the device and the ready/busy (R/B) signals are connected to PIO lines. The CE signal then remains asserted even when NCSx is not selected, preventing the device from returning to standby mode.