AN4035 datasheet(33/42 Pages) STMICROELECTRONICS

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AN4035

Nexus Read/Write access block

Doc ID 022669 Rev 2

33/41

is buffered internally by the burst data buffer. The endianess of the 32-bit data written to

RWD needs to be little endian.

-Value of 0xDEADBEEF to be written to memory: RWD = 0xEFBEADDE

The Nexus block then arbitrates for the system bus and transfer the burst data from the

burst data buffer to the memory starting at the address in RWA. When the access has

completed without error, then RWCS[ERR] = 0 and RWCS[DV] = 0. This indicates that

the device is ready for the next access. Nexus also asserts the RDY pin when the

transaction has completed without error. The external tool can use this as an

alternative to polling the RWCS status bits.

4.4

Single memory read access

The steps to perform a single memory read access via the Nexus R/W access block are:

Initialize RWA with the address to be read using the register index 0x9.

Initialize RWCS using Nexus register index 0x7.

Access Control (AC) = 0b1 (to indicate start access)

Map Select (MAP) = 0b000 (primary memory map)

Access Priority (PR) = 0b00 (lowest priority)

Read/Write (RW) = 0 (read access)

Word Size (SZ) = 0b000 (8-bit) or 0b001 (16-bit) or 0b010 (32-bit)

Access Count (CNT) = 0b00_0000_0000_0000 or 0b00_0000_0000_0001 (single

access)

Burst Control (BST) = 0b0 (burst disabled)

The Nexus block then arbitrates for the system bus and the read data is transferred to

RWD from the memory mapped address in RWA. When the access has completed

without error, then RWCS[ERR] = 0 and RWCS[DV] = 1. This indicates that the device

is ready for the next access. Nexus also asserts the RDY pin when the transaction has

completed without error. The external tool can use this as an alternative to polling the

RWCS status bits.

The data can then be read from the RWD register using Nexus register index 0xA. The

data in RWD are right-justified little endian.

- 8-bit value of 0xDE read from memory: RWD = 0x000000DE

- 16-bit value of 0xDEAD read from memory: RWD = 0x0000ADDE

- 32-bit value of 0xDEADBEEF read from memory: RWD = 0xEFBEADDE

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