AN2647 datasheet(14/34 Pages) STMICROELECTRONICS

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Interfacing with asynchronous memory

AN2647

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bit bus cycles. The bus cycle is identical to the standard 8-bit cycle except the EMI_CS chip

select signal stays low during the consecutive write cycles. Figure 9. shows the bus timings

when CPU executes a halfword write instruction (STRH) followed by a LDRH instruction.

Note the EMI_CS signal stays low throughout the two write bus cycles. The two bytes that

are read in the LDRH cycles are combined into one halfword in the EMI block before being

transferred to the CPU.

Figure 9.

Halfword write and read bus cycles in 8-bit non-multiplexed mode

Page mode timing

The EMI bus in non-Multiplexed mode can be configured to support memory with Page

mode. When in Page mode data are transferred in burst of four, eight or sixteen bytes. The

Page mode bit and the data transfer size is specified in the EMI_BCRx register.

The configuration of the EMI_RDn signal timing is the same as in asynchronous read

cycles. When page mode is enabled, the read cycle consists of an asynchronous read,

followed by multiple burst reads of one byte per BCLK clock. The EMI_RDn and chip select

signal will automatically be extended and stay active until the end of the last page read.

If the page mode access time of your memory device plus the EMI data setup time (tRDS) is

more than one BCLK period, you need to insert burst read wait state in the EMI_BRDCRx

1.2

Other EMI bus configurations

After the bus read/write bus timing configuration is determined, the next step is to set up the

Bus Control register and other GPIO port control registers to support the bus type that you

selected (Mux or Non_Mux).

1.2.1

Bus control register (BCR)

Each of the memory banks has its own Bus Control register that specifies the bus operating

mode. The bits that need to be configured for asynchronous memory read and write are

listed in Table 3.. Some of the register bits that are dedicated for synchronous bus mode are

not included in the table.

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