Texas Instruments TMS320DM646X DMSOC manual Read and Write Operations in Select Strobe Mode

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2.5.5Read and Write Operations in Select Strobe Mode

Select Strobe mode is the EMIF'ssecond mode of operation. The SS mode is selected when the SS bit in the asynchronous configuration register (ACFGn) is set to 1. In this mode, the EM_CS pin functions as a strobe signal and is therefore only active during the strobe period of an access cycle.

2.5.5.1Asynchronous Read Operations (Select Strobe Mode)

An asynchronous read is performed when any of the requesters mentioned in Section 2.2 request a read from the attached asynchronous memory. In the event that the read request cannot be serviced by a single access cycle to the external device, multiple access cycles will be performed by the EMIF until the entire request is fulfilled. The details of an asynchronous read operation in Select Strobe mode are described in Table 9 and an example timing diagram of a basic read operation is shown in Figure 6.

NOTE: During the entirety of an asynchronous read operation, the EM_WE and EM_RW pins are driven high.

	Table 9. Asynchronous Read Operation in Select Strobe Mode
Time Interval	Pin Activity in Select Strobe Mode
Turnaround	Once the EMIF receives a read request, the EMIF waits for the programmed number of turnaround cycles before
period	proceeding to the setup period of the operation. The number of wait cycles is taken directly from the TA field of
	the asynchronous configuration register (ACFGn). There are two exceptions to this rule:

•If the current read operation was directly proceeded by another read operation to the same CS space, no turnaround cycles are inserted.

•If the current read operation was not directly proceeded by a read operation to the same CS space and the TA field has been cleared to 0, one turnaround cycle will be inserted.

After the EMIF has waited for the turnaround cycles to complete, it proceeds to the setup period of the operation.

Start of setup	At the beginning of the setup period:
period	• The setup, strobe, and hold values are set according to the R_SETUP, R_STROBE, and R_HOLD values
		in ACFGn.
	• The address pins EM_A and EM_BA become valid.
Start of strobe	At the beginning of the strobe period:
period
	• EM_CS and EM_OE fall at the start of the strobe period
Start of hold	At the beginning of the hold period:
period
	• EM_CS and EM_OE rise
	• The EMIF samples the data on the EM_D bus
End of hold	At the end of the hold period:
period	• The address pins EM_A and EM_BA become invalid
	The EMIF may be required to issue additional read operations to a device with a small data bus width in order to
	complete an entire word access. In this case, the EMIF immediately re-enters the setup period to begin another
	operation without incurring the turnaround cycle delay. The setup, strobe, and hold values are not updated in this
	case. If the entire word access has been completed, the EMIF returns to its previous state unless another
	asynchronous request has been submitted. If this is the case, the EMIF instead enters directly into the
	turnaround period for the pending read or write operation.

18 Asynchronous External Memory Interface (EMIF)	SPRUEQ7C –February 2010
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