AT91M55800A_0212 Datasheet, PDF (31/227 Pages) ATMEL Corporation – AT91 ARM Thumb Microcontrollers

Data Float Wait State
AT91M55800A
Some memory devices are slow to release the external bus. For such devices it is nec-
essary to add wait states (data float waits) after a read access before starting a write
access or a read access to a different external memory.
The Data Float Output Time (t_DF) for each external memory device is programmed in the
TDF field of the EBI_CSR register for the corresponding chip select. The value (0 - 7
clock cycles) indicates the number of data float waits to be inserted and represents the
time allowed for the data output to go high impedance after the memory is disabled.
Data float wait states do not delay internal memory accesses. Hence, a single access to
an external memory with long t_DF will not slow down the execution of a program from
internal memory.
The EBI keeps track of the programmed external data float time during internal
accesses, to ensure that the external memory system is not accessed while it is still
busy.
Internal memory accesses and consecutive accesses to the same external memory do
not have added Data Float wait states.
Figure 16. Data Float Output Time
MCK
ADDR
NCS
NRD
(1)
(2)
tDF
D0-D15
Notes: 1. Early Read Protocol
2. Standard Read Protocol
31
1745C–ATARM–12/02