Rainbow Electronics AT91CAP9S250A - Manual (Page 185)

185
6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
Figure 22-16. Chip Select Wait State between a Read Access on NCS0 and a Write Access on NCS2
22.9.2 Early Read Wait State
In some cases, the SMC inserts a wait state cycle between a write access and a read access to
allow time for the write cycle to end before the subsequent read cycle begins. This wait state is
not generated in addition to a chip select wait state. The early read cycle thus only occurs
between a write and read access to the same memory device (same chip select).
An early read wait state is automatically inserted if at least one of the following conditions is
valid:
• if the write controlling signal has no hold time and the read controlling signal has no setup
time ( Figure 22-17 ).
• in NCS write controlled mode (WRITE_MODE = 0), if there is no hold timing on the NCS
signal and the NCS_RD_SETUP parameter is set to 0, regardless of the read mode ( Figure
22-18 ). The write operation must end with a NCS rising edge. Without an Early Read Wait
State, the write operation could not complete properly.
• in NWE controlled mode (WRITE_MODE = 1) and if there is no hold timing (NWE_HOLD =
0), the feedback of the write control signal is used to control address, data, chip select and
byte select lines. If the external write control signal is not inactivated as expected due to load
capacitances, an Early Read Wait State is inserted and address, data and control signals are
maintained one more cycle. See
Figure 22-19 .
A[25:2]
NBS0, NBS1,
NBS2, NBS3,
A0,A1
NCS0
NRD_CYCLE
Chip Select
Wait State
NWE_CYCLE
MCK
NCS2
NRD
NWE
D[31:0]
Read to Write
Wait State