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R_SETUP ) R_STROBE w
ǒ
t
EM_A
) t
ACC
(m) ) t
SU
) t
EM_D
Ǔ
t
cyc
* 1
R_SETUP ) R_STROBE ) R_HOLD w
t
RC
(m)
t
cyc
* 3
R_HOLD w
ǒ
t
H
* t
EM_D
* t
OH
(m) * t
EM_A
Ǔ
t
cyc
* 1
TA w
ǒ
t
EM_CS
) t
COD
(m) ) t
EM_D
Ǔ
t
cyc
* 1
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3.1.3 Taking Into Account PCB Delays
The equations described in Section 3.1.2 are for the ideal case, when board design does not contribute
delays. Board characteristics, such as impedance, loading, length, number of nodes, etc., affect how the
device driver behaves. Signals driven by the EMIF will be delayed when they reach the ASRAM and
conversely. Table 19 lists the delays shown in Figure 14 and Figure 15 due to PCB affects. The PCB
delays are board specific and must be estimated or determined though the use of IBIS modeling. The
signals denoted (ASRAM) are the signals seen at the ASRAM. For example, EM_CS represents the signal
at the EMIF and EM_CS (ASRAM) represents the delayed signal seen at the ASRAM.
Table 19. ASRAM Timing Requirements With PCB Delays
Parameter Description
Read Access
t
EM_CS
Delay on EM_CS from EMIF to ASRAM. EM_CS is driven by EMIF.
t
EM_A
Delay on EM_A from EMIF to ASRAM. EM_A is driven by EMIF.
t
EM_OE
Delay on EM_OE from EMIF to ASRAM. EM_OE is driven by EMIF.
t
EM_D
Delay on EM_D from ASRAM to EMIF. EM_D is driven by ASRAM.
Write Access
t
EM_CS
Delay on EM_CS from EMIF to ASRAM. EM_CS is driven by EMIF.
t
EM_A
Delay on EM_A from EMIF to ASRAM. EM_A is driven by EMIF.
t
EM_WE
Delay on EM_WE from EMIF to ASRAM. EM_WE is driven by EMIF.
t
EM_D
Delay on EM_D from EMIF to ASRAM. EM_D is driven by EMIF.
From Figure 14, the following equations may be derived. t
cyc
is the period at which the EMIF operates. The
R_SETUP, R_STROBE, and R_HOLD fields are programmed in terms of EMIF cycles where as the data
sheet specifications are typically given in nano seconds. This is explains the presence of t
cyc
in the
denominator of the following equations. A minus 1 is included in the equations because each field in
ACFGn is programmed in terms of EMIF clock cycles, minus 1 cycle. For example, R_SETUP is equal to
R_SETUP width in EMIF clock cycles minus 1 cycle.
34
Asynchronous External Memory Interface (EMIF) SPRUEQ7C–February 2010
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