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3.1.3Taking 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
tEM_CS
tEM_A
tEM_OE
tEM_D
Delay on EM_CS from EMIF to ASRAM. EM_CS is driven by EMIF. Delay on EM_A from EMIF to ASRAM. EM_A is driven by EMIF.
Delay on EM_OE from EMIF to ASRAM. EM_OE is driven by EMIF. Delay on EM_D from ASRAM to EMIF. EM_D is driven by ASRAM.
Write Access
tEM_CS
tEM_A
tEM_WE
tEM_D
Delay on EM_CS from EMIF to ASRAM. EM_CS is driven by EMIF. Delay on EM_A from EMIF to ASRAM. EM_A is driven by EMIF.
Delay on EM_WE from EMIF to ASRAM. EM_WE is driven by EMIF. Delay on EM_D from EMIF to ASRAM. EM_D is driven by EMIF.
From Figure 14, the following equations may be derived. tcyc 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 tcyc 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.
R_SETUP ) R_STROBE w tEM_A ) tACC(m) ) tSU ) tEM_D * 1
tcyc
R_SETUP ) R_STROBE ) R_HOLD w tRC(m) * 3
tcyc
R_HOLD w tH * tEM_D * tOH(m) * tEM_A * 1
tcyc
TA w tEM_CS ) tCOD(m) ) tEM_D * 1 tcyc
34 | Asynchronous External Memory Interface (EMIF) | SPRUEQ7C |
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