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Tips for configuring OMAP35x, AM35x, and AM-DM37x GPMC registers

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GPMC datapath overview[edit]

The figure below gives a simplified overview of the GPMC datapath. In synchronous mode, data from the flash are first captured on a GPMC_CLK rising edge, and then on a GPMC_FCLK rising edge. The second capture is enabled by logic that takes RDACCESSTIME, WAITMONITORING and GPMCFCLKDIVIDER into account.

NOTE: When the GPMC is configured for synchronous mode, the GPMC_CLK signal (which is an output) must also be set as an input (CONTROL.CONTROL_PADCONF_GPMC_NCS7[24] INPUTENABLE1 = 1). GPMC_CLK is looped back through the output and input buffers of the corresponding GPMC_CLK pad at the device boundary. The looped-back clock is used to synchronize the sampling of the memory signals.

The important point is that the second capture will occur at cycle number: · RDACCESSTIME+1 if GPMCFCLKDIVIDER is 0 (GPMC_CLK = GPMC_FCLK) , · RDACCESSTIME+2 if GPMCFCLKDIVIDER is greater than 0 (GPMC_CLK = GPMC_FCLK divided by 2, 3 or 4)

As the maximum GPMC_CLK speed is 100MHz, the logical path between the first sampling flop (on GPMC_CLK) and the second one (on GPMC_FCLK) was characterized to be 10ns long.

OMAP35x GPMC datapath.jpg

Rules for Setting up Synchronous Accesses[edit]

Rule 1. PAGEBURSTACCESSTIME must be a multiple of GPMCFCLKDIVIDER+1.

For synchronous accesses, your access time per burst must take into consideration the clock divider for the GPMC module.

For example, if your L3 Clock is 166MHz, you have to reduce the GPMC_CLK by setting GPMCFCLKDIVIDER=1 (ie, divide by 2).

GPMC_FCLK = 166MHz GPMC_CLK = 83MHz

Thus PAGEBURSTACCESSTIME must be a multiple of 2 (which implies a minimum of 2 as well). Thus, if you set PAGEBURSTACCESSTIME to 4, your burst will be 2 GPMC_CLK cycles long.

Rule 2. (RDCYCLETIME – CLKACTIVATIONTIME) must be a multiple of GPMCFCLKDIVIDER+1.

Similar to Rule 1, this also ensures the synchronous access ends with a rising edge on both GPMC_CLK and GPMC_FCLK.


Rule 3. (RDACCESSTIME – CLKACTIVATIONTIME) modulus (GPMCFCLKDIVIDER + 1) must be different from GPMCFCLKDIVIDER

This is a consequence of the internal sampling scheme in OMAP3 (see block diagram above). Violating this rule will not result in broken functionality, but rather in a limitation in the maximum GPMC_CLK frequency.

Figure 1 illustrates the sampling timing when Rule 3 is followed. The blue traces show the case where (RDACCESSTIME - CLKACTIVATIONTIME)%2 = 0, the red traces show the case when (RDACCESSTIME - CLKACTIVATIONTIME)%2 = 1.

Rule3A.jpg

Figure 2 shows an example where Rule 3 is violated. The data is re-sampled only one clock cycle after the initial sample.

Rule3b.jpg

Figure 3 shows an example where Rule 3 is violated. Again, the data is re-sampled only one clock cycle after the initial sample.

Rule3c.jpg


Rule 4. If GPMCFCLKDIVIDER is greater than 0, RDCYCLETIME must be greater or equal to RDACCESSTIME+2. If GPMCFCLKDIVIDER is 0, RDCYCLETIME must be greater or equal to RDACCESSTIME+1.

This is also a consequence of the sampling scheme described above. If Rule 4 is not obeyed, then the end of the access (determined by RDCYCLETIME) will occur before the last data is sampled on the second flop (at RDACCESSTIME+1 or RDACCESTIME+2 depending on GPMCFCLKDIVIDER). This will cause undefined behavior of the GPMC state machine.


Rule 5. If GPMCFCLKDIVIDER is greater than 0 and WAITREADMONITORING is enabled, CSRDOFFTIME must be greater or equal to RDACCESSTIME+2. If GPMCFCLKDIVIDER is 0 and WAITREADMONITORING is enabled, CSRDOFFTIME must be greater than or equal to RDACCESSTIME+1

This rule is another consequence of the sampling scheme described above. In synchronous mode of the GPMC, when wait monitoring is enabled, the internal GPMC state machine looks for wait assertion/deassertion. The state machine is designed so that in this case, one extra clock cycle is needed. If CSRDOFFTIME does not take this extra clock cycle into account, the GPMC state machine will go to an undefined state.


Rule 6. If GPMCFCLKDIVIDER is greater than 0 and WAITREADMONITORING is enabled, OEOFFTIME must be greater or equal to RDACCESSTIME+2. If GPMCFCLKDIVIDER is 0 and WAITREADMONITORING is enabled, OEOFFTIME must be greater than or equal to RDACCESSTIME+1.

The reasoning for this is the same as Rule 5, but it applies to the OE signal.


Rule 7. Regardless of WAITREADMONITORING and GPMCFCLKDIVIDER, OEOFFTIME and CSRDOFFTIME must be greater than or equal to RDACCESSTIME+1.

This rule is a consequence of the GPMC state machine implementation: for synchronous burst access, the GPMC state machine takes one cycle after AccessTime to freeze the counters controlling the CsRdOffTime and OeOffTime.

Take the example if the software is doing a two word burst, and we have RDACCESSTIME = OEOFFTIME. At RDACCESSTIME+1, the GPMC state machine adds PAGEBURSTACCESSTIME to OEOFFTIME, But it is already too late to deassert OE at the end of this cycle. This causes OE to be deasserted on cycle later. The same is true for CS and for longer burst access.


Rule 8. RdCycleTime and WriteCycleTime must be strictly greater than all the Off times of the control signals (OeOffTime CsRdOffTime, CsWrOffTime, AdvRdOffTime, AdvWrOffTime, WeOffTime), plus the possible extra delays added (CSExtraDelay, AdvExtraDelay, WeExtraDelay, OeExtraDelay, CsExtraDelay).

It is due to the internal logic of the GPMC, and rather a logical consequence of the waveform calculations than a real constraint. It is mainly listed in this note for reference.


Rule 9. In case of burst writes, if WaitMonitoring is enabled, CsWrOffTime must be greater than or equal to AccessTime+1.(there is no dependency with ClkDivider for writes).

This is for giving enough hold time for the last data to be corretly sampled.

More Tips[edit]

  • If you need to lengthen the timing parameters, consider setting the TIMEPARAGRANULARITY bit to 1. This doubles the programmed time for each of the following parameters: RD/WRCYCLETIME, RD/WRACCESSTIME, PAGEBURSTACCESSTIME, CSONTIME, CSRD/WROFFTIME, ADVONTIME, ADVRD/WROFFTIME, OEONTIME, OEOFFTIME, WEONTIME, WEOFFTIME,CYCLE2CYCLEDELAY, BUSTURNAROUND, TIMEOUTSTARTVALUE.
  • If you are multiplexing address and data (ie, MUXADDDATA=1), ensure WRDATAONADMUXBUS is set appropriately to tell the controller when to output data on the multiplexed bus.
  • For Write transactions with bursts, important parameters are WRDATAONADMUXBUS and WRACCESSTIME. WRDATAONADMUXBUS will define the clock cycle in which the bus will transition from address to data. WRACCESSTIME should be set up to end on the first write data in the burst. When WRACCESSTIME completes, the PAGEBURSTACCESSTIME should take over and control signal timings are “frozen” until the burst is complete. So even though you set up your WEOFFTIME as 6 cycles, for example, if that falls with the burst, it will automatically extend until the burst is finished. OEOFFTIME works the same way.
  • All rules similarly apply to write transactions. Keep a similar relationship between GPMCCLKDIVIDER and WRCYCLETIME and WRACCESSTIME.
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