2.6.2. Clocking and resets

This section describes:

Clocking

All configurations of the SMC support at least two clock domains, and have the following clock inputs:

  • smc_aclk

  • smc_mclk0

  • smc_mclk0n.

These clocks can be grouped into two clock domains:

AHB domain

smc_aclk is in this domain. You can only stop the smc_aclk domain signals when the SMC is in low-power mode.

Memory clock domain

The smc_mclk0 and smc_mclk0n are in this domain. smc_mclk0n is an inverted version of smc_mclk0. smc_mclk0 is used for timing and control signals.

You can tie off the smc_async0 and smc_msync0 pins so that the smc_aclk and smc_mclk0 clock domains can operate synchronously or asynchronously with respect to each other.

Synchronous clocking

The benefit of synchronous clocking is that you can reduce the read and write latency by removing the synchronization registers between clock domains. However, because of the integer relationship of the clocks, you might not be able to get the maximum performance from the system because of constraints placed on the bus frequency by the external memory clock speed. In synchronous mode, the handshaking between the smc_aclk and smc_mclk0 domains enables synchronous operation of the two clocks at multiples of each other, that is, ratios of n:1 and 1:m.

Asynchronous clocking

The main benefit of asynchronous clocking is that you can maximize the system performance, while running the memory interface at a fixed system frequency. Additionally, in sleep-mode situations when the system is not required to do much work, you can lower the frequency to reduce power consumption.

Reset

The SMC has two reset inputs:

hresetn

This is the reset signal for the smc_aclk domain.

smc_mreset0n

This is the reset signal for the smc_mclk0 domain.

You can change both reset signals asynchronously to their respective clock domain. Internally to the SMC the deassertion of the hresetn signal is synchronized to smc_aclk. The deassertion of smc_mreset0n is synchronized internally to smc_mclk0 and smc_mclk0n.

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