How to Resolve Clock Park Low of ICG

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How to Resolve Clock Park Low of ICG

As mentioned in our other post, Is Violation of Retention Flip-Flop Clock Parking a Real Issue, clock parked low is normally a requirement for retention flip flops for them to retain value properly during power down sleep. Here is a typical implementation of retention FF. Note the “AND” gate is NOT inside retention FF. As a matter of fact, this “AND” gate is actually a power isolation low cell and RETAIN# is active low iso enable signal. The “CLK” signal, input to AND/isolow cell, is required to be parked low before sleep or before RETAIN# is asserted low.

Let’s take a look of a very simple scenario. There is a clock source at chip top level. Its clk output is wired to core0 and core1. Inside core0 we add a ICG cell, clock gating cell, to gate off clock before sleep.

A typical ICG cell works as below.

But using this ICG cell in above simple use case does NOT work. Next we will discuss why and several ways to resolve it.

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We are a small design team with many years of experience in ASIC/FPGA IP and system designs which involve UART, SPI, AHB, AXI, ACE, USB, PCIE, etc.
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