Article ID: 000084417 Content Type: Troubleshooting Last Reviewed: 01/01/2015

Can the clock outputs generated from the altclklock Megafunction clock negative-edge triggered registers in the input/output elements (IOEs) for APEX series devices?

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Description

Yes, for APEX series devices you can clock negative-edge triggered registers in the IOE from a PLL, but all IOE registers must be clocked with the same polarity from a given PLL output. Specifically, for any given PLL output, you can only clock either all positive-edge triggered IOE registers or all negative-edge triggered IOE registers. See Figure 1.

 

Figure 1. PLL IOE Clocking

Figure 1. PLL IOE Clocking

Therefore, if you want to clock some IOEs on the positive edge and some on the negative edge, you would need to use one PLL output for each.

If you are getting the error "The <PLL port name> port of <PLL cell name> cannot feed IO cells both positively and negatively", then you have mixed polarity clocks for IOE registers (input and output registers) from a single PLL output.

Related Products

This article applies to 3 products

APEX™ 20KE
Apex™ II
Apex™ 20KC