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OR3C55 Datasheet(PDF) 39 Page - Agere Systems |
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OR3C55 Datasheet(HTML) 39 Page - Agere Systems |
39 / 210 page Lucent Technologies Inc. 39 Data Sheet June 1999 ORCA Series 3C and 3T FPGAs Programmable Input/Output Cells (continued) Zero-Hold Input There are two options for zero-hold input capture in the PIO. If input delay mode is selected to delay the signal from the input pin, data can be either registered or latched with guaranteed zero-hold time in the PIO using a system clock. To guarantee zero hold, the system clock spine structure must be used for clocking, as will be discussed later. The fast zero-hold mode of the PIO input takes advantage of the latch/FF combination and sources the input FF data from a dedicated latch that is clocked by the ExpressCLK from the PIC. The ExpressCLK is a clock from a dedi- cated input pin designed for fast, low-skew operation at the I/Os and is described more fully in the Clock Distribu- tion Network and PIC Interquad (MID) Routing sections that follow. The combination of ExpressCLK latch and system clock FF guarantees a zero-hold capture of input data in the PIO FF, while at the same time reducing input setup time. Figure 23 shows a schematic of the fast-capture latch/FF and a sample timing diagram. 5-5974(F) Note: CE and LSR signals not shown. Figure 23. Fast-Capture Latch and Timing D Q INPUT DATA LATCH CLK O I EXPRESSCLK O I SYSTEM CLK CD = 1 CLOCK ENABLE LOCAL SET/RESET DQ FF S/R CE DATA OUT TO PIC ROUTING EXPRESSCLK SYSTEM CLK INPUT DATA QLATCH QFF B AC D E B AC D E AB C D |
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