Implementations may also make use of a double-buffered register to avoid metastability when transferring data between clock domains. The simplest form of the PISO has a single shift register that receives the parallel data once per parallel clock, and shifts it out at the higher serial clock rate. It may use an internal or external phase-locked loop (PLL) to multiply the incoming parallel clock up to the serial frequency. The PISO (Parallel Input, Serial Output) block typically has a parallel clock input, a set of data input lines, and input data latches. There are 4 different SerDes architectures: (1) Parallel clock SerDes, (2) Embedded clock SerDes, (3) 8b/10b SerDes, (4) Bit interleaved SerDes. Thus, we expect to clock in data at SER when shifting as opposed to parallel loading.The basic SerDes function is made up of two functional blocks: the Parallel In Serial Out (PISO) block (aka Parallel-to-Serial converter) and the Serial In Parallel Out (SIPO) block (aka Serial-to-Parallel converter). The legend 1, 3D implies that SER is controlled by M1 and C3 clock. The upper stage at A is a wider block than the others to accommodate the input SER. In this case, we can conclude that the parallel data is loaded synchronously with the clock C3. The internal 2, 3D indicates that data, D, is controlled by M2 and C3 clock. The 8-shift stages, as indicated by title SRG8, are identified by the external inputs A, B, C, to H. The slash (/) is a separator between these two functions. Second, whenever M1 is asserted, as indicated by the 1 of C3/1 (arrow), the data is shifted as indicated by the right pointing arrow. First, C3 for shifting parallel data wherever a prefix of 3 appears. There are three control signals: M1 (Shift), M2 (Load), and C3/1 (arrow) (inhibited clock). The large notched block at the top of the ‘74ASL166 is the control section of the ANSI symbol.
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