Multi-bit flip-ops (MBFFs) are widely used in modern circuit designs because of their lower power consumption and smaller footprint. However, conventional MBFFs have routability issues due to the dense intra-cell connections. Since many horizontal connections are populated in the typical MBFF layouts, metal-2 (M2) tracks are highly occupied inside the cell. Accordingly, routers cannot leverage the M2 tracks for inter-cell connections. The conventional MBFFs also show a limited impact on the cell area reduction. Since the cell area saving of an MBFF mainly comes from the clock driver sharing, the layouts of other ip-op modules remain almost the same. In this paper, we propose a compact MBFF with metal-less clock routing and smaller height implementation. To achieve a sparse population of M2 routing tracks, we vertically place MBFF modules and interconnect them using the poly layer. As a result, the wire length of M2 layer inside a cell is significantly reduced. We also propose the smaller cell height implementation for compact MBFF layouts. Assuming the default standard cell height of 9 tracks, we present a 6-track MBFF implementation and the glue logic which makes legal cell placement with the 9-track logic cells. Experiments with a few test circuits show that the number of routing grids having congestion overflow is reduced by 16% and 73%, on average, compared to the single-bit flip-op and conventional MBFF based designs, respectively. Total cell area is also reduced by 8% and 2%, on average, compared to the single-bit flip-op and conventional MBFF based designs, respectively.
Bidirectional cell refers to a standard cell, in which metal-1 is used for both horizontal and vertical connections. Unidirectional cell, on the other hand, assumes reserved routing, e.g. metal-1 for only horizontal and metal-2 for only vertical connections. It has been introduced to take advantage of regular metal patterns, which are easier to print and can overcome the lithography limitations in sub-32nm technology. In unidirectional cell, metal-2 is laid out following the cell placement pitch. Since metal-2 pitch is usually different from placement pitch, some within-cell metal-2 become off track. This significantly degrades metal-2 routability. We propose a unidirectional cell with floating metal-2. After initial cell placement, metal-2 segment within each cell is snapped to nearest metal-2 track and is fixed. In addition, we propose cell redesign and post-placement optimization to enhance metal-1 routability. Metal-1 connections are forced to populate in limited number of tracks, so that remaining tracks are exposed during routing. Combined with post-placement optimization, this allows many longer metal-1 tracks to be available for horizontal connection. Experiments with test circuits show that routing errors are reduced by 7% and 60% with the proposed metal-1 considerations and floating metal-2 together.