In SX Microcontrollers, SX/B Compiler and SX-Key Tool, phipi wrote:

Symmetrical drive is considered a "feature" because it's more difficult/expensive to fabricate PMOS transistors on-chip with drive characteristics equivalent to their NMOS counterparts. (I'm not sure why, though. It might be a size issue or something to do with symmetrical gate thresholds and/or doping issues.) Consequently, budget microcontrollers often have more pull-down capability than pull-up capability. It also explains why some MOSFET H-bridges use n-channel devices in both their lower and upper legs, as it's easier to include a gate-voltage-boosting charge pump than to incorporate high-current p-channel drivers. (I'm sure Beau or Chip could explain this [i]much[/i] better than I just did!)

-Phil

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