It's trivial but true, the 11.592MHz x 2 works very well too... Besides other it allows to achieve^* 115200 baud. Many modern '51 derivatives work up to 24MHz and higher.

However, it's a pity that most members of the 'RD2 sub-family are rated for 20MHz only in 6-clock mode - usually they work also when slightly overclocked to 22.1184MHz, but there is no guarantee for them to work so (I would expect that they migh fail when operating at wide temperature range). That would lead to the 18.432MHz as Oleg said (with maximum^* of 19200 baud).

A quite nice crystal for ocassional use is also the 14.31818MHz crystal which can be "excavated" from any old PC motherboard and many old VGA boards, too. The "ideal" frequency for baudrate generation (up to^* 38400 baud) is 14.7456MHz, but the difference (2.9%) is small enough to work in not demanding applications.

Jan Waclawek

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^* maximum of "standard" baudrates that can be generated on a "standard" PC COM port = an integer fraction of 115200; on a standard 12-clock 8051 using timer 1 as baudrate generator