Stephen C. Holland Says:
You have to consider that making a resistor in the CMOS process is difficult at best, and so the weak pullups can be anywhere in a range of values. From the SX datasheet, the weak pullup current at Vdd = 5.5V can be between 100 and 190uA. This means a resistance of 28.9K - 55Kohm. Pretty wide range, and also pretty weak. If you need either a stronger pullup, or an accurate value, an external pullup should be used.
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