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Card #c523IR // #c523Synthesis

Some instructions read the external pin state, but read-modify-write instructions read the internal port latch, modify it, and write it back — so a bit toggled this way reflects what the port was last driven to, not what is happening on the pin. Port 0 is open-drain and needs external 10K pull-ups; it also multiplexes address/data (AD0-AD7), while P2 carries the high address byte (A8-A15) and P3 pins double as RxD/TxD, INT0/INT1, T0/T1, WR, and RD.

— In our words · Source: 8051 Microcontroller Notes

[Use when]

Toggling 8051 port bits or driving external buses — know whether your instruction reads the latch or the pin, add the pull-ups on P0, and account for which pins are stolen by timers, interrupts, or memory access.

[Why it matters]

This latch-vs-pin split is the classic source of 8051 port bugs — toggling a bit can resurrect a stale value — and the notes treat the ports' alternate functions as the price of the chip's integration: every pin does two jobs.

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