The Microcontroller Idea Book - Jan Axelson's Lakeview Research

To set or clear a different bit, determine which bits to set and clear for the result you want,
convert the value to decimal or hexadecimal, and write the value to 0FC03h.
Adding Handshaking
For many applications, Mode 0 is all you need. Modes 1 and 2 add handshaking, or control,
signals for components that require them.
Mode 1 is Strobed Input/Output. It uses Port C for handshaking signals that let the
8052-BASIC and the peripheral tell each other whether or not they’re ready to send or receive
data, and to confirm that data has been received. Mode 1 also latches the input data, so you
can use an external signal to latch data into U15, and save it until the 8052-BASIC has time
to read it. In Mode 1, you can configure Ports A and B to be inputs, outputs, or one of each.
Each port has its own set of handshaking signals on Port C, as described in Table 6-2.
Input Control. For input ports, these are the added signals:
STB (strobe input) causes data to load into the 8255’s input latch. In Mode 1, the 8052-BASIC
can’t read the data at U15’s ports until STB’s rising edge latches the data in. You can use an
output of a clock or counter chip to latch data into U15 at timed intervals, or latch data when
a user presses a key.
IBF (input buffer full output) goes high to indicate that the 8255 has loaded data in response
to STB. When the 8052-BASIC reads the data, the rising edge of RD brings IBF low again.
You can use IBF to clear or reset the device that generated STB.
INTE (interrupt enable) is an internal signal that you must set to enable INTR, described next.
For Group A, you set INTE by writing 1 to Port C, bit 4. For Group B, write 1 to Port C, bit
2. The BASIC-52 statement to set INTE for Port A is this:
XBY(0F03h)=9
For Port B, it’s this:
XBY(0F03h)=5
Inputs and Outputs
Writing to these locations sets internal bits in U12. However, these write operations have
no effect on the STB inputs, which share the same bit addresses at Port C.
INTR (interrupt request output) goes high when INTE is set and both STB and IBF are high,
to signal that the 8255 has data waiting to be read. INTR can connect to an interrupt input on
the 8052-BASIC (pin 13, for example), to cause it to jump to an interrupt routine that reads
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