Changes
The Zilog [[Z80 PIO]] chip is a '''Dual 8bit''' I/O Port (not to be confused with the '''Triple 8bit''' 8255 PPI, which is sometimes referred to as PIO, too).
== Register Addressing ==
The chip has B/A (Port B/A Select) and C/D (Control/Data Select) pins, which may be connected to the CPUs A0 and A1 pins (or vice-versa, A1 and A0) (or other pins like A3 and A4). Anyways, no matter how the addressing is done:
There are 4 registers - 2 Data registers, and 2 Control registers.
== Data Registers A and B
Data direction can be configured in the corresponding control register. Depending on the direction, the data bits are inputs or outputs with following meaning:
7-0 Data (0=Low, 1=High)
Aside from the 8 data lines, each port additionaly supports automatic /STROBE and READY handshaking signals (whereas, unlike as in printer ports, STROBE can act as an input, not an output), the handshaking stuff isn't used in Mode3.
== Control Registers A and B ==
When D0=0:
7-1 Interrupt Vectors Bit7-1 (Vector Bit0 is fixed = zero)
0 Must be 00h
When D0..D3=03h:
7 Interrupt Enable (1=Enable) (same as when D0..D3=0Fh)
6-4 Not used (should be zero)
0-3 Must be 03h
When D0..D3=07h:
7 Interrupt Enable (1=Enable)
6 Interrupt AND/OR (0=OR, 1=AND) ;\
5 Interrupt High Low (1=High) ; used in Mode3 only
4 Interrupt Mask Follows ;/ ;<--- also ACK interrupt?
0-3 Must be 07h
When D0..D3=0Fh:
7-6 Mode (0=out, 1=in, 2=bidirectional,3=control) (mode2 on PortA only)
5-4 Not used (should be zero)
0-3 Must be 0Fh
After Mode3 (control mode) was set, then next control write is:
0-7 Direction bits, separately for each pin (0=out, 1=in)
After InterruptMaskFollows was set (in Mode3 only?), then next write is:
0-7 Select bits to be monitored (in OR mode: 0=Select)
== Usage in CPC ==
A [[Z80 PIO]] is used by [[ACU Real Time Clock (DIY)]], and [[Dk'tronics Real Time Clock]].
