Understanding the 6820/1 PIA by CLIVE JONES

The 6820/1 PIA can be classed as two seperate 8-bit ports with interrupt
generating pins in one device. A two "channel" device if you like.

Each channel has an 8-bit (byte wide) port and any of those 8 port pins
can be configured as an input or output in any configuration. The ports
are refered to as "port A" and "port B". The interrupt generating pins
are CA1 and CA2 for port A and CB1, CB2 for port B. These are inputs on
the the other side of the PIA (to the "outside world" - not the
processor bus side). Cx1 is *always* an interrupt input pin, Cx2 can be
either an interrupt generating pin or a basic output pin as per the

Any state change received on the Cxx pins appear on the IRQ (Interrupt
ReQuest) pins on the bus side of the PIA (providing Cx2 is set to
generate interrupts). These pins - IRQa/b are tied together and also
tied to all other PIA IRQ pins in the system. This IRQ line is tied to
the 680x processor with a pull-up resistor (wire OR operation). When the
PIA acknowledges an interrupt on it's own Cxx pins, it sets the IRQa/b
output pins low and the processor starts the IRQ service routine.

The PIA also has three Chip Select pins (CSx) which bring the PIA out of
standby or "tri-state" operation via address decoding. In it's tri-
state, the data bus lines are high impedance - virtually open circuit so
that they do not present any weight or draw current from the processor
data bus whilst it's carrying out other system hardware operations. The
IRQa/b output pins and the port sides/Cxx pins remain active even during
tri-state operation so that they can still assert IRQs or acknowledge
incomming data on the port side.

There are also two Register Select pins (RS0/1) and these two bits
determine which of the internal six registers are being read from or
written too. However, you don't need to be a rocket scientist to work
out that you can't address six registers with two bits (00,01,10,11 = 4
address combinations). So to understand how the PIA works - you need to
look inside...

RS0/1 Description Function

0 0 Peripheral Register A Port data bits themselves
0 0 Data Direction Reg. A Sets I/O pin. 0=input, 1=output

...the first thing you see here as that both registers share the same
address. You determine which register you want to access by setting bit
2 of the "control" register. A "control" register exists for each

0 1 Control Register A

 |      |    |    |    |    |    |_____|
 |      |    |    |    |    |       |
 |      |    |    |    |    |       '-- CA1 control (set-up)
 |      |    |    |    |    '-- =0 Direction. =1 Port itself/Periph. A
 |      |    '----'----'--- CA2 control (set-up)
 |      '--- (read only) Interrupt via CA2 (if set to gen. interrupts)
 '--- (read only) Interrupt via CA1

...bits 0-5 can be written too and read by the processor but the IRQ
bits (6 and 7) are set by the PIA internally and the processor reads
these two bits to determine which CAx pin asserted the interrupt and the
appropriate action is then taken. The processor cannot write to bits 6
and 7. When the PIA sets bit 6 or 7 to "1" - it will lower the
appropriate IRQa/b output pin, but since they're both tied together -
they both go low. The PIA will clear bit 6 and 7 itself after the
processor has read the control port, ready for the next interrupt.

Port B has exactly the same configuration but substisute "B" for "A" and
"2" for "1" on the CA/B inputs...

1 0 Peripheral Register B Port data bits themselves
1 0 Data Direction Reg. B Sets I/O pin. 0=input, 1=output
1 1 Control Register B Sets CBx config. Port Register
access etc. that's your register addressing taken care of.

Out of reset. The PIA will always configure itself to both ports being
inputs and all interrupts disabled. The processor has to reconfigure
this for proper in-game operation.

So if you were to set up port A using 6800/2/8 assembly, it would look
like this (for example);

A_direct equ $2000
A_data equ $2000
A_cntrl equ $2001

clr A_cntrl ;access port A direction register (bit2=0)
ldaa #$0f ;upper 4 bits=input, lower 4 bits=output
staa A_direct ;set them.
tab ;accumulator B=accumulator A
ldaa #$04 ;now access the...
staa A_cntrl ; port iself
stab A_data ;set outputs (lower 4 lines PA0/3 high out)

... ;set port B...

The set-up and programming of the PIA depends on it's function. For
instance, port "A" may set set to all outputs and each bit individually
set high in sequence to sweep the switch matrix. Port "B"" may be
configured as all inputs to read the returns from the matrix and the
code will determine the valid switch closures and their scoring etc.

Interupt Handling

Cx2 is the only pin that can be configured as I/O. Cx1 is interrupt
generating only.

* Cx1 configuration *

bit1 bit0 Cx1 state IRQa/b status bit7 state
--------- --------- ------------- ----------

0 0 High>Low Disabled (High) =1 on low transition Cx1 pin
0 1 High>Low low when bit7=1 =1 on low transition Cx1 pin
1 0 Low>High Disabled (High) =1 on High transition Cx1 pin
1 1 Low>High low when bit7=1 =1 on High transition Cx1 pin

* Cx2 as Interrupt Generating *
(Bit 5 cleared.)

bit5 bit4 bit3 Cx2 state IRQa/b status bit6 state
-------------- --------- ------------- ----------
0 0 0 High>Low Disabled (High) =1 on low transition Cx2
0 0 1 High>Low low when bit6=1 =1 on low transition Cx2
0 1 0 Low>High Disabled (High) =1 on high transition Cx2
0 1 1 Low>High low when bit6=1 =1 on high transition Cx2

* Cx2 as I/O *
(Bit 5 set.)

bit5 bit4 bit3 bit2 Function
------------------- --------

1 0 0 0 Cx2 low after CPU reads port A data next low clock
1 0 0 1 Cx2 high when bit 7 is set (interrupt pending Cx1)
1 1 0 x Cx2 = contents of bit 3 (Cx2=output 0 this case)
1 1 1 x Cx2 = contents of bit 3 (Cx2=output 1 this case)


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