II.4.2 GAL22V10
The next circuit can't be realized with a GAL16V8 or GAL20V8. The reason
for this is that there are different tristate controls for the register
outputs. A GAL16V8 and GAL20V8 has only one tristate control for all
register outputs, this is the /OE (output enable) pin in mode 3.
Enable1 ---------------------------------+
|
|\|
+----+ | \
Clock ---o---------------|> Q|------| +------- Y1
| +----+ | | | /
A ---+--| | +---|D | |/
| | & |---+ +----+
B ---+--| |
| +----+ |\
| +----+ | \
+---------------|> Q|------| +------- Y2
| | | /
C -------------------|D | |/|
+----+ |
|
+----+ |
Enable2 -------------------| | |
| & |--------o-------- Y3
+---| |
| +----+
+----+ |
D ----| | |
| | |
E ----| >1 |-----o-------------------------- Y4
| = |
F ----| |
+----+
Futhermore all register outputs should be reseted asynchronously
when the inputs F and AsyncRe are high and they should be preseted
synchronously when the input SyncPre is high.
(To keep the circuit diagram as simple as possible the inputs
AsyncRe and SyncPre are not drawn in it.)
Don't think about what the function of this circuit is - there is non.
It is just an example.
Well, and this is one of many possible pin designations:
GAL22V10
---- ----
Clock 1| |24 +5V
A 2| |23 Y1
B 3| |22 Y2
C 4| |21 Y3
D 5| |20 Y4
E 6| |19 Enable1
F 7| |18 Enable2
NC 8| |17 NC
NC 9| |16 NC
NC 10| |15 NC
NC 11| |14 AsyncRe
GND 12| |13 SyncPre
---------
The only thing which you have to keep in mind here is that the clock
signal for the register outputs must be at pin 1 and that the outputs
must be at OLMC pins (pin 14-23).
To define the asynchronous reset and the synchronous preset for the register
outputs the GAL assembler offers you the keywords AR (asynchronous reset) and
SP (synchronous preset). Since AR and SP are keywords it is not allowed to
use them as pin names when a GAL22V10 is used.
The source file of this example looks like this:
******************************************************
GAL22V10
22V10
Clock A B C D E F NC NC NC NC GND
SyncPre AsyncRe NC NC NC Enable2 Enable1 Y4 Y3 Y2 Y1 VCC
Y1.R = A * B ; Y1 is a registered output => .R
Y1.E = Enable1
Y2.R = C
Y2.E = Enable2 * Y4 ; Attention: there is a feedback of Y4
; (Y4 is defined as output but it's
; used as input again)
Y3 = Enable2 * Y4 ; there is a feedback again
Y4 = D + E + F
AR = F * AsyncRe ; define asynchronous reset
SP = SyncPre ; define synchronous preset
DESCRIPTION
******************************************************
Instead of Y3 = Enable2 * Y4 you could also write:
Y3 = Enable2*D + Enable2*E + Enable2*F.
But using a feedback is much more comfortable than using this long
equation. Another reason for using a feedback is for example the
tristate control of Y2. In tristate controls there is just one
product term allowed, this means no ORs.