Design Entry Techniques
CPLD Schematic Design Guide
2-9
You can prevent the fitter from automatically using the global tristate
control pins.
1.
In the Design Manager, go to the Flow Engine and select
Setup
Options
2.
The Design Implementation Option menu appears. Select
Edit
Template
3.
Then select
Basic Options
4.
Disable the box adjacent to
Use Global Output Enables
.
If global output enable optimization is disabled, IBUF inputs used for
tristate control always pass through the logic array. You can still use
BUFGTS symbols or the BUFG=OE attribute to explicitly specify
global tristate control inputs.
Asynchronous Clear and Preset
To use a device input as an asynchronous clear or preset source, you
can simply connect an IBUF to the CLR or PRE input of one or more
flip-flops in your design. The fitter automatically uses the global set/
reset pin (GSR) of the CPLD whenever possible. A global set/reset
input signal may pass through an inverter to perform active-low clear
or preset. A global set/reset inputs may also be used as an ordinary
input signal to other logic elsewhere in the design.
If a device input passes through any logic function other than an
inverter before it is used as an asynchronous clear or preset by any
flip-flop, the input cannot be routed using the global set/reset path.
Instead, the clear or preset signal will be routed through the logic
array for all flip-flops controlled by such an input. Routing a clear or
preset through the logic array uses up one extra p-term for each flip-
flop.
There is only one global set/reset pin on each XC9500 device. If you
need to explicitly control the use of the global set/reset pin, you can
use the BUFGSR symbol in place of an IBUF. You can alternatively
apply the attribute BUFG=SR to an IBUF symbol or the pad net.
You can prevent the fitter from using the global set/reset pin. In the
Design Manager, disable the
Use Global Set/Reset
option in
the
Basic Options
template of the Flow Engine.
Summary :
Design Entry Techniques CPLD Schematic Design Guide 2-9 You can prevent the fitter from automatically using the global tristate control pins. If a device input passes through any logic function other than an inverter before it is used as an asynchronous clear or preset by any flip-flop, the input cannot be routed using the global set/reset path.
Tags :
input,design,setreset,use,preset,clear,logic,through,pin,ibuf,control,select,asynchronous