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September 18, 1996 (Version 1.04)
4-47
Table 18: Pin Descriptions
Pin Name
I/O
During
Config.
I/O
After
Config.
Pin Description
Permanently Dedicated Pins
VCC
I
I
Eight or more (depending on package) connections to the n5 V supply voltage
(+3.3 V for low-voltage devices). All must be connected, and each must be decoupled
with a 0.01 - 0.1
µ
F capacitor to Ground.
GND
I
I
Eight or more (depending on package type) connections to Ground. All must be con-
nected.
CCLK
I or O
I
During configuration, Configuration Clock (CCLK) is an output in Master modes or Asyn-
chronous Peripheral mode, but is an input in Slave mode, Synchronous Peripheral
mode, and Express mode. After configuration, CCLK has a weak pull-up resistor and
can be selected as the Readback Clock. There is no CCLK High time restriction on
XC4000-Series devices, except during Readback. See
and Low Time Specification for the Readback Clock” on page 65
for an explanation of
this exception.
DONE
I/O
O
DONE is a bidirectional signal with an optional internal pull-up resistor. As an output, it
indicates the completion of the configuration process. As an input, a Low level on
DONE can be configured to delay the global logic initialization and the enabling of out-
puts.
The optional pull-up resistor is selected as an option in MakeBits, the XACT
step pro-
gram that creates the configuration bitstream. The resistor is included by default.
PROGRAM
I
I
PROGRAM is an active Low input that forces the FPGA to clear its configuration mem-
ory. It is used to initiate a configuration cycle. When PROGRAM goes High, the FPGA
finishes the current clear cycle and executes another complete clear cycle, before it
goes into a WAIT state and releases INIT.
The PROGRAM pin has a permanent weak pull-up, so it need not be externally pulled
up to Vcc.
User I/O Pins That Can Have Special Functions
RDY/BUSY
O
I/O
During Peripheral mode configuration, this pin indicates when it is appropriate to write
another byte of data into the FPGA. The same status is also available on D7 in Asyn-
chronous Peripheral mode, if a read operation is performed when the device is selected.
After configuration, RDY/BUSY is a user-programmable I/O pin.
RDY/BUSY is pulled High with a high-impedance pull-up prior to INIT going High.
RCLK
O
I/O
During Master Parallel configuration, each change on the A0-A17 outputs (A0 - A21 for
XC4000EX) is preceded by a rising edge on RCLK, a redundant output signal. RCLK
is useful for clocked PROMs. It is rarely used during configuration. After configuration,
RCLK is a user-programmable I/O pin.
M0, M1, M2
I
I (M0),
O (M1),
I (M2)
As Mode inputs, these pins are sampled after INIT goes High to determine the configu-
ration mode to be used. After configuration, M0 and M2 can be used as inputs, and M1
can be used as a 3-state output. These three pins have no associated input or output
registers.
During configuration, these pins have weak pull-up resistors. For the most popular con-
figuration mode, Slave Serial, the mode pins can thus be left unconnected. The three
mode inputs can be individually configured with or without weak pull-up or pull-down re-
sistors. A pull-down resistor value of 4.7 k
Ω
is recommended.
These pins can only be used as inputs or outputs when called out by special schematic
definitions. To use these pins, place the library components MD0, MD1, and MD2 in-
stead of the usual pad symbols. Input or output buffers must still be used.