R
XC5200 Series Field Programmable Gate Arrays
Z = D • A + D • B + D • C + D • N
A
B
C
N
~100 kΩ
D
D
D
D
N
A
B
C
BUFT
BUFT
BUFT
BUFT
A
B
C
N
X6466
"Weak Keeper"
Figure 10: 3-State Buffers Implement a Multiplexer
Input/Output Blocks
User-configurable input/output blocks (IOBs) provide the
interface between external package pins and the internal
logic. Each IOB controls one package pin and can be con-
figured for input, output, or bidirectional signals.
Table 5: Supported Sources for XC5200-Series Device
Inputs
XC5200 Input Mode
5 V,
TTL
5 V,
CMOS
Source
The I/O block, shown in Figure 11, consists of an input
buffer and an output buffer. The output driver is an 8-mA
full-rail CMOS buffer with 3-state control. Two slew-rate
control modes are supported to minimize bus transients.
Both the output buffer and the 3-state control are invertible.
The input buffer has globally selected CMOS or TTL input
thresholds. The input buffer is invertible and also provides a
programmable delay line to assure reliable chip-to-chip
set-up and hold times. Minimum ESD protection is 3 KV
using the Human Body Model.
Any device, Vcc = 3.3 V,
CMOS outputs
√
√
√
Unreliable
Data
Any device, Vcc = 5 V,
TTL outputs
Any device, Vcc = 5 V,
CMOS outputs
√
Optional Delay Guarantees Zero Hold Time
XC5200 devices do not have storage elements in the IOBs.
However, XC5200 IOBs can be efficiently routed to CLB
flip-flops or latches to store the I/O signals.
7
Vcc
Delay
Input
Buffer
The data input to the register can optionally be delayed by
several nanoseconds. With the delay enabled, the setup
time of the input flip-flop is increased so that normal clock
routing does not result in a positive hold-time requirement.
A positive hold time requirement can lead to unreliable,
temperature- or processing-dependent operation.
Pullup
I
Output
Buffer
PAD
Pulldown
O
T
The input flip-flop setup time is defined between the data
measured at the device I/O pin and the clock input at the
CLB (not at the clock pin). Any routing delay from the
device clock pin to the clock input of the CLB must, there-
fore, be subtracted from this setup time to arrive at the real
setup time requirement relative to the device pins. A short
specified setup time might, therefore, result in a negative
setup time at the device pins, i.e., a positive hold-time
requirement.
Slew Rate
Control
X9001
Figure 11: XC5200 I/O Block
IOB Input Signals
The XC5200 inputs can be globally configured for either
TTL (1.2V) or CMOS thresholds, using an option in the bit-
stream generation software. There is a slight hysteresis of
about 300mV.
When a delay is inserted on the data line, more clock delay
can be tolerated without causing a positive hold-time
requirement. Sufficient delay eliminates the possibility of a
data hold-time requirement at the external pin. The maxi-
mum delay is therefore inserted as the software default.
The inputs of XC5200-Series 5-Volt devices can be driven
by the outputs of any 3.3-Volt device, if the 5-Volt inputs are
in TTL mode.
The XC5200 IOB has a one-tap delay element: either the
delay is inserted (default), or it is not. The delay guarantees
a zero hold time with respect to clocks routed through any
of the XC5200 global clock buffers. (See “Global Lines” on
page 96 for a description of the global clock buffers in the
XC5200.) For a shorter input register setup time, with
Supported sources for XC5200-Series device inputs are
shown in Table 5.
November 5, 1998 (Version 5.2)
7-91
ETC [ ETC ]
