Zynq-7000 PCB Design Guide
67
UG933 (v1.8) November 7, 2014
Chapter 5:
Processing System (PS) Power and Signaling
QSPI
The clock, data, and SS lines are recommended to have matched lengths to facilitate
meeting setup and hold times. PCB and package delay skew for QSPI_IO[0:3] and QSPI_SS
lines relative to QSPI_SCLK should be less than ±50 ps. Keeping the clock and data lines
equal provides greater immunity to undesirable setup and hold time effects. It is highly
recommended to perform a signal integrity analysis on the clock line at the near (close to
Zynq-7000 AP SoC device) and far ends.
The overall trace delays of the clock and data lines affect the maximum frequency at which
the QSPI interface can run.
IMPORTANT:
An important rule that must be followed is that the clock-to-out time of the flash device
(tckoflash) plus twice the maximum PCB trace length delay (Tpd) must be greater than the maximum
hold time requirement of the Zynq-7000 AP SoC device. In other words:
TQSPICKD <
Tckomin
flash
+ 2 *
T
pd
(requirement)
For example, with a Zynq-7000 AP SoC hold time requirement of 1.3 ns, and a flash
clock-to-out of 1.0 ns, the propagation delay of the clock and data lines must be at least
0.15 ns. With a higher hold time requirement, the PCB trace delays will need to increase.
The following table shows minimum trace delays depending on various timing scenarios:
Maximum Operating Frequency (Feedback Mode Enabled)
Note:
To operate QSPI at the highest possible frequency, feedback mode must be enabled.
To enable this mode, MIO pin 8 must be programmed as the feedback output clock and
must only be connected to a pull-up/pull-down resistor on the PCB for boot strapping (see
As long as the clock and data lines are matched and at least their minimum length per the
rule above, the maximum frequency for the QSPI interface is the
lesser
of:
Equation 5-1
Equation 5-2
TQSPICKD
(ns)
Tckominflash
(ns)
Minimum Tpd
(ns)
1.3
0.0
0.65
1.3
1.0
0.15
3.0
0
1.50
3.0
1.0
1.0
Fmax
1
1
2
TQSPICKOMAX Tsu
flash
+
(
)
×
-----------------------------------------------------------------------
=
Fmax
2
1
Tcko
flash
TQSPIDCK
2
Tpd
×
(
)
+
+
----------------------------------------------------------------------------
=
