Spartan-6 66mhz pci

[jonpry]

I'm trying to make a simple PCI interface in a XC6SLX45. In the end
I'll probably go with the premade Xilinx core but I wanted to get
familiar with the details first. It seems that the chip simply cannot
meet the timing for this under any circumstances. I've use the UCF
pinouts created for the xilinx core hoping this includes some magic.
Creating a simple do nothing design that essentially registers frame_n
and pipes it out of trdy_n using a BUFIO2 clock i was able to achieve
this:

Slack (slowest paths): -0.671ns (requirement - (clock arrival + clock
path + data path + uncertainty))
Source: Inst_pci_wrapped/pci_trdy_o_int (FF)
Destination: TRDY_N (PAD)
Source Clock: clk_buf rising at 0.000ns
Requirement: 7.000ns
Data Path Delay: 4.597ns (Levels of Logic = 1)
Clock Path Delay: 3.049ns (Levels of Logic = 2)
Clock Uncertainty: 0.025ns

Clock Uncertainty: 0.025ns ((TSJ^2 + TIJ^2)^1/2 + DJ) / 2
+ PE
Total System Jitter (TSJ): 0.050ns
Total Input Jitter (TIJ): 0.000ns
Discrete Jitter (DJ): 0.000ns
Phase Error (PE): 0.000ns

Maximum Clock Path at Slow Process Corner: PCLK to Inst_pci_wrapped/
pci_trdy_o_int
Location Delay type Delay(ns) Physical
Resource
Logical
Resource(s)
-------------------------------------------------
-------------------
L4.I Tiopi 1.040 PCLK
PCLK
PCLK_IBUF

ProtoComp8.IMUX.12
BUFIO2_X1Y15.I net (fanout=1) 0.413 PCLK_IBUF
BUFIO2_X1Y15.IOCLK Tbufcko_IOCLK 0.096 bufio2_inst
bufio2_inst
OLOGIC_X0Y59.CLK0 net (fanout=2) 1.500 clk_buf
-------------------------------------------------
---------------------------
Total 3.049ns (1.136ns
logic, 1.913ns route)
(37.3% logic,
62.7% route)

Maximum Data Path at Slow Process Corner: Inst_pci_wrapped/
pci_trdy_o_int to TRDY_N
Location Delay type Delay(ns) Physical
Resource
Logical
Resource(s)
-------------------------------------------------
-------------------
OLOGIC_X0Y59.OQ Tockq 0.842
Inst_pci_wrapped/pci_trdy_o_int

Inst_pci_wrapped/pci_trdy_o_int
M3.O net (fanout=1) 0.234
Inst_pci_wrapped/pci_trdy_o_int
M3.PAD Tioop 3.521 TRDY_N

Inst_pci_wrapped/trdy_buf_inst/OBUFT
TRDY_N
-------------------------------------------------
---------------------------
Total 4.597ns (4.363ns
logic, 0.234ns route)
(94.9% logic,
5.1% route)

It seems unlikely to me that any of that can be removed, and it is not
even close to the 6ns clock to out that I need. Am I missing
something? How can 66mhz pci timing possibly be met in a device with
3.5ns Tioop?

[smi...@gmail.com]

> It seems unlikely to me that any of that can be removed, and it is not
> even close to the 6ns clock to out that I need. Am I missing
> something? How can 66mhz pci timing possibly be met in a device with
> 3.5ns Tioop?

You're right. Spartan 6 only supports 33 MHz PCI. See table 1 in DS206, LogiCORE IP 32-Bit Initiator/Target v3 & v4 for PCI.

www.xilinx.com/support/documentation/ip_documentation/pci32/v4_16/pci_32_ds206.pdf

[jonpry]

> www.xilinx.com/support/documentation/ip_documentation/pci32/v4_16/pci...

Thanks. That would explain it! I had assumed that since it could be
done in S3E that it would be a breeze in S6.

[Gabor]

In the Spartan 3E and some earlier parts, there were special
pins for TRDY and IRDY with built in logic to get the timing
down to something possible. Only Xilinx was allowed to use this
feature, and it was part of every Xilinx PCI core. Because
modern computers have mostly gone away from PCI for high
performance peripherals, Xilinx dropped the IRDY and TRDY
logic in the S6 and newer parts. S6 LXT parts can easily
do 1 lane PCIe, which is about twice PCI-66 bandwidth at
a much lower pin count. 66 MHz PCI has pretty much gone the
way of PCI-X, and other high performance parallel buses.
(Anyone remember EISA?) Almost no new systems use it.

- Gabor