Issues to be aware of when configuring Virtex and XC4000X devices in a daisy chain.
Virtex and XC4000X devices may be mixed in a serial daisy chain, with the following conditions.
1. All DONE pins are wired together with an external pullup to 3.3 V.
2. XC4000X devices must have the bitstreams generated with the following options:
SyncToDone = Yes
OutputsActive = DI, DI_PLUS_1, or DI_PLUS_2
GSRInactive = DI, DI_PLUS_1, or DI_PLUS_2
3. If programming by a micro-device, at least 3 clocks must be issued after DONE has gone High or the devices must use a USER clock for the startup sequence.
4. The STARTUP block is used in the 4000 device.
5. The GTS signal on the STARTUP block is asserted.
6. The daisy chain consists of Virtex and XC4000-series architectures.
There are no ordering requirements for the daisy chain. For ease of design, families should be grouped together. This also reduces the bitstream size.
When a XC4000XL, XLA, or XV device is combined with a Virtex device in a daisy chain, the length count (LC) of the XC4000-series device will be disabled. This means that once the device has filled its configuration memory without error, it attempts to start up. In a daisy chain application, full start-up is not achieved until all the DONE pins go High (they should be tied together).
When the LC of the XC4000 device is disabled, part of the device starts up before DONE goes High. If the GTS signal on the STARTUP block is asserted, the DOUT pin goes into a 3-state condition (it becomes a regular I/O after configuration). This means that the device no longer passes data down the chain, and configuration fails. Eliminating any of the three conditions listed above alleviates the problem. Also, if design changes are possible, "AND*" the input signal with the DONEIN signal, so the GTS signal is asserted only when the pin is asserted and DONE is High.
The following devices are permitted in the serial chain:
All other devices are NOT permitted in the serial chain