MCV architecture for FIR v6.1

If you are thinking about using FIR v6.1 (especially the MCV

architecture), then be warned. Seems like there's a bug with the FIR

6.1 core that is not documented in the errata sheet.

FIR settings:

- Multiple cycle variable (MCV) architecture

- 150 taps interpolating filter (contains both positive and negative

coefficients).

- 2 cycles/output

- No coefficient reload.

- Coefficients are stored in M4K

I inputted a delta function, expecting the FIR to output a delayed

version of the filter response. However, it seems like some outputs are

dropped while the others are repeated twice.

While there is an errata that seems somewhat related to the issue, but

the errata is only applicable when all coefficients are positive (and

MCV with no coefficient reloading). The suggested work arounds in

errata sheet are:

1. Use other architectures

2. Turn on coefficient reload, but disassert coef_we (to disable

reloading coefficient).

Since my FIR has two out of the three criteria, I thought I would give

the work-arounds a try... 1) works for me, but 2) still does not work.

Checked with Altera and this is indeed a bug. They promised to have

this fix in the next release that is coming out later this year.

Btw, the above error is also reproducable with other cycles/output.