Calculating the depth of the FIFO so that no data is dropped.

[ASIC_intl]

SEnder sends data at a rate of 80 words/ 100 clocks and receiver consume at the rate of 8 words/ 10 clocks. Calculate the depth of the FIFO so that no data is dropped.

 

[dcreddy1980]

Re: FIFO problem

For worst case, The write data scenarion lokks like : 20-80-80-20 for 200 clock cycles and from it we knew that we will write 160 writes in 160 clock cycles.

For Read, with 8 reads per 10 clock cycles, we can have 128 reads in 160 clock cycles, So FIFO depth = 160 writes - 128 reads = 32

Comments are welcome !

 

[ASIC_intl]

Re: FIFO problem

What do u mean by "write data scenarion lokks like : 20-80-80-20 for 200 clock cycles" ? What is 20-80-80-20 here? According to the question in 200 cycles 80x2 = 160 words can be written in worst case.

Why are u considering 200 cycles for the calculation? You can use 100 clock cycles, or 300 or even 400 clock cycles also.

 

[dcreddy1980]

Re: FIFO problem

20 Nops- 80 writes- 80-writes-20 Nops --> this is the worst case where u can have 160 writes at a strech...Thas why i have choosen this sequence.

If you have better answer let me know!!

 

[ASIC_intl]

Re: FIFO problem

What does Nops stand for? Is it 'no reading of data'? How can it be 20 Nops at the last of the sequence 20-80-80-20 ? Because in such a case the 20 words will be read in the last 20 cycles of the 200 cycles.

 

[AdvaRes]

Re: FIFO problem

What does Nops stand for? Is it 'no reading of data'? How can it be 20 Nops at the last of the sequence 20-80-80-20 ? Because in such a case the 20 words will be read in the last 20 cycles of the 200 cycles.

Nops: In assembly a nop is a No Operation. In our case 20 Nops means waiting for 20 cycles.

 

[ASIC_intl]

Re: FIFO problem

How can so it be 20 Nops after 20-80-80 ? The reading operation will still go on after 180 cycles because of unread data present.

Added after 11 minutes:

See:

Let in First 100 cycles writing to occur from 20th to 100 cycles. since there will be no data in FIFO to read in first 20 cycles the reading of data will actually start from 21st cycle. So in First 100 cycles 64 words can be read. Nextly in next 80 cycles writing will go on. The reading will also go on for the next 80 cycles and in next 80 cycles ( 100th to 180th cycle) 64 words can be read. So FIFO size = 160-(2x64) = 32. Is that what u mean dcreddy1980?

Added after 3 minutes:

Hi dcredy1980

Can u please let me know the way we are calculating the FIFO depth is the general way or not? Is it we always need to see the worst rate? It is sometimes difficult to find out the worst rate. As it is in this case also and we are not sure whether we are correct or not at the end also. We still think we may be wrong in finding the worst case.

 

[dcreddy1980]

Re: FIFO problem

Let in First 100 cycles writing to occur from 20th to 100 cycles. since there will be no data in FIFO to read in first 20 cycles the reading of data will actually start from 21st cycle. So in First 100 cycles 64 words can be read. Nextly in next 80 cycles writing will go on. The reading will also go on for the next 80 cycles and in next 80 cycles ( 100th to 180th cycle) 64 words can be read. So FIFO size = 160-(2x64) = 32. Is that what u mean dcreddy1980?

Yes, Thats what i was trying to explain you.I did take the worst case, so that we don't loose any data.

 

[ASIC_intl]

Re: FIFO problem

reddy1980
u did not reply to my mast query.

 

[dcreddy1980]

Re: FIFO problem

If you are aware of u r write frequency and read frequency, There is no chance of missing the data if u have calculated the fifo depth correctly. If you just dont know the exact frequencies, we may loose data if the FIFO depth was not enough.

 

[Nir Dahan]

FIFO problem

it seems to me that DCreddy's calculation is correct.

ND

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