PDSP16510A MA

13

the minimum allowable sampling period must be increased to

allow for the extra dumping time. Thus when DIS and DOS

have equal periods and, for example, there is no overlapping;

(n - 4)S must be greater than or equal to PK

The maximum sampling rates given in Table 5 allow for the

extra dumping time.

The load and dump operations are not concurrent with

transforms in the 1024 point modes, and an external input

buffer will be needed if loss of incoming data is to be avoided.

This is loaded at the sampling rate and then data is transferred

to the PDSP16510 at a user defined rate. The time taken to

load this external buffer must be at least equal to the sum of

the time to transfer data in and out of the FFT processor and

the transform time itself. When data blocks are overlapped by

50% or 75%, no more than one half or one quarter of the block,

respectively, must have been loaded in the same time. In the

1024 point modes the dump time can be any user defined

value, and need not be increased to allow for block overlap-

ping. The dump time, however , does directly

effect the

maximum sampling rates which can be accommodated with-

out loss of incoming data.

The maximum sampling rates for 1024 point transforms

at any load and dump rate can be calculated from the following

relationship:

1024S or 512S or 256S > 1024B + PK + D

for 0%, 50%, or 75% overlapping respectively. S, P, and K

were defined opposite. B is the clock period in which data is

read from the input buffer and loaded into the device, D is the

total dump time allowing for the four extra DOS periods. The

periods of the load and dump clocks cannot be less than the

system clock period. The maximum sampling rates given in

Table 5 assume that a 40 MHz I/O rate is used, and that all

results are dumped.

MULTIPLE DEVICE SYSTEMS

In real time applications several devices may be used in

parallel in order to increase the sampling rate, but not to

increase the transform size. When all outputs are commoned

together, and feed a single output processor, then the data

dump time must always be less than or equal to the time taken

to load the data block ( or 50% or 25% of the time with block

overlapping ). In most configurations with block overlapping

the dump rate requirements will limit the maximum input rate,

if only one output processor is provided. This can be avoided

if the system provides separate output processors for every

device. The system clock used for internal calculations then

ultimately imposes a limit on the maximum sampling rate

possible.

A multiple device system performing complex transforms

with a single output processor is shown in Figure 9. The INEN/

LFLG signals are used to co-ordinate the segmentation of

data between devices. The in-active going edge of LFLG

instigates the load procedure in the next device, and, since

this edge can be programmed to occur either 25%, 50%, or

100% through the load operation, it can cause the next device

to commence loading before the previous one has finished. In

this manner data block overlapping is achieved. When mul-

SINGLE DEVICE SAMPLING RATES

In a single device system the maximum sampling rate is

dependent on the transform size, the data overlap, and

whether real or complex data is applied. Table 4 gives the

times taken to complete the transforms for the various block

sizes, which include an allowance for synchronisation be-

tween the DIS strobe and the system clock. If continuous data

is to be transformed, the time to acquire a new block of data

(or partial block with overlapping) must be at least equal to

these transform times. Load and dump times must also be

added in the 1024 modes. For non continuous transforms the

peak rate is limited by the system clock rate and the factor , F,

given previously.

The time taken to dump the transformed data must be no

more than the load time, if continuous inputs are to be

supported and I/O operations are concurrent with transforms.

With block overlapping the dump time must be reduced to the

time taken to load the partial block. This dump time must

include four extra DOS strobes needed to prime the output

circuitry when a transform is complete. These, in effect, can be

added to the transform time such that with concurrent I/O and

0%, 50%, or 75% overlapping;

nS or nS or nS must be greater than or equal to PK + 4W

2

4

where n is the transform size, S is the input DIS period, P is

the number of clock periods given in Table 4, K is the system

clock period, and W is the DOS period which can be less than

S if necessary.

When DIS and DOS are produced from a common source

Figure 9. Multiple Device Configuration