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TM1300 Data Book
Philips Semiconductors
6-12
PRODUCT SPECIFICATION
6.7
HIGHWAY LATENCY AND HBE
Refer to
Chapter 20, “Arbiter,”
for a description of the ar-
biter terminology used here. The VI unit uses internal
buffering before writing data to SDRAM. There are two
internal buffers, each 16 entries of 32 bits.
In fullres mode, each internal buffer is used for 128 Y
samples, 64 U samples, and 64 V samples. Once the
first internal buffer is filled, 4 highway transactions must
occur before the second buffer fills completely. Hence,
the requirement for not losing samples is:
4 requests must be served within 256 VI clock cycles.
For the typical CCIR601-resolution NTSC or PAL 27-
MHz VI clock rate, the latency requirement is 4 requests
in 9481 ns (25600/27). This can be used as one request
every 2370 ns or, with a TM1300 SDRAM clock speed of
100 MHz, every 237 SDRAM clock cycles. The one re-
quest latency is used to define the priority raising value
(see
Section 20.6.3 on page 20-8
).
In halfres mode, the Y, U, and V decimation by 2 takes
place before writing to the internal buffers. So, the re-
quirement for not loosing samples is:
4 requests served within 512 VI clock cycles.
For halfres subsampling, NTSC or PAL 27-MHz VI clock
rate and TM1300 SDRAM clock speed of 100 MHz, la-
tency is 4 requests in 51200/27 = 18962 ns (1896 high-
way clock cycles) or one request every 4740 ns (474
SDRAM clock cycles).
For raw8 capture and message passing modes, each in-
ternal buffer stores 64 samples at the incoming VI clock
rate. The latency requirement is one request served ev-
ery 64 VI clock cycles.
For the raw10 capture modes, each internal buffer stores
32 samples. Hence, the requirement for not losing sam-
ples is one request served every 32 VI clock cycles.
For a 38-MHz data rate on the incoming 10-bit samples
and a TM1300 SDRAM clock speed of 100 MHz, high-
way latency should be set to guarantee less than 3200/
38 = 842 ns (84 SDRAM clock cycles) per clock cycle.
This cannot be met if any other peripherals are enabled.
Table 6-4
summarizes the maximum allowed highway
latency (in SDRAM clock cycles) needed to guarantee
that no samples are lost. The general formula uses ‘F’ to
represent the VI clock frequency (in MHz).
ACTIVE = BUF2
BUF1FULL
ACTIVE = BUF1
ACTIVE = BUF2
ACTIVE = BUF1
BUF2FULL
BUF1FULL
BUF2FULL
raise OVERRUN*
* OVERRUN and OVERFLOW are sticky flags. They are set,
but do notaffect operation. They can only be cleared by soft-
ware, by writing a ‘1’ to ACK_OVR or ACK_OVF.
(See text in
Section 6.6
)
ACK1 & ~ACK2
ACK1 & ACK2
~ACK1 & ACK2
EOM
EOM
EOM
ACK1
EOM
ACK2
RESET
No EOM
raise OVERFLOW*
(
See text in
Section 6.6
)
No EOM
raise OVERFLOW*
(
See text in
Section 6.6
)
Figure 6-18. VI message passing mode major states.
Table 6-4. VI highway latency requirements (27-MHz
data rate, 100-MHz TM1300 highway clock)
Mode
Max latency setting
(27 MHz, 100 MHz)
Formula
fullres capture
halfres capture
raw8
raw10s
raw10u
message passing
237
474
237
118
118
237
6,400/F
12,800/F
6,400/F
3,200/F
3,200/F
6,400/F