SM320VC33 datasheet(17/54 Pages) TI

17 / 54 page

SM320VC33, SMJ320VC33

DIGITAL SIGNAL PROCESSOR

SGUS034E - FEBRUARY 2001 - REVISED OCTOBER 2002

POST OFFICE BOX 1443

• HOUSTON, TEXAS 77251-1443

example 2 (continued)

Table 5. MUX Select (Bus Control Register Bits 4 and 3)

BIT 4

BIT 3

RESULTS

Ignore internal wait counter and use only external READY

Use only internal wait counter and ignore ready pin

Logically AND internal wait counter with ready pin

Logically OR internal wait counter with ready pin (reset default)

posted writes

External writes are effectively “posted” to the bus, which then acts like an output latch until the write completes.

Therefore, if the application code is executing internally, it can perform a very slow external write with no penalty

since the bus acts like it has a one-level-deep write FIFO.

data bus I/O buffer

The circuit shown in Figure 9 is incorporated into each data pin to lightly “hold” the last driven value on the data

bus pins when the DSP or an external device is not actively driving the bus. Each bus keeper is built from a

three-state driver with nominal 15 k

Ω output resistance which is fed back to the input in a positive feedback

configuration. The resistance isolated driver then pulls the output in one direction or the other keeping the last

driven value. This circuit is enabled in all functional modes and is only disabled when SHZ is pulled low.

Ω

15 k

Ω

SHZ

Bus keeper

Internal

Data Bus

R/W

External Data

Bus Pin

Figure 9. Bus Keeper Circuit

For an external device to change the state of these pins, it must be able to drive a small dc current until the driver

threshold is crossed. At the crossover point, the driver changes state, agreeing with the external driver and

assisting the change. The voltage threshold of the bus keeper is approximately at 50% of the DVDD supply

voltage. The typical output impedance of 30

Ω for all SM/SMJ320VC33 I/O pins is easily capable of meeting

this requirement.

SM320VC33 Datasheet(PDF) 17 Page - Texas Instruments

SM320VC33 Datasheet(HTML) 17 Page - Texas Instruments

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