Cypress Semiconductor STK14C88-5 Manual do Utilizador descarregar pdf (Página 13)

STK14C88

Document Number: 001-52038 Rev. *D Page 13 of 21

Best Practices

nvSRAM products have been used effectively for over 15 years.

While ease-of-use is one of the product’s main system values,

experience gained working with hundreds of applications has

resulted in the following suggestions as best practices:

■ The nonvolatile cells in an nvSRAM are programmed on the

test floor during final test and quality assurance. Incoming

inspection routines at customer or contract manufacturer’s

sites sometimes reprogram these values. Final NV patterns are

typically repeating patterns of AA, 55, 00, FF, A5, or 5A. The

end product’s firmware should not assume an NV array is in a

set programmed state. Routines that check memory content

values to determine first time system configuration, cold or

warm boot status, and so on, should always program a unique

NV pattern (for example, complex 4-byte pattern of 46 E6 49

53 hex or more random bytes) as part of the final system

manufacturing test to ensure these system routines work

consistently.

■ Power-up boot firmware routines should rewrite the nvSRAM

into the desired state (such as autostore enabled). While the

nvSRAM is shipped in a preset state, best practice is to again

rewrite the nvSRAM into the desired state as a safeguard

against events that might flip the bit inadvertently (program

bugs, incoming inspection routines, and so on).

■ The V

CAP

value specified in this datasheet includes a minimum

and a maximum value size. Best practice is to meet this

requirement and not exceed the max V

CAP

value because the

nvSRAM internal algorithm calculates V

CAP

charge time based

on this max V

CAP

value. Customers who want to use a larger

CAP

value to make sure there is extra store charge and store

time should discuss their V

CAP

size selection with Cypress to

understand any impact on the V

CAP

voltage level at the end of

a t

RECALL

period.

Preventing STORES

The STORE function can be disabled on the fly by holding HSB

high with a driver capable of sourcing 30 mA at a V

of at least

2.2 V, because it must overpower the internal pull down device

that drives HSB

low for 20 ms at the onset of a STORE. When

the STK14C88 is connected for AutoStore operation (system

connected to V

and a 68 uF capacitor on V

CAP

) and V

crosses V

SWITCH

on the way down, the STK14C88 attempts to

pull HSB

low; if HSB does not actually get below V

, the part

stops trying to pull HSB

low and abort the STORE attempt.

Hardware Protect

The STK14C88 offers hardware protection against inadvertent

STORE operation and SRAM writes during low-voltage condi-

tions. When V

CAP

< V

SWITCH

, all externally initiated STORE

operations and SRAM writes are inhibited.

AutoStore can be completely disabled by tying V

to ground

and applying + 5 V to V

CAP

. This is the AutoStore Inhibit mode;

in this mode STOREs are only initiated by explicit request using

either the software sequence or the HSB

pin.

Low Average Active Power

The STK14C88 draws significantly less current when it is cycled

at times longer than 50 ns. Figure 13 shows the relationship

between I

and read cycle time. Worst case current

consumption is shown for both CMOS and TTL input levels

(commercial temperature range, V

= 5.5 V, 100% duty cycle

on chip enable). Figure 14 shows the same relationship for write

cycles. If the chip enable duty cycle is less than 100%, only

standby current is drawn when the chip is disabled. The overall

average current drawn by the STK14C88 depends on the

following items:

■ CMOS versus TTL input levels

■ The duty cycle of chip enable

■ The overall cycle rate for accesses

■ The ratio of reads to writes

■ The operating temperature

■ The V

level

■ I/O loading.

Figure 13. Icc (max) Reads

Figure 14. Icc (max) Writes

100

50 100 150 200

Cycle Time (ns)

TTL

CMOS

Average Active Current (mA)

100

50 100 150 200

Cycle Time (ns)

TTL

CMOS

Average Active Current (mA)

Not Recommended for New Designs

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