PCA8802 datasheet(17/34 Pages) NXP

17 / 34 page

PCA8802_1

Product data sheet

Rev. 01 — 19 February 2009

17 of 34

NXP Semiconductors

PCA8802

Smartcard RTC

This instruction takes only 3 parameters in one command. Data after the 3rd parameter

will be interpreted as the next instruction.

Accurate setting and start-up can be implemented using the dvs_cmd instruction in

cooperation with the set_cmd instruction. An example is shown in Table 8.

9.6.8 Instruction rd_cmd

With the read instruction (rd_cmd) the counter value can be read at any time. When the

counter value is read, the counter is frozen so that there will be no changes during the

read back. After a read is terminated, the counter will be allowed to increment again. Any

increment that was scheduled during the frozen period will then be effected.

Reading the counter is cyclic i.e. the device will repeatedly return the present counter

value until the read is terminated. Reading the counter more than once may be useful in

the case that the application is subject to a strong Electromagnetic Interference (EMI)

environment so that read back values can be compared.

Read back must be terminated within 32 seconds else a count will be dropped.

Table 8.

Example of accurate setting of the counter

Step

Action

Code sequence Note

initiate transfer

START condition -

send wrt_cmd

1010 0000

enable dvs_cmd

0001 0001

set counter with set_cmd

1000 0000

set the counter = 1

0000 0000

P1[23:16]

0000 0000

P2[15:8]

0000 0001

P3[7:0]

end transfer

STOP condition

wait for an external time

marker

initiate transfer

START condition -

send wrt_cmd

1010 0000

disable dvs_cmd

0001 0000

counter starts on the ACK cycle of this

instruction

end transfer

STOP condition

Fig 25. Read bit order

001aaj208

b23 b22

MSB

b21 b20 b19 b18 b17 b16 b15 b14 b13 b12 b11 b10 b9

LSB

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