PCA9641PW datasheet(32/55 Pages) NXP

32 / 55 page

PCA9641

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Product data sheet

Rev. 2.1 — 27 October 2015

32 of 55

NXP Semiconductors

PCA9641

2-channel I2C-bus master arbiter

15. Shared mailbox

Shared mailbox contains two 8-bit bidirectional mailboxes used for communication

between the two masters. Each master has MB_HI and MB_LO registers on their address

map. The mailbox uses a MBOX_FULL and MBOX_EMPTY status to assist in the flow of

data and prevent data loss or corruption.

When a master is sending data via the mailbox, it must check the MBOX_EMPTY status

bit. If the MBOX_EMPTY status bit is zero (not EMPTY), then it contains data for the other

master that has not been read, and writing at this time would result in data loss/corruption.

When the MBOX_EMPTY status bit is one (EMPTY), the master may write to the mailbox.

In order to send data through the mailbox, the master must write the entire 16 bits, starting

with MB_LO and finishing with MB_HI. If the mailbox is written in reverse order, it will not

activate the FULL flag on the receiving master. Once the mailbox has been written, the

transmitting master’s MBOX_EMPTY status bit is cleared (0), and the receiving master’s

MBOX_FULL status bit is set (1).

When a master’s MBOX_FULL status bit is set, it means that there is data in the mailbox

from the other master that has not been read. The master may read the mailbox in any

order, but the FULL flag will not be cleared until both MB_LO and MB_HI have been read.

When they have been read, the sending master’s MBOX_EMPTY status bit is set,

indicating the data has been read and the mailbox is ready for more data. When they have

been read, the receiving master’s MBOX_FULL status bit is cleared, indicating there is no

new data in the mailbox to be read.

When a master writes the mailbox registers, it is sending data to the other master’s

mailbox. When a master reads the mailbox, it is reading from its own mailbox. It is not

possible to write data into the mailbox and read it back.

Fig 19. Shared mailbox byte arrangement

aaa-008573

MB_LOW

D14 D13 D12 D11 D10 D9

MB_HI

D15

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