P87LPC778FDH datasheet(29/79 Pages) PHILIPS

29 / 79 page

Philips Semiconductors

P87LPC778

CMOS single-chip 8-bit microcontroller

Product data

Rev. 01 — 31 March 2004

29 of 79

9397 750 12378

ARL — ‘Arbitration Loss’ is ‘1’ when transmit Active was set, but this device lost

arbitration to another transmitter. Transmit Active is cleared when ARL is ‘1’. There

are four separate cases in which ARL is set:

1. If the program sent a ‘1’ or repeated start, but another device sent a ‘0’, or a stop,

so that SDA is ‘0’ at the rising edge of SCL. (If the other device sent a stop, the

setting of ARL will be followed shortly by STP being set.)

2. If the program sent a ‘1’, but another device sent a repeated start, and it drove

SDA LOW before SCL could be driven LOW. (This type of ARL is always

accompanied by STR = 1.)

3. In master mode, if the program sent a repeated start, but another device sent a

‘1’, and it drove SCL LOW before this device could drive SDA LOW.

4. In master mode, if the program sent stop, but it could not be sent because

another device sent a ‘0’.

STR — ‘STaRt’ is set to a ‘1’ when an I2C-bus start condition is detected at a non-idle

slave or at a master. (STR is not set when an idle slave becomes active due to a start

bit; the slave has nothing useful to do until the rising edge of SCL sets DRDY.)

STP — ‘SToP’ is set to 1 when an I2C-bus stop condition is detected at a non-idle

slave or at a master. (STP is not set for a stop condition at an idle slave.)

MASTER — ‘MASTER’ is ‘1’ if this device is currently a master on the I2C-bus.

MASTER is set when MASTRQ is ‘1’ and the bus is not busy (i.e., if a start bit hasn’t

been received since reset or a ‘Timer I’ time-out, or if a stop has been received since

the last start). MASTER is cleared when ARL is set, or after the software writes

MASTRQ = 0 and then XSTP = 1.

8.7.4

Writing I2CON

Typically, for each bit in an I2C-bus message, a service routine waits for ATN = 1.

Based on DRDY, ARL, STR, and STP, and on the current bit position in the message,

it may then write I2CON with one or more of the following bits, or it may read or write

the I2DAT register.

CXA — Writing a ‘1’ to ‘Clear Xmit Active’ clears the Transmit Active state. (Reading

the I2DAT register also does this.)

8.7.5

Regarding Transmit Active

Transmit Active is set by writing the I2DAT register, or by writing I2CON with

XSTR = 1 or XSTP = 1. The I2C-bus interface will only drive the SDA line low when

Transmit Active is set, and the ARL bit will only be set to ‘1’ when Transmit Active is

set. Transmit Active is cleared by reading the I2DAT register, or by writing I2CON with

CXA = 1. Transmit Active is automatically cleared when ARL is ‘1’.

IDLE — Writing ‘1’ to ‘IDLE’ causes a slave’s I2C-bus hardware to ignore the I2C-bus

until the next start condition (but if MASTRQ is ‘1’, then a stop condition will cause

this device to become a master).

CDR — Writing a ‘1’ to ‘Clear Data Ready' clears DRDY. (Reading or writing the

I2DAT register also does this.)

CARL — Writing a ‘1’ to ‘Clear Arbitration Loss’ clears the ARL bit.

CSTR — Writing a ‘1’ to ‘Clear STaRt’ clears the STR bit.

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