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P87LPC764 Datasheet(PDF) 16 Page - NXP Semiconductors

Part No. P87LPC764
Description  Low power, low price, low pin count (20 pin) microcontroller with 4 kbyte OTP
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Manufacturer  PHILIPS [NXP Semiconductors]
Direct Link  http://www.nxp.com
Logo PHILIPS - NXP Semiconductors

P87LPC764 Datasheet(HTML) 16 Page - NXP Semiconductors

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Product data
P87LPC764
Low power, low price, low pin count (20 pin)
microcontroller with 4 kbyte OTP
2003 Sep 03
15
BIT
SYMBOL
FUNCTION
I2CFG.7
SLAVEN
Slave Enable. Writing a 1 this bit enables the slave functions of the I2C subsystem. If SLAVEN and
MASTRQ are 0, the I2C hardware is disabled. This bit is cleared to 0 by reset and by an I2C
time-out.
I2CFG.6
MASTRQ
Master Request. Writing a 1 to this bit requests mastership of the I2C bus. If a transmission is in
progress when this bit is changed from 0 to 1, action is delayed until a stop condition is detected. A
start condition is sent and DRDY is set (thus making ATN = 1 and generating an I2C interrupt).
When a master wishes to release mastership status of the I2C, it writes a 1 to XSTP in I2CON.
MASTRQ is cleared by an I2C time-out.
I2CFG.5
CLRTI
Writing a 1 to this bit clears the Timer I overflow flag. This bit position always reads as a 0.
I2CFG.4
TIRUN
Writing a 1 to this bit lets Timer I run; a zero stops and clears it. Together with SLAVEN, MASTRQ,
and MASTER, this bit determines operational modes as shown in Table 1.
I2CFG.2, 3
Reserved for future use. Should not be set to 1 by user programs.
I2CFG.1, 0 CT1, CT0
These two bits are programmed as a function of the CPU clock rate, to optimize the MIN HI and LO
time of SCL when this device is a master on the I2C. The time value determined by these bits
controls both of these parameters, and also the timing for stop and start conditions.
CT0
SU01474
CT1
TIRUN
CLRTI
MASTRQ
SLAVEN
0
1
2
3
4
5
6
7
I2CFG
Reset Value: 00h
Bit Addressable
Address: C8h
Figure 8. I2C Configuration Register (I2CFG)
Regarding Software Response Time
Because the P87LPC764 can run at 20 MHz, and because the I2C
interface is optimized for high-speed operation, it is quite likely that
an I2C service routine will sometimes respond to DRDY (which is set
at a rising edge of SCL) and write I2DAT before SCL has gone low
again. If XDAT were applied directly to SDA, this situation would
produce an I2C protocol violation. The programmer need not worry
about this possibility because XDAT is applied to SDA only when
SCL is low.
Conversely, a program that includes an I2C service routine may take
a long time to respond to DRDY. Typically, an I2C routine operates
on a flag-polling basis during a message, with interrupts from other
peripheral functions enabled. If an interrupt occurs, it will delay the
response of the I2C service routine. The programmer need not worry
about this very much either, because the I2C hardware stretches the
SCL low time until the service routine responds. The only constraint
on the response is that it must not exceed the Timer I time-out.
Values to be used in the CT1 and CT0 bits are shown in Table 2. To
allow the I2C bus to run at the maximum rate for a particular
oscillator frequency, compare the actual oscillator rate to the f OSC
max column in the table. The value for CT1 and CT0 is found in the
first line of the table where CPU clock max is greater than or equal
to the actual frequency.
Table 2 also shows the machine cycle count for various settings of
CT1/CT0. This allows calculation of the actual minimum high and
low times for SCL as follows:
SCL min high low time (in microseconds)
+ 6 * Min Time Count
CPU clock (in MHz)
For instance, at an 8 MHz frequency, with CT1/CT0 set to 1 0, the
minimum SCL high and low times will be 5.25
µs.
Table 2 also shows the Timer I timeout period (given in machine
cycles) for each CT1/CT0 combination. The timeout period varies
because of the way in which minimum SCL high and low times are
measured. When the I2C interface is operating, Timer I is pre-loaded
at every SCL transition with a value dependent upon CT1/CT0. The
pre-load value is chosen such that a minimum SCL high or low time
has elapsed when Timer I reaches a count of 008 (the actual value
pre-loaded into Timer I is 8 minus the machine cycle count).


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