UM10503
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
1116 of 1269
NXP Semiconductors
UM10503
Chapter 43: LPC43xx I2C-bus interface
43.7.5 I
2
C SCL HIGH and LOW duty cycle registers
43.7.5.1 Selecting the appropriate I
2
C data rate and duty cycle
Software must set values for the registers SCLH and SCLL to select the appropriate data
rate and duty cycle. SCLH defines the number of C_PCLK cycles for the SCL HIGH time,
SCLL defines the number of I2C_PCLK cycles for the SCL low time. The frequency is
determined by the following formula (I2C_PCLK is the frequency of the peripheral I2C
clock):
(9)
The values for SCLL and SCLH must ensure that the data rate is in the appropriate I
2
C
data rate range. Each register value must be greater than or equal to 4.
gives
some examples of I
2
C-bus rates based on I2C_PCLK frequency and SCLL and SCLH
values.
SCLL and SCLH values should not necessarily be the same. Software can set different
duty cycles on SCL by setting these two registers. For example, the I
2
C-bus specification
defines the SCL low time and high time at different values for a Fast-mode and Fast-mode
Plus I
2
C.
Table 986. I
2
C SCL HIGH Duty Cycle register (SCLH - address 0x400A 1010 (I2C0) and
0x400E 0010 (I2C1)) bit description
Bit
Symbol
Description
Reset value
15:0
SCLH
Count for SCL HIGH time period selection.
0x0004
31:16
-
Reserved. The value read from a reserved bit is not defined.
-
Table 987. I
2
C SCL Low duty cycle register (SCLL - address 0x400A 1014 (I2C0) and
0x400E 0014 (I2C1)) bit description
Bit
Symbol
Description
Reset value
15:0
SCLL
Count for SCL low time period selection.
0x0004
31:16
-
Reserved. The value read from a reserved bit is not defined.
-
Table 988. SCLL + SCLH values for selected I
2
C clock values
I
2
C mode
I
2
C bit
frequency
I2C_PCLK (MHz)
6
8
10
12
16
20
30
40
50
SCLH + SCLL
Standard mode
100 kHz
60
80
100
120
160
200
300
400
500
Fast-mode
400 kHz
15
20
25
30
40
50
75
100
125
Fast-mode Plus
1 MHz
-
8
10
12
16
20
30
40
50
I2C
bitfrequency
I2CPCLK
I2CSCLH
I2CSCLL
+
---------------------------------------------------------
=