HT80C51 User Manual
Peripheral Modules – I2C Interface (SIO1)
Handshake Solutions
© Philips Electronics N.V. 2005
Page 57 of 132
The Clock Rate Bits CR0, CR1, and CR2
These three bits determine the serial clock frequency when SIO1 is in a Master mode. The various
serial rates are shown in [Table 9].
A 12.5 kHz bit rate may be used by devices that interface to the I2C bus via standard I/O port lines
which are software driven and slow. 100 kHz is usually the maximum bit rate and can be derived from
a 16 MHz, 12 MHz, or a 6 MHz oscillator. A variable bit rate (0.5 kHz to 62.5 kHz) may also be used if
Timer 1 is not required for any other purpose while SIO1 is in a Master mode.
The frequencies shown in [Table 9] are unimportant when SIO1 is in a Slave mode. In the Slave
modes, SIO1 will automatically synchronize with any clock frequency up to 100 kHz.
The Status Register, S1STA
S1STA is an 8-bit read-only special function register. The three least significant bits are always zero.
The five most significant bits contain the status code. There are 26 possible status codes. When S1STA
contains F8H, no relevant state information is available and no serial interrupt is requested. All other
S1STA values correspond to defined SIO1 states. When each of these states is entered, a serial inter-
rupt is requested (SI = “1”). A valid status code is present in S1STA one machine cycle after SI is set by
hardware and is still present one machine cycle after SI has been reset by software.
bit rate (kbit/s) at fiClk
CR2
CR1
CR0
6 MHz
12 MHz
16 MHz
24 MHz
30 MHz
fiClk divided by
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
23
27
31
37
6.25
50
100
0.24..62.5
0..255
47
54
63
75
12.5
100
200
0.49..62.5
0..254
62.5
71
83.3
100
17
133
267
0.65..55.6
0..253
94
107
125
150
25
200
400
0.98..50.0
0..251
117
134
155
188
31
250
500
1.22..52.1
0..250
256
224
192
160
960
120
60
256-(reload value timer 1)
reload value timer 1, mode 2
[Table 9]
Serial Clock Rates (needs update)
NOTES:
1.
These frequencies exceed the upper limit of 100 kHz of the I2C-bus specification and cannot be
used in an I2C-bus application.
2.
At fOSC = 24 MHz/30 MHz the maximum I2C bus rate of 100 kHz cannot be realized due to the
fixed divider rates.
5.5.4.4.
More Information on SIO1 Operating Modes
The four operating modes are:
•
Master Transmitter
•
Master Receiver
•
Slave Receiver
•
Slave Transmitter
Data transfers in each mode of operation are shown in [Figure 21] to [Figure 24].
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