HT80C51 User Manual
Peripheral Modules – I2C Interface (SIO1)
Handshake Solutions
© Philips Electronics N.V. 2005
Page 53 of 132
into the five most significant bits of the status register when the serial interrupt flag is set (by hard-
ware) and remains stable until the interrupt flag is cleared by software. The three least significant bits
of the status register are always zero. If the status code is used as a vector to service routines, then
the routines are displaced by eight address locations. Eight bytes of code are sufficient for most of the
service routines.
5.5.4.3.
The Four SIO1 Special Function Registers
The microcontroller interfaces to SIO1 via four special function registers. These four SFRs (S1ADR,
S1DAT, S1CON, and S1STA) are described individually in the following sections.
The Address Register, S1ADR
The CPU can read from and write to this 8-bit, directly addressable SFR. S1ADR is not affected by the
SIO1 hardware. The contents of this register are irrelevant when SIO1 is in a Master mode. In the
Slave modes, the seven most significant bits must be loaded with the microcontroller’s own slave ad-
dress, and, if the least significant bit is set, the general call address (00H) is recognized; otherwise it is
ignored.
7
6
5
4
3
2
1
0
S1ADR (DBH)
own slave address
GC
The most significant bit corresponds to the first bit received from the I2C bus after a start condition. A
logic 1 in S1ADR corresponds to a high level on the I2C bus, and a logic 0 corresponds to a low level
on the bus.
The Data Register, S1DAT
S1DAT contains a byte of serial data to be transmitted or a byte, which has just been received. The
CPU can read from and write to this 8-bit, directly addressable SFR while it is not in the process of
shifting a byte. This occurs when SIO1 is in a defined state and the serial interrupt flag is set. Data in
S1DAT remains stable as long as SI is set. Data in S1DAT is always shifted from right to left: the first bit
to be transmitted is the MSB (bit 7), and, after a byte has been received, the first bit of received data is
located at the MSB of S1DAT. While data is being shifted out, data on the bus is simultaneously being
shifted in; S1DAT always contains the last data byte present on the bus. Thus, in the event of lost arbi-
tration, the transition from master transmitter to slave receiver is made with the correct data in S1DAT.
7
6
5
4
3
2
1
0
S1DAT (DAH)
SD7
SD6
SD5
SD4
SD3
SD2
SD1
SD0
← shift direction
SD7 - SD0:
Eight bits to be transmitted or just received. A logic 1 in S1DAT corresponds to a high level on the I2C
bus, and a logic 0 corresponds to a low level on the bus. Serial data shifts through S1DAT from right to
left. [Figure 19] shows how data in S1DAT is serially transferred to and from the SDA line.
S1DAT and the ACK flag form a 9-bit shift register which shifts in or shifts out an 8-bit byte, followed by
an acknowledge bit. The ACK flag is controlled by the SIO1 hardware and cannot be accessed by the
CPU. Serial data is shifted through the ACK flag into S1DAT on the rising edges of serial clock pulses
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