enCoRe™ V CY7C643xx, enCoRe™ V LV CY7C604xx TRM, Document No. 001-32519 Rev *H
109
I
2
C Slave
15.3.4
I2C_SCR Register
The I
2
C Status and Control Register (I2C_SCR) is used by
the slave to control the flow of data bytes and to keep track
of the bus state during a transfer.
This register contains status bits to determine the state of
the current I
2
C transfer, and control bits to determine the
actions for the next byte transfer. At the end of each byte
transfer, the I
2
C hardware interrupts the M8C microcon-
troller and stalls the I
2
C bus on the subsequent low of the
clock, until the enCoRe V device intervenes with the next
command. This register may be read as many times as nec-
essary; but on a subsequent write to this register, the bus
stall is released and the current transfer continues.
There are five status bits: Byte Complete, LRB, Address,
Stop Status, and Bus Error. These bits have Read/Clear
(RC) access, which means that they are set by hardware but
may be cleared by a write of ‘0’ to the bit position. Under
certain conditions, status is cleared automatically by the
hardware.
There are two control bits: Transmit and ACK. These bits
have RW access and may be cleared by hardware.
Bit 7: Bus Error.
The Bus Error status detects misplaced
Start or Stop conditions on the bus. These may be due to
noise, rogue devices, or other devices that are not yet syn-
chronized with the I
2
C bus traffic. According to the I
2
C spec-
ification, all compatible devices must reset their interface
upon a received Start or Stop. This is a natural thing to do in
slave mode because a Start initiates an address reception
and a Stop idles the slave.
A bus error is defined as follows. A Start is only valid if the
block is idle or a slave receiver is ready to receive the first bit
of a new byte after an ACK. Any other timing for a Start con-
dition sets the Bus Error bit. A Stop is only valid if the block
is idle or a slave receiver is ready to receive the first bit of a
new byte after an ACK. Any other timing for a Stop condition
sets the Bus Error bit.
Bit 5: Stop Status.
Stop status is set upon detection of an
I
2
C Stop condition. This bit is sticky, which means that it
remains set until a ‘0’ is written back to it by the firmware.
This bit may only be cleared if the Byte Complete status bit
is set. If the Stop Interrupt Enable bit is set, an interrupt is
also generated upon Stop detection. It is never automatically
cleared. Using this bit, a slave can distinguish between a
previous Stop or Restart upon a given address byte inter-
rupt.
The selections are shown in the following table:
Bit 4: ACK.
This control bit defines the acknowledge data bit that is
transmitted out in response to a received byte. When receiv-
ing, a byte complete interrupt is generated after the eighth
data bit is received. Upon the subsequent write to this regis-
ter to continue (or terminate) the transfer, the state of this bit
determines the next transmitted data bit. It is
. A
‘1’ sends an ACK and a ‘0’ sends a NACK. A slave receiver
sends a NACK to inform the master that it cannot receive
any more bytes.
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Access
0,D7h
Bus Error
Stop
Status
ACK
Address
Transmit
LRB
Byte
Complete
# : 00
Legend
# Access is bit specific.
Bit Access
Description
7
RC
Bus Error
1 = A misplaced Start or Stop condition was detected.
This status bit must be cleared by firmware with a write of ‘0’
to the bit position. It is never cleared by the hardware.
5
RC
Stop Status
1 = A Stop condition was detected.
This status bit must be cleared by firmware with a write of ‘0’
to the bit position. It is never cleared by the hardware.
4
RW
ACK: Acknowledge Out
0 = NACK the last received byte.
1 = ACK the last received byte.
This bit is automatically cleared by hardware upon the fol-
lowing byte complete event.
3
RC
Address
1 = The transmitted or received byte is an address.
This status bit must be cleared by firmware with a write of ‘0’
to the bit position.
2
RW
Transmit
0 = Receive Mode.
1 = Transmit Mode.
This bit is set by firmware to define the direction of the byte
transfer.
Any Start detect automatically clears this bit.
1
RC
LRB: Last Received Bit
The value of the ninth bit in a transmit sequence, which is
the acknowledge bit from the receiver.
0 = Last transmitted byte was ACKed by the receiver.
1 = Last transmitted byte was NAKed by the receiver.
Any Start detect automatically clears this bit.
0
RC
Byte Complete
Transmit Mode:
1 = 8 bits of data have been transmitted and an ACK or
NACK is received.
Receive Mode:
1 = 8 bits of data have been received.
Any Start detect automatically clears this bit.
