National CP3BT26 - Manual (Page 185)

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CP3
BT26
Power Down
When this device is in Power Save, Idle, or Halt mode, the
ACB module is not active but retains its status. If the ACB is
enabled (ACBCTL2.ENABLE = 1) on detection of a Start
Condition, a wake-up signal is issued to the MIWU module.
Use this signal to switch this device to Active mode.
The ACB module cannot check the address byte for a match
following the start condition that caused the wake-up event
for this device. The ACB responds with a negative acknowl-
edge, and the device should resend both the Start Condition
and the address after this device has had time to wake up.
Check that the ACBCST.BUSY bit is inactive before entering
Power Save, Idle, or Halt mode. This guarantees that the de-
vice does not acknowledge an address sent and stop re-
sponding later.
24.2.3 SDA and SCL Pins Configuration
The SDA and SCL pins are driven as open-drain signals.
For more information, see the I/O configuration section.
24.2.4 ACB Clock Frequency Configuration
The ACB module permits software to set the clock frequen-
cy used for the ACCESS.bus clock. The clock is set by the
ACBCTL2.SCLFRQ field. This field determines the SCL
clock period used by this device. This clock low period may
be extended by stall periods initiated by the ACB module or
by another ACCESS.bus device. In case of a conflict with
another bus master, a shorter clock high period may be
forced by the other bus master until the conflict is resolved.
24.3 ACCESS.BUS INTERFACE REGISTERS
The ACCESS.bus interface uses the registers listed in
Table 75.
24.3.1 ACB Serial Data Register (ACBSDA)
The ACBSDA register is a byte-wide, read/write shift regis-
ter used to transmit and receive data. The most significant
bit is transmitted (received) first and the least significant bit
is transmitted (received) last. Reading or writing to the ACB-
SDA register is allowed when ACBST.SDAST is set; or for
repeated starts after setting the START bit. An attempt to
access the register in other cases produces unpredictable
results.
24.3.2 ACB Status Register (ACBST)
The ACBST register is a byte-wide, read-only register that
maintains current ACB status. At reset, and when the mod-
ule is disabled, ACBST is cleared.
XMIT The Direction Bit bit is set when the ACB mod-
ule is currently in master/slave transmit mode.
Otherwise it is cleared.
0 – Receive mode.
1 – Transmit mode.
MASTER The Master bit indicates that the module is
currently in master mode. It is set when a re-
quest for bus mastership succeeds. It is
cleared upon arbitration loss (BER is set) or
the recognition of a Stop Condition.
0 – Slave mode.
1 – Master mode.
NMATCH The New match bit is set when the address
byte following a Start Condition, or repeated
starts, causes a match or a global-call match.
The NMATCH bit is cleared when written with
1. Writing 0 to NMATCH is ignored. If the
ACBCTL1.INTEN bit is set, an interrupt is sent
when this bit is set.
0 – No match.
1 – Match or global-call match.
STASTR The Stall After Start bit is set by the successful
completion of an address sending (i.e., a Start
Condition sent without a bus error, or negative
acknowledge), if the ACBCTL1.STASTRE bit
is set. This bit is ignored in slave mode. When
the STASTR bit is set, it stalls the bus by pull-
ing down the SCL line, and suspends any oth-
er action on the bus (e.g., receives first byte in
master receive mode). In addition, if the
ACBCTL1.INTEN bit is set, it also sends an
interrupt to the core. Writing 1 to the STASTR
bit clears it. It is also cleared when the module
is disabled. Writing 0 to the STASTR bit has
no effect.
0 – No stall after start condition.
1 – Stall after successful start.
Table 75 ACCESS.bus Interface Registers
Name Address Description
ACBSDA FF F2A0h
ACB Serial Data
Register
ACBST FF F2A2h ACB Status Register
ACBCST FF F2A4h
ACB Control Status
Register
ACBCTL1 FF F2A6h
ACB Control
Register 1
ACBCTL2 FF F2AAh
ACB Control
Register 2
ACBCTL3 FF F2AEh
ACB Control
Register 3
ACBADDR1 FF F2A8h
ACB Own Address
Register 1
ACBADDR2 FF F2ACh
ACB Own Address
Register 2
7 0
DATA
7 6 5 4 3 2 1 0
SLVSTP SDAST BER NEGACK STASTR NMATCH MASTER XMIT