PC87570 datasheet(103/168 Pages) NSC

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ACCESS.bus (ACB) Interface

103

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Bit 3 - Stall After Start (STASTR)

This bit is set by the successful completion of an ad-

dress sending (i.e., a Start Condition sent without a bus

error, or negative acknowledge), if ACBCTL1.STAS-

TRE is set. This bit is ignored in slave mode. When

STASTR is set, it stalls the ACCESS.bus by pulling

down the SCL line, and suspends any further action on

the bus (e.g., receive of first byte in Master Receive

mode). In addition, if ACBCTL1.INTEN is set, it also

causes the ACB module to send an interrupt to the

CR16A. Writing 1 to STASTR clears it. It is also cleared

when the module is disabled and is always cleared

when STASTRE is cleared. Writing 0 to STASTR has no

effect.

Bit 4 - Negative Acknowledge (NEGACK)

This bit is set by hardware when a transmission is not

acknowledged on the ninth clock. (In this case SDAST

is not set.) Writing 1 to NEGACK clears it. It is also

cleared when the module is disabled. Writing 0 to

NEGACK is ignored.

Bit 5 - Bus Error (BER)

BER is set by the hardware when a Start or Stop Condi-

tion is detected during data transfer (i.e., Start or Stop

Condition during the transfer of bits 2 through 8 and ac-

knowledge cycle), or when an arbitration problem is de-

tected. Writing 1 to BER clears it. It is also cleared when

the module is disabled. Writing 0 to BER is ignored.

Bit 6 - SDA Status (SDAST)

When set, this bit indicates that the SDA data register is

waiting for data (transmit - master or slave) or holds data

that should be read (receive - master or slave). This bit

is cleared when reading from the ACBSDA register dur-

ing a receive, or when written to during a transmit.]

When ACBCTL1.START is set, reading ACBSDA regis-

ter does not clear SDAST. This enables the ACB to

send a repeated start in master receive mode.

Bit 7 - Slave Stop (SLVSTP)

If set, SLVSTP indicates that a Stop Condition was de-

tected after a slave transfer (i.e., after a slave transfer in

which MATCH or GCMATCH was set). Writing 1 to

SLVSTP clears it. It is also cleared when the module is

disabled. Writing 0 to SLVSTP is ignored.

13.4.3

ACB Control Status Register (ACBCST)

The ACBCST Register is a byte-wide, read/write register

that maintains current ACB status and controls several ACB

module functions. The functions of the CBCST are de-

scribed below. Upon reset and when the module is disabled

the non-reserved bits of ACBCST are cleared (0).

Bit 0 - BUSY

When BUSY is set (1), this indicates that the ACB mod-

ule is in one of the following states:

Generating a Start Condition

In Master mode (ACBST.MASTER is set)

In Slave mode (ACBCST.MATCH or ACBCST.GMATCH

is set)

In the period between detecting a Start condition

and completing the reception of the address byte.

After this, the ACB either becomes not busy or en-

ters slave mode.

The BUSY bit is cleared by the completion of any of the

above states, and by disabling the module. BUSY is a

read only bit. It should always be written 0.

Bit 1 - Bus Busy (BB)

When set (1), BB indicates the bus is busy. It is set when

the bus is active (i.e., a low level on either SDA or SCL),

or by a Start Condition. It is cleared when the module is

disabled, on detection of a Stop Condition, or when writ-

ing ‘1’ to this bit. See Section 13.5 for a description of

the use of this bit.

Bit 2 - Address Match (MATCH)

In slave mode, MATCH is set (1) when ACBAD-

DR.SAEN is set and the first seven bits of the address

byte (the first byte transferred after a Start Condition)

matches the 7-bit address in the ACBADDR register. It

is cleared by Start Condition, a repeated start and a

Stop Condition (including illegal Start or Stop Condi-

tion).

Bit 3 - Global Call Match (GCMTCH)

In slave mode, GCMTCH is set (1) when ACBCTL1.GC-

MEN is set and the address byte (the first byte trans-

ferred after a Start Condition) is 00h. It is cleared by

Start Condition, a repeated Start and a Stop Condition

(including illegal Start or Stop Condition).

Bit 4 - Test SDA Line (TSDA)

Reads the current value of the SDA line. This bit can be

used while recovering from an error condition in which

the SDA line is constantly pulled low by a slave that

went out of synch. This bit is a read-only bit. Data written

to it is ignored.

Bit 5 - Toggle SCL Line (TGSCL)

This bit enables toggling the SCL line during the process

of error recovery. When the SDA line is low, writing 1 to

this bit toggles the SCL line for one cycle. Writing 1 to

TGSCL while SDA is high, is ignored. The bit is cleared

when the clock toggle is completed.

Reserved

TGSCL

TSDA

3210

GCMTCH

MATCH

BUSY

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