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PSoC CY8CTMG20x and CY8CTST200 TRM, Document No. 001-53603 Rev. *C

I2C Slave

15.3.9

I2C_CFG Register

The I

C Configuration Register (I2C_CFG) is used to set the

basic operating modes, baud rate, and selection of inter-
rupts. The bits in this register control baud rate selection and
optional interrupts. The values are typically set once for a
given configuration. The bits in this register are all RW.

Bit 6: PSelect.

Pin Select. With the default value of zero,

the I

C pins are P1[7] for clock and P1[5] for data. When this

bit is set, the pins for I

C switch to P1[1] for clock and P1[0]

for data. You cannot change this bit while the Enable bit is
set. However, the PSelect bit may be set at the same time
as the enable bits. The two sets of pins used on I

C are not

equivalent. The default set, P1[7] and P1[5], are the pre-
ferred set. The alternate set, P1[1] and P1[0], are provided
so that I

C may be used with 8-pin PSoC devices.

If In-System Serial Programming (ISSP) is used and the
alternate I

C pin set is also used, you must consider the

interaction between the PSoC Test Controller and the I

bus. The interface requirements for ISSP must be reviewed
to ensure that they are not violated.

Even if ISSP is not used, pins P1[1] and P1[0] respond dif-
ferently than other I/O pins to a POR or XRES event. After
an XRES event, both pins are pulled down to ground by
going into the resistive zero drive mode before reaching the
High Z drive mode. After a POR event, P1[0] drives out a
one, then goes to the resistive zero state for some time, and
finally reaches the High Z drive mode state. After POR,
P1[1] goes into a resistive zero state for a while before going
to the High Z drive mode.

Bit 4: Stop IE.

Stop Interrupt Enable. When this bit is set, a

slave can interrupt upon Stop detection. The status bit asso-
ciated with this interrupt is the Stop Status bit in the

I2C_SCR Register

. When the Stop Status bit transitions

from ‘0’ to ‘1’, the interrupt is generated. It is important to
note that the Stop Status bit is not automatically cleared.
Therefore, if it is already set, no new interrupts are gener-
ated until it is cleared by firmware.

Bits 3 and 2: Clock Rate[1:0].

These bits offer a selection

of three sampling and bit rates. All block clocking is based
on the SYSCLK input, which is nominally 12 MHz or 6 MHz
or 24 MHz for CY8CTMG20x and CY8CTST200 (unless the
PSoC device is in external clocking mode). The sampling
rate and the baud rate are determined as follows:

■

Sample Rate = SYSCLK/Prescale Factor

■

Baud Rate = 1/(Sample Rate x Samples per Bit)

When clocking the input with a frequency other than 6/12
MHz (for example, clocking the device with an external
clock), the baud rates and sampling rates scale accordingly.
Whether the block works in a Standard Mode or Fast Mode
system depends upon the sample rate.

The sample rate must be sufficient to resolve bus events,
such as Start and Stop conditions. See the Philips Semicon-
ductors (now NXP Semiconductors) I

C-Bus Specification,

version 2.1, for minimum start and stop hold times.

Bit 0: Enable.

When the slave is enabled, the block gener-

ates an interrupt upon any Start condition and an address
byte that it receives indicating the beginning of an I

C trans-

fer. When operating as a slave, the block is clocked from an
external master. Therefore, the block works at any fre-
quency up to the maximum defined by the currently selected
clock rate. The internal clock is only used in slave mode, to
ensure that there is adequate setup time from data output to
the next clock upon the release of a slave stall. When the
Enable bit is ‘0’, the block is held in reset and all status is
cleared. CPU access to the HW buffer (EZI2C mode) is
blocked when the bit is '0'. Block enable is synchronized to
the SYSCLK clock input (

see “Timing Diagrams” on

page 130

). If the block is enabled when there is an I

transaction already on the I

C bus, an erroneous start is

detected if SCL is high and SDA is low. (SDA in this case is
low because of a data/ack bit.) Thus, to avoid this erroneous
start detection, enable the block only when the bus is idle
(SCL and SDA are high when idle) or only when both SCL
and SDA are low.

The other option is to change the drive modes of the I

pins to be other than open drain mode and then enable the

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0