Revision Affected

B, C

Details

The I2C peripheral present on the MCU is a Fast-mode device; it will clock-stretch the SCL (Clock) line when used with a Standard-mode host.

There is a requirement from the I2C Specification for a Fast-mode device used in a Standard-mode system to meet t_SU:DAT (data set-up time) + t_r(max) (rise time) before releasing the SCL line. See Footnote 4 of the "Characteristics of the SDA and SCL bus lines for Standard, Fast, and Fast-mode Plus I²C-bus devices" table in the NXP Semiconductors I²C-bus specification and user manual (UM10204).

However, the C2000 I2C clock-stretches the SCL line by a fixed amount = 6 * f_mod Clock (I2C Clock rate of the C2000) in the above scenario. When the C2000™ microcontroller is acting as a target transmitter with a Standard-mode host, it is possible for the clock line (SCL) to be released by the C2000 before the data (SDA) is ready, if the t_r of SDA is too long.

The "Pull-up resistor sizing" section in the NXP Semiconductors I²C-bus specification and user manual (UM10204) gives more details on choosing the appropriate PU resistor (R_p), based on the rise time (t_r) and bus capacitance (C_b) shown in Equation 1.

Workaround

1. Reducing t_r with a strong pullup
   In order to ensure that t_SU:DAT + t_r(max) is met, the user can configure the pullup resistance on the SDA line such that it meets the constraints listed in the SDA Data Rise Time Requirement column of Table 3-3 based on the value of f_mod Clock in their system. This will ensure that the data present on the SDA line is valid when the C2000 releases the SCL signal. Table 3-4 gives suggested R_p resistor values for a given f_mod Clock (MHz) and C_b (bus capacitance). For other values of C_b, please use Equation 1 to calculate the value of R_p needed in the system.
   Table 3-3 Data Rise Time Requirements for C2000 as Target Transmitter with Standard-Mode Host

   fmod Clock (MHz)	fmod Period (ns)	SCL Clock- Stretch Delay from C2000 I2C (ns): (6*fmod Clock)	Data Set-up Time (ns): tSU:DAT (Standard Mode)	SDA Data Rise Time Requirement (ns): tr
   7	142.9	857	250	607
   8	125	750		500
   9	111	666		416
   10	100	600		350
   11	90.9	545		295
   12	83.3	500		250

   Table 3-4 Pullup Resistor (R_p) Values for Common Bus Capacitances (C_b)

   fmod Clock (MHz)	SDA Data Rise Time Requirement (ns): tr	Rp (kΩ) for Cb = 100 pF	Rp (kΩ) for Cb = 200 pF	Rp (kΩ) for Cb = 300 pF	Rp (kΩ) for Cb = 400 pF
   7	607	7.1	3.5	2.3	1.7
   8	500	5.9	2.9	1.9	1.4
   9	416	4.9	2.4	1.6	1.2
   10	350	4.1	2.0	1.3	1.0
   11	295	3.4	1.7	1.1	0.8
   12	250	2.9	1.4	0.9	0.7

2. t_r = 1000 ns
   This workaround is not preferred due to restrictions in general I2C usage, use Workaround 1 when possible.
   If the system is such that it requires 1000 ns of rise time on the SDA line, the C2000 I2C f_mod Clock can be configured to 4.8 MHz so the clock-stretching (6 * f_mod Clock) satisfies this requirement. This results in t_r = (1/4.8 MHz) * 6 = 1000 ns. This workaround is only valid in systems where the C2000 I2C is the target on the I2C bus. Note that 4.8 MHz is outside the data sheet's required range of 7 MHz to 12 MHz for f_mod Clock. Using f_mod at 4.8 MHz, even though it is outside of the data sheet's required range, will work for the C2000 I2C in Target mode on a Standard-mode host bus. Using f_mod = 4.8 MHz in any other configurations except the one listed in this workaround will cause other timing parameters to be violated and is not allowed.