10.1.5.2.1 Output Driver AC Characteristics

The following figure provides the timing definitions required to specify the maximum operating frequency of an output driver.

Figure 10-6. Timing Definitions of a Digital Output Signal

t_SIG: Period of the digital output signal

f = 1 / t_SIG: Frequency of the digital output signal

t_HIGH: Time during which the output waveform is greater than V_DD / 2

t_LOW = t_SIG - t_HIGH: Time during which the output waveform is less than V_DD / 2

d = t_HIGH / t_SIG: Output waveform duty cycle

V_O: Output waveform amplitude

In Table 10-19 and Table 10-20, the maximum operating frequency f_MAX guarantees that the driver’s output waveform fulfills the following characteristics:

t_R < 0.75 / f_MAX and t_F < 0.75 / f_MAX
d: the duty cycle of the output waveform is between 45% and 55%
V_O: the output waveform amplitude is greater than 95% VDD

The f_MAX parameter indicates the speed limit of an output driver across various operating conditions: supply voltage range, load capacitance, drive strength and slewrate programming. The effective maximum output frequency of a specific output line may be limited by the peripheral that drives this line.

Table 10-19 and Table 10-20 provide the AC output characteristics of the output drivers in the following conditions:

Output load: 10 pF capacitor to ground
Two VDD ranges:
- 1.7V < VDD < 3.6V
- 3.0V < VDD < 3.6V
Two Drive settings: 0 and 1
Two Slewrate settings for the GPIO type: 0 and 1. The DDRIO drivers do not have slewrate settings and rather use an autocalibration setting.

For the GPIO drivers, the table below shows the recommended Drive and Slewrate settings depending on the output switching frequency and the two commonly used VDD ranges (1.8V and 3.3V). Other settings are possible but they may lead to excessively fast rise and fall times (t_R, t_F), with a potentially negative impact on the electromagnetic emissions of the application.

Table 10-18. Recommended GPIO Drive and Slewrate Settings versus GPIO Use Case
VDD Range	Low-Speed (f_GPIO < 50 MHz)⁽¹⁾	High-Speed (f_GPIO ≥ 50 MHz)
1.7V – 1.9V	Drive = 1, Slewrate = 1	Drive = 1, Slewrate = 0
3.0V – 3.6V	Drive = 0, Slewrate = 1	Drive =0, Slewrate = 0

Note:

Indicative value. See Table 10-19 and Table 10-20 for accurate maximum frequency specifications.

Table 10-19. Output Driver AC Characteristics (1.7V < V_DD < 1.9V, C_L = 10 pF)
Symbol	Parameters	Conditions		Min	Max	Unit
t_{R /}t_F	Rise or fall time⁽¹⁾⁽²⁾	GPIO	Drive = 0, Slewrate = 0	1.5	5.1	ns
			Drive = 1, Slewrate = 0	1.4	4.8	ns
			Drive = 0, Slewrate = 1	4.1	10.3	ns
			Drive = 1, Slewrate = 1	2.1	7.0	ns
f_MAX	Maximum frequency^(2)(3)(4)	GPIO	Drive = 0, Slewrate = 0	95	–	MHz
			Drive = 1, Slewrate = 0	135	–	MHz
			Drive = 0, Slewrate = 1	25	–	MHz
			Drive = 1, Slewrate = 1	50	–	MHz

Note:

Symbol	Parameter	Conditions		Min	Max	Unit
t_Ror t_F	Rise or fall time⁽¹⁾⁽²⁾	GPIO	Drive = 0, Slewrate = 0	1.5	2.9	ns
			Drive = 1, Slewrate = 0	1.4	2.7	ns
			Drive = 0, Slewrate = 1	4.0	7.0	ns
			Drive = 1, Slewrate = 1	2.2	4.0	ns
f_MAX	Maximum frequency^(2)(3)(4)		Drive = 0, Slewrate = 0	90	–	MHz
			Drive = 1, Slewrate = 0	130	–	MHz
			Drive = 0, Slewrate = 1	25	–	MHz
			Drive = 1, Slewrate = 1	50	–	MHz