58.5.2.1 Output Driver AC Characteristics

The timing definitions necessary to specify the maximum operating frequency of an output driver are given in the following figure.

Figure 58-6. Timing Definitions of a Digital Output Signal

T: Period of the digital output signal

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

tHIGH: Time during which the output waveform is greater than VDD / 2

tLOW = T - tHIGH: Time during which the output waveform is less than VDD / 2

d = tHIGH / T: Output waveform duty cycle

VO: Output waveform amplitude

In tables Output Driver AC Characteristics (1.7V < VDD < 1.9V) and Output Driver AC Characteristics (3.0V < VDD < 3.6V), the maximum operating frequency fMAX ensures that the driver’s output waveform fulfills all the following characteristics:
  • tR < 0.75 / fMAX and tF < 0.75 / fMAX
  • d: the duty cycle of the output waveform is between 45% and 55%
  • VO: the output waveform amplitude is greater than 95% VDD

The fMAX 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.

Tables Output Driver AC Characteristics (1.7V < VDD < 1.9V) and Output Driver AC Characteristics (3.0V < VDD < 3.6V) give 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 < 1.9V
    • 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. See 7.2.4.1 DDR/SDR I/O Calibration.

For the GPIO drivers, the table below gives 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 (tR, tF), with a potentially negative impact on the electromagnetic emissions of the application.

For the DDRIO drivers, it is recommended to use Drive = 0 if the VDD range is 3.0V to 3.6V, and Drive = 1 if the VDD range is 1.7V to 1.9V.

Table 58-15. Recommended GPIO Drive and Slewrate Settings versus GPIO Use Case
VDD Range Low-speed fGPIO ≤ 50 MHz(1) High-speed 50 MHz ≤ fGPIO ≤ 150 MHz(1)
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:
  1. This is an indicative value. See tables Output Driver AC Characteristics (1.7V < VDD < 1.9V) and Output Driver AC Characteristics (3.0V < VDD < 3.6V) for accurate maximum frequency specifications.

Table 58-16. Output Driver AC Characteristics (1.7V < VDD < 1.9V)
Symbol Parameter I/O Type Conditions Min Max Unit
tR or tF Rise or fall time(1)(4) Any CL = 10 pF – – –
GPIO Drive = 0, Slewrate = 0 0.9 4.0 ns
Drive = 1, Slewrate = 0 0.8 2.8 ns
Drive = 0, Slewrate = 1 3.0 12.0 ns
Drive = 1, Slewrate = 1 1.6 5.6 ns
DDRIO Drive = 0 1.2 3.1 ns
Drive = 1 1.0 2.6 ns
fMAX Maximum frequency(2)(3)(4) Any CL = 10 pF – – –
GPIO Drive = 0, Slewrate = 0 90 – MHz
Drive = 1, Slewrate = 0 130 – MHz
Drive = 0, Slewrate = 1 25 – MHz
Drive = 1, Slewrate = 1 50 – MHz
DDRIO Drive = 0 90 – MHz
Drive = 1 100 – MHz
Note:
  1. Measured between VO = 10% VDD and VO= 90% VDD
  2. fMAX = 0.75 / (tR+tF). Frequency numbers are rounded for legibility.
  3. fMAX may be limited by the peripheral that drives the I/O line.
  4. Simulation data

Table 58-17. Output Driver AC Characteristics (3.0V < VDD < 3.6V)
Symbol Parameter I/O Type Conditions Min Max Unit
tR or tF Rise or fall time(1)(4) Any CL = 10 pF – – –
GPIO Drive = 0, Slewrate = 0 0.9 2.0 ns
Drive = 1, Slewrate = 0 0.8 1.8 ns
Drive = 0, Slewrate = 1 3.0 6.6 ns
Drive = 1, Slewrate = 1 1.6 3.3 ns
DDRIO Drive = 0 0.9 2.0 ns
Drive = 1 0.8 1.7 ns
fMAX Maximum frequency(2)(3)(4) Any CL = 10 pF – – –
GPIO Drive = 0, Slewrate = 0 150 – MHz
Drive = 1, Slewrate = 0 170 – MHz
Drive = 0, Slewrate = 1 50 – MHz
Drive = 1, Slewrate = 1 75 – MHz
DDRIO Drive = 0 140 – MHz
Drive = 1 160 – MHz
Note:
  1. Measured between VO = 10% VDD and VO= 90% VDD
  2. fMAX = 0.75 / (tR+tF). Frequency numbers are rounded for legibility.
  3. fMAX may be limited by the peripheral that drives the I/O line.
  4. Simulation data