11.1.6.2.1 Output Driver AC Characteristics

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

Figure 11-8. 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 11-21 and Table 11-22, the maximum operating frequency f_MAX ensures 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 programming. The effective maximum output frequency of a specific output line may be limited by the peripheral that drives this line. Conversely, higher output switching speeds may be achieved under different load conditions. Microchip recommends to perform an electrical simulation of the critical interfaces (GMAC, SDMMC, QSPI, etc.) with the provided IBIS models.

Table 11-21 and Table 11-22 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 < 1.9V and
- 3.0V < VDD < 3.6V
Two Drive settings: 0 and 1, and
Two Slewrate settings for the GPIO type: 0 and 1.

For the GPIO drivers, the following table provides 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 11-19. Recommended GPIO Drive and Slewrate Settings vs IO Use Case
VDD Range	Low Speed f_GPIO ≤ 50 MHz⁽¹⁾	High Speed 50 MHz ≤ f_GPIO ≤ 170 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:

This is an indicative value. See Table 11-21 and Table 11-22 for accurate maximum frequency specifications.

For the HSIO drivers (used in SDMMCx and QSPI0 peripherals), the following table provides their nominal output impedance with respect to the Drive settings. These drivers do not have a Slewrate setting but are rather calibrated against an external 1% resistor mounted on the SDMMCx_CAL or QSPI0_CAL pins. Depending on the target signal frequency and the external load, it is possible adjust their target output impedance.

Table 11-20. HSIO Output Impedance vs Drive Settings
VDD Range	Drive = 00 (Type D)	Drive = 01 (Type A)	Drive = 10 (Type C)	Drive = 11 (Type B)	Unit
1.7V – 1.9V⁽¹⁾	100	33	66	50	Ω
3.0V – 3.6V⁽²⁾	84	28	56	42	Ω

Symbol	Parameter	I/O Type	Conditions	Min	Max	Unit
t_Ror 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	12.3	ns
			Drive = 1, Slewrate = 1	2.1	7.0	ns
		HSIO	Drive = 11 (Type B)	1.6	1.8	ns
			Drive = 01 (Type A)	1.1	1.25	ns
			Drive = 10 (Type C)	2.2	2.3	ns
			Drive = 00 (Type D)	3.3	3.4	ns
		PIOBU	–	1.0	4.0	ns
f_MAX	Maximum frequency⁽²⁾⁽³⁾	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
		HSIO	Drive = 11 (Type B)	125	–	MHz
			Drive = 01 (Type A)⁽⁴⁾	150	–	MHz
			Drive = 10 (Type C)	100	–	MHz
			Drive = 00 (Type D)	70	–	MHz
		PIOBU	–	25	–	MHz