C8051F50x-F51x
Post-Tracking Mode is selected when AD0TM is set to 01b. A programmable tracking time based on
AD0TK is started immediately following the convert start signal. Conversions are started after the pro-
grammed tracking time ends. After a conversion is complete, ADC0 does not track the input. Rather, the
sampling capacitor remains disconnected from the input making the input pin high-impedance until the
next convert start signal.
Dual-Tracking Mode is selected when AD0TM is set to 11b. A programmable tracking time based on
AD0TK is started immediately following the convert start signal. Conversions are started after the pro-
grammed tracking time ends. After a conversion is complete, ADC0 tracks continuously until the next con-
version is started.
Depending on the output connected to the ADC input, additional tracking time, more than is specified in
Table 5.9, may be required after changing MUX settings. See the settling time requirements described in
Section “6.2.1. Settling Time Requirements” on page 57.
Convert Start
Pre-Tracking
AD0TM = 10
Track
Idle
Convert
Track
Convert ...
Post-Tracking
AD0TM= 01
Track
Track
Convert
Convert
Idle
Track Convert..
Track Convert..
Dual-Tracking
AD0TM = 11
Track
Track
Figure 6.2. ADC0 Tracking Modes
6.1.3. Timing
ADC0 has a maximum conversion speed specified in Table 5.9. ADC0 is clocked from the ADC0 Subsys-
tem Clock (FCLK). The source of FCLK is selected based on the BURSTEN bit. When BURSTEN is
logic 0, FCLK is derived from the current system clock. When BURSTEN is logic 1, FCLK is derived from
the Burst Mode Oscillator, an independent clock source with a maximum frequency of 25 MHz.
When ADC0 is performing a conversion, it requires a clock source that is typically slower than FCLK. The
ADC0 SAR conversion clock (SAR clock) is a divided version of FCLK. The divide ratio can be configured
using the AD0SC bits in the ADC0CF register. The maximum SAR clock frequency is listed in Table 5.9.
ADC0 can be in one of three states at any given time: tracking, converting, or idle. Tracking time depends
on the tracking mode selected. For Pre-Tracking Mode, tracking is managed by software and ADC0 starts
conversions immediately following the convert start signal. For Post-Tracking and Dual-Tracking Modes,
the tracking time after the convert start signal is equal to the value determined by the AD0TK bits plus 2
FCLK cycles. Tracking is immediately followed by a conversion. The ADC0 conversion time is always 13
SAR clock cycles plus an additional 2 FCLK cycles to start and complete a conversion. Figure 6.3 shows
timing diagrams for a conversion in Pre-Tracking Mode and tracking plus conversion in Post-Tracking or
Dual-Tracking Mode. In this example, repeat count is set to one.
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Rev. 1.1
SILICON [ SILICON ]
