C8051F120/1/2/3/4/5/6/7 C8051F130/1/2/3
92
Rev. 1.4
7.2.
ADC2 Modes of Operation
ADC2 has a maximum conversion speed of 500 ksps. The ADC2 conversion clock (SAR2 clock) is a
divided version of the system clock, determined by the AD2SC bits in the ADC2CF register. The maximum
ADC2 conversion clock is 6 MHz.
7.2.1. Starting a Conversion
A conversion can be initiated in one of five ways, depending on the programmed states of the ADC2 Start
of Conversion Mode bits (AD2CM2-0) in ADC2CN. Conversions may be initiated by:
1.
Writing a ‘1’ to the AD2BUSY bit of ADC2CN;
2.
A Timer 3 overflow (i.e. timed continuous conversions);
3.
A rising edge detected on the external ADC convert start signal, CNVSTR2;
4.
A Timer 2 overflow (i.e. timed continuous conversions);
5.
Writing a ‘1’ to the AD0BUSY of register ADC0CN (initiate conversion of ADC2 and ADC0 with
a single software command).
During conversion, the AD2BUSY bit is set to logic 1 and restored to 0 when conversion is complete. The
falling edge of AD2BUSY triggers an interrupt (when enabled) and sets the interrupt flag in ADC2CN. Con-
verted data is available in the ADC2 data word, ADC2.
When a conversion is initiated by writing a ‘1’ to AD2BUSY, it is recommended to poll AD2INT to determine
when the conversion is complete. The recommended procedure is:
Step 1. Write a ‘0’ to AD2INT;
Step 2. Write a ‘1’ to AD2BUSY;
Step 3. Poll AD2INT for ‘1’;
Step 4. Process ADC2 data.
When CNVSTR2 is used as a conversion start source, it must be enabled in the crossbar, and the corre-
sponding pin must be set to open-drain, high-impedance mode (see
Section “18. Port Input/Output” on
for more details on Port I/O configuration).
7.2.2. Tracking Modes
The AD2TM bit in register ADC2CN controls the ADC2 track-and-hold mode. In its default state, the ADC2
input is continuously tracked, except when a conversion is in progress. When the AD2TM bit is logic 1,
ADC2 operates in low-power track-and-hold mode. In this mode, each conversion is preceded by a track-
ing period of 3 SAR clocks (after the start-of-conversion signal). When the CNVSTR2 signal is used to ini-
tiate conversions in low-power tracking mode, ADC2 tracks only when CNVSTR2 is low; conversion
begins on the rising edge of CNVSTR2 (see Figure 7.2). Tracking can also be disabled (shutdown) when
the entire chip is in low power standby or sleep modes. Low-power Track-and-Hold mode is also useful
when AMUX or PGA settings are frequently changed, due to the settling time requirements described in
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