Microchip Technology dsPIC30F - Page 396

dsPIC30F Family Reference Manual
DS70064C-page 17-20 © 2004 Microchip Technology Inc.
17.12.2 Clocked Conversion Trigger
When SSRC<2:0> = 111, the conversion trigger is under A/D clock control. The SAMC bits
(ADCON3<12:8>) select the number of T
AD clock cycles between the start of sampling and the
start of conversion. This trigger option provides the fastest conversion rates on multiple channels.
After the start of sampling, the module will count a number of T
AD clocks specified by the SAMC
bits.
Equation 17-2: Clocked Conversion Trigger Time
When using only 1 S/H channel or simultaneous sampling, SAMC must always be programmed
for at least one clock cycle. When using multiple S/H channels with sequential sampling,
programming SAMC for zero clock cycles will result in the fastest possible conversion rate. See
Example 17-3 for code example.
Figure 17-6: Converting 1 Channel, Manual Sample Start, T
AD Based Conversion Start
Example 17-3: Converting 1 Channel, Manual Sample Start,
T
AD Based Conversion Start Code
TSMP = SAMC<4:0>*TAD
ADCLK
SAMP
ADCBUF0
TSAMP
TCONV
BSF ADCON1,SAMP
Instruction Execution
DONE
= 16 TAD
ADPCFG = 0xEFFF; // all PORTB = Digital; RB12 = analog
ADCON1 = 0x00E0; // SSRC bit = 111 implies internal
// counter ends sampling and starts
// converting.
ADCHS = 0x000C; // Connect RB12/AN12 as CH0 input ..
// in this example RB12/AN12 is the input
ADCSSL = 0;
ADCON3 = 0x1F02; // Sample time = 31Tad, Tad = internal 2 Tcy
ADCON2 = 0;
ADCON1bits.ADON = 1; // turn ADC ON
while (1) // repeat continuously
{
ADCON1bits.SAMP = 1; // start sampling then ...
// after 31Tad go to conversion
while (!ADCON1bits.DONE); // conversion done?
ADCValue = ADCBUF0; // yes then get ADC value
} // repeat // repeat