Spurious ADC values

[chrispflieger]

LPC 1227 running at 36 MHz and I need to sample a DC signal, so roughly 1/10 second will be fine, no IRQ required.

The problem is I see a lot of spurious values coming from the ADC - I have a buffer of 36 recent values, and nine are just way out of bounds - like should be ~24600 but some are ~32000 and some below 1000.

The setup code:

   LPC_IOCON->R_PIO0_30 = 0x00000003;
   LPC_SYSCON->PDRUNCFG      &= ~( 1ul <<  4 ); // Power up ADC block
   LPC_SYSCON->SYSAHBCLKCTRL |=  ( 1ul << 14 ); // Enable PCLK to ADC
   LPC_ADC->CR = 0x001010400 | (ENABLED_CHANNELS);     // set for burst mode with a 36MHz clock.

The sample code:

    while ((LPC_ADC->GDR & 0x80000000) == 0);
    result = (uint16_t)((LPC_ADC->GDR & 0x0000FFC0) >> 6);

I assume there's some weird sampling problem because of burst mode, but is there something I need to do to synchronize sampling?

[jeremyzhou]

Hi Chris Pflieger,

Thank you for your interest in NXP Semiconductor products and 

for the opportunity to serve you.

To provide the fastest possible support, I've attached a ADC demo and please refer to it for details.

Have a great day,
TIC

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[chrispflieger]

There's my problem. Even though I'm only looking at one channel, LPC_ADC->GDR gives wonky values, but LPC_ADC->DR[0] doesn't.