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Mar 8, 2012

SiM3U1xx-current-voltage-converter


Introduction
  SiM3U1xx and SiM3C1xx include the Current-to_Voltage Converter (IVC) module. The IVC module allows current sourced from a pin (sunk externally) to be measured by SARADCn modules on the device.

Function Description

    The IVC module consists of two channels of current-to-voltage conversion circuitry. Current is sourced from the IVC0.0 or IVC0.1 pins, creating a voltage drop at the pin and the ADC input. A simplified diagram of single channel of the IVC circuitry is shown in below Figure.


The IVC module has selectable full-scale current ranges of 1 mA to 6 mA, configurable in 1 mA steps. The transfer function of the IVC module is a curve that is 0 V with a 0 mA input, and 1.65 V at full scale.

We need to set PB0.7 pin as analog if want to use IVC function. 

And set IVC0 as SARADC input channel.

We need a little math to calculate the current, here is the equation.

Vout = (Vfullscale x code)/1023
Iin = Vout / slope


Where:
code = the left-justified 10 bit SARADC output code
Vfullscale =Full scale output, it 1.65V
Vout = IVC convert voltage output value

Iin = the current sunk externally from IVCn pin.
Slope = 275 mV/mA with condition input range 6mA

Here is the detailed register description for IVC0 registers

 

Code Implementation

   We used AppBuilder to generate basic code we need, and then added necessary code to make temperature sensor works.
void CLKCTRL_setup_default_mode_clock_gates(void)
{
  SI32_CLKCTRL_A_enable_apb_to_modules_0(SI32_CLKCTRL_0,
                                         SI32_CLKCTRL_A_APBCLKG0_PB0 |
                                         SI32_CLKCTRL_A_APBCLKG0_SARADC0);|
  SI32_CLKCTRL_A_enable_apb_to_modules_1(SI32_CLKCTRL_0,
                                         SI32_CLKCTRL_A_APBCLKG1_MISC1 |
                                         SI32_CLKCTRL_A_APBCLKG1_MISC0);
}
2.       Configure the input range 6mA of the IVC channel and enable the IVC channel.(gIVC0.c)
void IVC0_enter_default_mode_from_reset(void)
{
  SI32_IVC_A_select_channel_0_range(SI32_IVC_0, SI32_IVC_A_CONTROL_IN0RANGE_6_MA_VALUE);
  SI32_IVC_A_enable_channel_0(SI32_IVC_0);
}

3.       Enable SARADC module; Timeslot0 channel is “IVC0.0 Output”; Output packing mode is “Lower half-word only”. And keep default setting of reference GND select as “internal GND” and start of conversion source as “On Demand by writing 1to ADBUSY” (gSARADC.c)
void SARADC0_enter_default_mode_from_reset(void)
{
  SI32_SARADC_A_enable_module(SI32_SARADC_0);
  SI32_SARADC_A_select_output_packing_mode_lower_halfword_only(SI32_SARADC_0);
  SI32_SARADC_A_select_timeslot0_channel(SI32_SARADC_0, 16);
}
4.       Port bank configuration. PB0.0 and PB0.7 need to be set as analog pin and skipped in crossbar.(gPB.c)
void pb_enter_default_mode_from_reset(void)
{
  SI32_PBCFG_A_unlock_ports(SI32_PBCFG_0);
  // PB0 Setup
  SI32_PBSTD_A_set_pins_analog(SI32_PBSTD_0, 0x0081);
  SI32_PBSTD_A_write_pbskipen(SI32_PBSTD_0, 0x2081);
  // PB1 Setup
  SI32_PBSTD_A_set_pins_push_pull_output(SI32_PBSTD_1, 0x0008);
  SI32_PBSTD_A_write_pbskipen(SI32_PBSTD_1, 0x0008);
  // Enable Crossbar0 signals & set properties
  SI32_PBCFG_A_enable_crossbar_0(SI32_PBCFG_0);
  // PB2 Setup
  SI32_PBSTD_A_set_pins_push_pull_output(SI32_PBSTD_2, 0x0C00);
  SI32_PBSTD_A_set_pins_high_drive_strength(SI32_PBSTD_2, 0x0C00);
  // Enable Crossbar1 signals & set properties
  SI32_PBCFG_A_enable_crossbar_1(SI32_PBCFG_0);
}
5.       Caculate IVC output voltage and input current.(mySARADC0.c)
void mySARADC0_current_convert_voltage(void)
{
  uint32_t voltage, current, tmp;
  SI32_SARADC_A_start_conversion(SI32_SARADC_0);
  while((0x20 & SI32_SARADC_A_read_fifostatus(SI32_SARADC_0)) == 0){
  }
  tmp = SI32_SARADC_A_read_data(SI32_SARADC_0);
  // IVC0 full scale output is 1650 mV. SARADC is 10 bit, means max value is 1023
  voltage = 1650 * tmp / 1023;
  current = voltage * 1000 / myIVC0_get_range_slope();
  printf("The Vout=%d mV, Current = %d uA \n",voltage, current );
}

Function validation

   We use series-connected 1K resistors connect P0.7 (IVC0) pin to GND, by choosing difference value ohm resistors; we can get difference current output from P0.7 pin.

References

1.       SiM3U1xx/SiM3C1xx Reference Manual: downloadable from the Silicon Labs web site at http://www.silabs.com/pages/DownloadDoc.aspx?FILEURL=Support Documents/TechnicalDocs/SiM3U1xx_SiM3C1xx_RM.pdf&src=DocumentationWebPart

2.       SiM3U1xx Data Sheet: downloadable from the Silicon Labs web site at http://www.silabs.com/pages/DownloadDoc.aspx?FILEURL=Support Documents/TechnicalDocs/SiM3U1xx.pdf&src=DocumentationWebPart

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