The Si72xx-WD-Kit includes demonstrations for wheel position sensing, wheel rotation counting, and display of the magnetic field data from sensors and small postage stamp sized evaluation boards. This demonstration uses the MCU and display from a pre-programmed EFM32 Happy Gecko STK. The source code for the demo is included with Simplicity Studio which can be downloaded from https://www.silabs.com/products/development-tools/software/simplicity-studio
The Si1133 and Si115x Sensor parts share a common die layout with the same photodiode array. The article points out where the various photodiodes and photodiode combinations are located and selected. This die is centered in the 2x2 mm clear QFN package that the parts come in. If the module is used the visible and IR photodiode array is centered in beneath the module circular opening.
Figure 1 Photo Diode Locations on the die
The figure below shows that at each level there are 12 individual squares that are connected as one named photodiode for each two squares. The colors shown here are strictly for illustration purposes. They do not imply a filter or sensitivity. The photodiodes are named the same way except with a b suffix on the bottom. Thus, at the top there are: D1a pair, D3a pair … D6a pair while at the bottom there are D1b pair, D3b pair … D6b pair.
The black corner photodiodes are covered in metal and do not measure light. They are used to automatically compensate for photodiode current leakage.
Figure 2 Light Travel in the Stacked Photodiodes
The following #define table shows all the photodiode selections available. The figures below illustrate some examples with different groups of PDs selected.
#define ADCCONFIG_ADC_MUX_D1a_D4a_minus_DARK 0x0D // (D1a+D4a) - (D5a+D6a)
#define ADCCONFIG_ADC_MUX_D1a_minus_DARK 0x0B // (D1a - D5a)
#define ADCCONFIG_ADC_MUX_L_IR 0x02 // (D1b + D2b + D3b + D4b) - 2*(D5b + D6b)
#define ADCCONFIG_ADC_MUX_M_IR 0x01 // (D1b + D2b) - (D5b + D6b)
#define ADCCONFIG_ADC_MUX_S_IR 0x00 // D1b(w) - D5
#define ADCCONFIG_ADC_MUX_UV_SHALLOW_minus_DARK 0x18 // UV shallow - UV shallow-dark
Table 1 Si1133/53 photodiode selections: ADCMUX[4:0] field of ADCCONFIGx register
Figure 3 The RED area shown in this figure is the one active with the ADC mux field set to 0x00
Figure 4 The RED area shown in this figure is the one active with the ADC mux field set to 0x02
Figure 5 The RED area shown in this figure is the one active with the ADC mux field set to 0x01
Photodiodes used in the Si1133/4x/5x sensors are arranged in a 3D stack, one set shallow and one set deep. The figure below shows that at each of the two levels level there are 12 individual squares that are connected as one named photodiode for each two squares. The colors shown here are strictly for illustration purposes. They do not imply a filter or sensitivity. The photodiodes are named the same way except with a “b” suffix on the bottom instead of “a”. Thus, at the top there are: D1a pair, D3a pair … D6a pair while at the bottom there are D1b pair, D3b pair … D6b pair.
The black corner photodiodes are covered in metal and do not measure light. They are used to compensate for photodiode current leakage.
Figure 1 Light Travel in the Stacked Photodiodes
As a result of this arrangement, the spectral response of the shallow and deep photodiodes is different. The shallow photodiodes are about 4X less sensitive in the green but about 20 times less sensitive in the IR which is can be an advantage in some applications (e.g. Ambient light sensing or ALS that suffer from IR interference.
The resultant response curves for both shallow and deep photodiodes are shown in the figure above. The deep one is much more sensitive with a peak at about 800 nm while the shallow one is less sensitive with a peak in the blue.
The advantage of the less sensitive one is the relative response of IR region where it suppresses relative IR response by a large factor this can be seen in the figure below that show that comparing 550 nm (green) response to the 900 nm IR response the shallow PDs suppress 900 nm IR by a factor of 7.5 while the deep PDs accentuate the 900 nm IR by a factor of 2.
The shallow PD is more effective for visible light sensing (ALS) since IR response is a major problem. Th reduced overall sensitivity is usually acceptable especially with si1133/5x parts since they have good dark current compensation and can reach ~0.01 lux with the deep PD and ~0.1 lux with the shallow PDs. Note that the older Si114x part does not have the dark current compensation and is limited to higher light levels.
What’s the major difference between Si114x and Si115x proximity sensor? As an existing Si114x customer, is there any guidance to migrate to Si115x?
Both Si114x and Si115x optical sensors can be used to measure proximity and ambient light. However, Si114x has dedicated PS and ALS tasks while Si115x provides 6 configurable channels with the flexibility of setting multiple PS or ALS measurements in any order as users would like to.
In general, Si115x has a few improvements over Si114x.
On the other hand, Si114x has one feature that’s missing on Si115x: interrupt when a set number of consecutive samples exceeding the threshold. On Si115x, if the host configures the sensor to interrupt on a certain threshold, the sensor will generate the interrupt as soon as 1 sample exceeds that threshold.
Although the basic principle of operation is the same for Si114x and Si115x, the host interface, including the I2C registers and parameter registers, are quite different. Existing customers moving from Si114x to Si115x will have to re-write the host software. We’d always recommend customers to check out the example project we have on Si115x/33 OPT EXP board first. The demo provides the driver code as well as initialization example code for Si115x. Using that as a starting point and then follow the register table definition in the datasheet, customers should be able to do the migration without much difficulty. If customers run into issues or have questions porting the code, they’re welcome to create technical support tickets in our system.