Firmware implementation

AN2415

6/11

Note:

At start-up, the evaluation board works in Normal mode.

2.2

Error detection mode

After entering Error Detection mode as shown in Figure 3, it is recommended to send all

data to the drivers as “1”.

As long as the OE pin is high, the serial data can still be shifted to the Shift Register via the

SDI pin, and out via the SDO pin. The LE pin can also send the data in the Shift Register to

the output latch.

When the state of the OE pin is pulled down to low voltage for at least 2 µs, the drivers

execute the error detection function and load the error status into the Shift Register. The

error status codes saved in the Shift Register can then be shifted out via the SDO pin bit-by-

bit along with the clock. The SDO pin of the last driver of the chain is connected to the MISO

pin of the microcontroller. Incoming data can be sent to the PC through the SCI and

displayed on the GUI. For more information, refer to the timing diagram shown in Figure 5.

3

Firmware implementation

The purpose of this application note is to explain how to manage the error detection features

of the drivers. Refer to AN2141 for an explanation of the basic driver features.

3.1

Timing diagram

At power-up, the microcontroller sends data to the drivers’ Shift Register via the SPI

configured at 250 kHz. The maximum communication frequency for this driver is 25 MHz to

satisfy high volume data transmissions. (Please refer to the device datasheet for more

details). The two potentiometers are connected to ADC inputs. The analog voltage inputs

are converted to a digital value. According to this digital value, it will change the PWM

duty-cycle signal for brightness and data time delay for text speed. By default, the

microcontroller works in Normal mode and only enters Error Detection mode when an

external interrupt pin connected to SW2 is triggered. The related ISR disables the SPI I/O

function and sends a specific sequence to the driver via the CLK, OE, and LE pins as shown

in Figure 5.

After the driver has received the specific sequence, the microcontroller enables the SPI and

sends 0xFF data for each driver. The drivers have already entered Error Detection mode

and the microcontroller maintains pin OE low for detecting the error status. Drivers send

back the LED status to the microcontroller via the MISO pin (connected to the SDO pin).

Then, the microcontroller resumes Normal mode operation, sending the specific sequence

to the drivers. Figure 5 shows each phase, switching from Error Detection to Normal modes.