User manual

Low-power Application Kit (Oryx) - User’s Guide

Page 28

The SPI clock is set to 750kHz. One byte transfer takes about 13.4uS. During SPI transfers the

LPC11U37 does not have much to do, other than waiting for the SPI transfer to complete. The

processor core frequency is haled during this time, which results in about half the normal current

consumption. Note that the clock frequency to the SPI peripheral block is not affected.

Toggling the VCOM bit takes 70uS all in all including all interrupt handling. 48uS of this, the current

consumption is half (7.6mA). The rest, 22uS, the current consumption is 14mA. Generating the 0.5Hz

VCOM signal to the LCD generate a mean current consumption of 0.7uA.

The conclusion is that having the LPC11U37 in power down mode and having the LCD active cost

about 3.2uA (2.5+0.7) in current consumption. On top of this the LPC11U37 must do something useful

to generate something on the LCD.

Updating the complete LCD (96x96 pixels) takes about 18mS in total. Current consumption is 7.6mA

during this time. It is not necessary to update all 96x96 pixels on the display. Selected rows (96 pixels)

of the display can be updated. The clock application in the pre-loaded demo application on the board

demonstrates when only part of the display is updated – the part where the time is presented. For the

specific case of the clock application, it takes 10.4mS to generate the updated bitmap and 3.8mS to

send it to the LCD via SPI transfers. Updating a clock on the LCD gives a mean current consumption

of (10.4mS*14mA + 3.8mS*7.6mA) = 174.5uA. Updating only once per minute would give a significant

current consumption. There is a setting on the clock application that demonstrates this also.

There is an alternative way to generate the 0.5Hz VCOM signal. It can be a hardware signal directly

from the RTC, the CLKOUT signal. R24 and R25 must then be mounted and R26 removed. The RTC

must also be programmed to generate a suitable CLKOUT signal.

5.3.3 Real-Time Clock

The RTC can interact with the LPC11U37 in some different ways. The selected way in the design is to

have the RTC generate a 1Hz signal to the LPC11U37. The signal is high 63/64 parts of a second and

low 1/64 part of a second (15.6mS). The output from the RTC is an open-drain output and requires the

input pin on the LPC11U37 to have a pull-up resistor enabled. Because the signal from the RTC is only

low for 15.6mS each second, the current consumption from the pull-up resistor is kept to an average

on 0.625uA.

The alternative solution is to have the RTC generate a real interrupt signal. The LPC11U37 must then

reset the interrupt flag inside the RTC via I2C commands. The LPC11U37 must then be active (about

15mA current) for about 350uS to accomplish this. This results in a mean current consumption of about

5.25uA (which is clearly higher than the used solution).

The 1Hz signal from the RTC generates a 1Hz interrupt at the LPC11U37. This can be used for time

keeping and refreshing the LCD.

There are sample applications that used the WDT to get a time keeping interrupt. The RTC is much,

much more accurate than the WDT.

5.3.4 SPI Flash

There is a supply voltage switch for the SPI flash. This means that the current consumption of the SPI

flash is zero when not used. The SPI clock is 2.9MHz. Each byte transfer takes about 4.5uS. Reading

a bmp file for filling the 96x96 pixels takes 5.5mS.

5.3.5 Temperature Sensor

The temperature sensor can be placed in a suspended mode where the current consumption is 0.2uA

maximum. Therefore the current consumption can be kept very low. Read the datasheet for the sensor

to get information about startup times from suspended mode until an accurate temperature reading is

possible.