ADUC: Should I connect devices analog and digital ground pins to separate ground planes?

Q The MicroConverter has separate pins for analog and digital grounds. Should I connect these to two separate ground planes on my board? A No. At least, not unless the two ground planes are connected together very near the chip. In all other situations, it is best to keep the MicroConverter on a single ground plane. If you have two separate ground planes on your board, then place the MicroConverter on the quieter of the two (usually the analog plane) to obtain optimum ADC and DAC performance.

Program halts or operates incorrectly with ADuC842 at 16.78MHz

Q Program halts or operates incorrectly with ADuC842 at 16.78MHz. What might be the problem? A The ADuC842 can only operate at 16.78MHz when the supply voltage is 5V. Maximum frequency is limited to 8.38MHz at 3V operation. If the power supply decoupling, grounding of the printed circuit board design is marginal, then increasing the clock rate may bring these problems to light, causing the the program to stall more frequently. => with an analogue oscilloscope, scope the voltage on the Vdd and Ground lines to assess how much noise is present and how it changes with frequency. If it is more than a 100mV you have a problem.

Using ADuC842 as master on a multimaster I2C bus

Q Can I use the ADuC842 in an I2C-multimaster environment? A The ADuC842 does not implement I2C master mode in hardware. You need to write the software to implement the code. See Application note uC001 at www.analog.com Page 7 of this application note states clearly that it is not possible to readback the status of SCL or SDA pins unless MDE is low (configured as an input). Therefore you need to build in the multimaster functionality into your software master by using extra input pins.

ADUC842 port 3 internal pull-up electrical characteristics

Q I'm using an ADuC842 and I noticed that on the datasheet there are few and no specific indications about the electrical characteristics of the internal pull-up on port 3, can you provide more information? A The port 3 pins have a finite and non-linear output impedance i.e. the voltage will change according to how much current you draw. We specify this in the following way (see page 5 of the datasheet, section on digital outputs). Voh output voltage high: > 2.4V when Isource < 80uA and power supply >4.5V > 4.0V when Isource < 20uA and power supply >4.5V so basically, when you draw > 80uA from the part we can't guarantee that the "high" voltage will stay above the 2.4V required to be a logic high in TTL. This makes it a fairly weak output and shouldn't be used to drive anything that needs significant current. Port 3, Vol output voltage low: < 0.4V when Isink < 8mA This is a much more realistic situation for driving any external load, so you should always refer the load to Vdd and drive the pin low to turn on the load, rather than referring the load to ground and trying to drive the pin high. In any case the part shouldn't sink more than 10mA per pin.

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