Maxim > Design Support > Technical Documents > Application Notes > Microcontrollers > APP 1918
Keywords: 87C752, data conversion, analog to digital converter, convertor, ADC, A to D, upgrade, 12
bits, 8 bits, A/D, converters
APPLICATION NOTE 1918
12-Bit ADC Upgrades µC's Internal 8-Bit ADC
Jul 01, 2001
Abstract: This article explains how to upgrade the ADC conversion resolution of an 87C752
microcontroller from 8 bits to 12 bits. The analog-to-digital converter is replaced automatically by sensing
the presence of the external MAX186 or MAX188 A to D.
The simple circuit of Figure 1 (plus a software routine) lets you substitute a multichannel, 12-bit A/D
converter for the 8-bit A/D converter internal to an 87C752 microcontroller. Thus, a single assembly can
implement both the low- and high-performance versions of a system. The software mentioned can be
downloaded from EDN's free electronic bulletin board service (BBS)*.
Figure 1. The connections shown enable this µC to subsistute an external 12-bit A/D converter for its
internal 8-bit converter. Replacing the external converter with the resister network causes the µC to
resume use of its converter.
A socket lets you plug in the external A/D converter when you need it; otherwise you plug in the network
of ten 10Ω resistors. At power-up, the µC executes a routine that looks for the external converter. If
present, it is used; if not, the internal 8-bit converter is used. (Internally, the chip handles all conversion
results as 12-bit values.)
This idea relies on the fact that the 87C752's five A/D-input pins can also serve as the bidirectional pins
of an 8051 port (port 1). The resistor network connects the internal A/D converter directly to the applied
analog inputs. Or, replacing the network with the external A/D converter connects those inputs to
corresponding channels on that converter, and the µC's A/D-input pins (now acting as a bidirectional
port) serve as a digital interface to the converter. By fortune and design, the change from external to
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