p87lpc778-01 NXP Semiconductors, p87lpc778-01 Datasheet - Page 15

• Manufacturer Part Number: p87lpc778-01
• Description: P87lpc778 Cmos Single-chip 8-bit 80c51 Microcontroller With 128-byte Data Ram, 8 Kb Otp
• Manufacturer: NXP Semiconductors

Philips Semiconductors
9397 750 12378
Product data
8.4.1 The A/D in Power-down and Idle modes
Table 8:
While using the CPU clock as the A/D clock source, the Idle mode may be used to
conserve power and/or to minimize system noise during the conversion. CPU
operation will resume and Idle mode terminate automatically when a conversion is
complete if the A/D interrupt is active. In Idle mode, noise from the CPU itself is
eliminated, but noise from the oscillator and any other on-chip peripherals that are
running will remain.
The CPU may be put into Power-down mode when the A/D is clocked by the on-chip
RC oscillator (RCCLK = 1). This mode gives the best possible A/D accuracy by
eliminating most on-chip noise sources.
If the Power-down mode is entered while the A/D is running from the CPU clock
(RCCLK = 0), the A/D will abort operation and will not wake up the CPU. The
contents of DAC0 will be invalid when operation does resume.
When an A/D conversion is started, Power-down or Idle mode must be activated
within two machine cycles in order to have the most accurate A/D result. These two
machine cycles are counted at the CPU clock rate. When using the A/D with either
Power-down or Idle mode, care must be taken to insure that the CPU is not restarted
by another interrupt until the A/D conversion is complete. The possible causes of
wake-up are different in Power-down and Idle modes.
A/D accuracy is also affected by noise generated elsewhere in the application, power
supply noise, and power supply regulation. Since the P87LPC778 power pins are
also used as the A/D reference and supply, the power supply has a very direct affect
CPU clock rate
32 kHz
1 MHz
4 MHz
11.0592 MHz
12 MHz
16 MHz
20 MHz
Fig 5. A/D converter connections.
AD0 (P0.3)
AD1 (P0.4)
AD2 (P0.5)
AD3 (P0.6)
Example A/D conversion times
RCCLK = 0
NA
186 s
46.5 s
16.8 s
15.5 s
11.6 s
9.3 s
Rev. 01 — 31 March 2004
ADCON
ADCON
00
01
10
11
DAC0
RCCLK = 1
minimum
563.4 s
16 s
15.9 s
15.5 s
15.3 s
32.4 s
18.9 s
ADCON
A/D CONVERTER
CMOS single-chip 8-bit microcontroller
(A/D result, read DAC0)
DAC0
nominal
659 s
39.3 s
23.6 s
20.2 s
20.1 s
19.7 s
19.4 s
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
P87LPC778
V REF + = V DD
V REF - = V SS
maximum
757 s
48.9 s
30.1 s
27.1 s
26.9 s
26.4 s
26.1 s
002aaa616
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