DS80C400 Maxim, DS80C400 Datasheet - Page 95

• Manufacturer Part Number: DS80C400
• Description: The DS80C400 network microcontroller offers the highest integration available in an 8051 device
• Manufacturer: Maxim

DS80C400 Network Microcontroller
source to exit stop mode, resuming operation in less than 100ns. After 65,536 oscillations of the external clock
source (not the ring oscillator), the device clears the ring oscillator mode bit, RGMD (EXIF.2), to indicate that the
device has switched from the ring oscillator to the external clock source.
The ring oscillator runs at approximately 15MHz, but varies over temperature and voltage. As a result, no serial
communication or precision timing should be attempted while running from the ring oscillator, since the operating
frequency is not precise. Likewise, the Ethernet and CAN controllers derive their timing from the system clock and
should not be enabled until RGMD = 0. The reset (default) state of the RGSL bit is logic 0, which does not result in
use of the ring oscillator to exit stop mode.
EMI Reduction
One of the major contributors to radiated noise in an 8051-based system is the toggling of ALE. The microcontroller
allows software to disable ALE when not used by setting the ALEOFF (PMR.2) bit to a 1. When ALEOFF = 1, ALE
automatically toggles during off-chip program and data memory accesses. However, ALE remains static when
performing on-chip memory access. The default state of ALEOFF is 0, so ALE normally toggles at a frequency of
XTAL/4.
Software Breakpoint Mode
The DS80C400 provides a special software-breakpoint mode for code-debug purposes. Breakpoint mode can be
enabled by setting the BPME bit (ACON.4) to a logic 1. Once enabled, the A5h op code can be used to create a
break in code execution. When the break op code (A5h) is executed, all clocks to the timer 0, 1, 2, 3, and watchdog
timer blocks are stopped and any serial port operation (when derived from a timer) is halted. Additionally, the state
machine controlling access to timed-access-protected SFRs is suspended. Much like an interrupt, the CPU
generates a hardware LCALL and vector to address location 000083h. Unlike an interrupt, however, the return
address is not pushed onto the stack, but is placed into the BPA1 (LSB), BPA2 (MSB), and BPA3 (XSB) SFRs, and
the A5h op code is used to exit breakpoint mode and return to the address contained in the BPA3:1 SFRs.
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