DS89C420-QCL Datasheet, PDF (100/139 Pages) Maxim Integrated Products – Ultra-High-Speed Flash Microcontroller User’s Guide

Ultra-High-Speed Flash
Microcontroller User’s Guide
When serving as an I/O port, the drive varies as follows: for logic 0, the port invokes a strong pulldown; for logic 1, the port invokes a
strong pullup for two oscillator cycles to assist with the logic transition. Then the port reverts to a weak pullup. This weak pullup is main-
tained until the port transitions from logic 1 to logic 0. External circuits can overdrive the weak pullup. This allows the logic 1 output
state to serve as the input state as well.
Substantial DC current is available in both the high and low levels. However, the power dissipation limitations make it inadvisable to heav-
ily load multiple pins. In general, sink and source currents should not exceed 10mA total per port (8 bits) and 25mA total per package.
Input Functions
The input state of the I/O ports is the same as that of the output logic 1. That is, the pin is pulled weakly to logic 1. This logic 1 state is
easily overcome by external components. Thus, after software writes a 1 to the port pin, the port is configured for input. When the port
is read by software, the state of the pin is read. The only exception is the read-modify-write instructions, discussed earlier. If the exter-
nal circuit is driving logic 1, then the pin is logic 1. If the external circuit is driving logic 0, then it overcomes the internal pullup. Thus,
the pin is the same as the driven logic state. Note that the port latch is not altered by a read operation. Therefore, if logic 0 is driven
onto a port pin from an external source, then removed, the pin reverts to the weak pullup, as determined by the internal latch.
SECTION 11: PROGRAMMABLE TIMERS
The ultra-high-speed microcontroller incorporates three 16-bit programmable timers and has a watchdog timer with a programmable
interval. Because the watchdog timer is significantly different from the other timers, it is described separately. The 16-bit timers are
referred to as timers.
The three timers offer the same controls and I/O functions that were available in the 80C32. As mentioned, the actual timing of these func-
tions is user selectable to be compatible with the instruction cycle of the older generation of 8051 family (12 clocks per tick) or the new
generation (1 clock per tick). The timing for each of the three timers can be selected independently and can be changed dynamically.
In most modes, the timers can be used as either counters of external events or timers. When functioning as a counter, 1 to 0 transi-
tions on a port pin are monitored and counted. When functioning as timers, they effectively count oscillator or system clock cycles. The
time base for the timer function is detailed later in this section. Because an input clock pulse must be sampled high for two system
clock cycles and low for two system clock cycles in order to be recognized, this sets the maximum sampling frequency on any timer
input at one-fourth of the main system clock frequency.
Since the ultra-high-speed microcontroller timers have a variety of features, the following lists summarize the capabilities:
Timer 0
Timer 1
Timer 2
13-bit timer/counter
13-bit timer/counter
16-bit timer/counter
16-bit timer/counter
16-bit timer/counter
16-bit timer with capture
8-bit timer w/ autoreload
8-bit timer w/ autoreload 1
6-bit autoreload timer/counter
Two 8-bit timer/counters
External control pulse timer/counter
16-bit up/down autoreload
External control pulse timer/counter
Baud-rate generator
Timer/counter
Baud-rate generator
Timer output clock generator
16-BIT TIMERS
Timers 0 and 1 are nearly identical. Timer 2 has several additional features such as up/down counting, capture values, and an option-
al output pin that make it unique. The following table summarizes the SFR bits that control operation of timers 0, 1, and 2. Detailed bit
descriptions can be found in Section 4. After the table, timers 0 and 1 are described first, followed by a separate description for timer
2. As mentioned above, the time base for each timer can be varied, which is discussed in more detail in the following pages.
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