TMS370Cx36
8-BIT MICROCONTROLLER
SPNS039B – JANUARY 1996 – REVISED FEBRUARY 1997
16
POST OFFICE BOX 1443
HOUSTON, TEXAS 77251–1443
low-power and IDLE modes (continued)
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X = Don’t care
(SCCR2.6)
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(SCCR2.7)
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MODE SELECTED
1
1
HALT
0
X
IDLE
When low-power modes are disabled through a programmable contact in the mask-ROM devices, writing to the
SCCR2.6-7 bits is ignored. In addition, if an IDLE instruction is executed when low-power modes are disabled
through a programmable contact, the device always enters the IDLE mode.
To provide a method for always exiting low-power modes for mask-ROM devices, INT1 is enabled automatically
as a nonmaskable interrupt (NMI) during low-power modes when the hard watchdog mode is selected. This
means that the NMI is generated always, regardless of the interrupt enable flags.
The following information is preserved throughout both the STANDBY and HALT modes: RAM (register file),
CPU registers (SP, PC, and ST), I/O pin direction and output data, and status registers of all on-chip peripheral
functions. Since all CPU instruction processing is stopped during the STANDBY and HALT modes, the clocking
of the WD timer is inhibited.
clock modules
The ’x36 family provides two clock options that are referred to as divide-by-1 (phase-locked loop) and
divide-by-4 (standard oscillator). Both the divide-by-1 and divide-by-4 options are configurable during the
manufacturing process of a TMS370 microcontroller. The ’x36 masked-ROM devices offer both options to meet
system engineering requirements. Only one of the two clock options is allowed on each ROM device. The ’736A
EPROM has only the divide-by-4.
The divide-by-1 clock module option provides the capability for reduced electromagnetic interference (EMI) with
no added cost.
The divide-by-1 provides a one-to-one match of the external resonator frequency (CLKIN) to the internal system
clock (SYSCLK) frequency, whereas the divide-by-4 produces a SYSCLK which is one-fourth the frequency of
the external resonator. Inside of the divide-by-1 module, the frequency of the external resonator is multiplied
by four, and the clock module then divides the resulting signal by four to provide the four-phased internal system
clock signals. The resulting SYSCLK is equal to the resonator frequency. These are formulated as follows:
Divide-by-4 option : SYSCLK
external resonator frequency
4
external resonator frequency
CLKIN
4
4
Divide-by-1 option : SYSCLK
4
CLKIN
The main advantage of choosing a divide-by-1 oscillator is the reduced EMI. The harmonics of low-speed
resonators extend through fewer of the emissions spectrum than the harmonics of faster resonators. The
divide-by-1 provides the capability of reducing the resonator speed by four times, and this results in a steeper
decay of emissions produced by the oscillator.