MOTOROLA
4-10
ON-CHIP MEMORY
M68HC11
REFERENCE MANUAL
lect (CSEL) control bit in the OPTION register is zero, the E clock is used; when CSEL
is one, an on-chip resistor-capacitor (RC) oscillator is used. The frequency of this on-
chip RC oscillator is about 2.5 MHz but varies with processing.
The recommended programming and erase time is 10 ms when V
DD
is 5 Vdc
±
10 per-
cent and the E clock is 2 MHz. If the E clock is 1 MHz or less, the CSEL bit should be
written to one to enable the on-chip RC oscillator to drive the V
PP
charge pump. For
an E clock between 1 and 2 MHz, the programming and erase times can be increased
to 20 ms, or the RC oscillator can be selected. Experimentation has shown the EE-
PROM is programmable with V
DD
equal to 3 Vdc and CSEL equals one to enable the
on-chip RC clock.
CSEL also enables a separate RC oscillator associated with the A/D converter sys-
tem. The E-clock frequency (where switchover to CSEL equals one is recommended)
is lower for the A/D than it is for EEPROM operations. In the A/D system, switching to
CSEL equals one can increase conversion errors; thus, it is better to perform A/D con-
versions with CSEL equals zero. In some applications, it is worthwhile to switch CSEL
on and off, depending on whether A/D or EEPROM programming/erase operations are
occurring. Refer to
12.2.2 A/D Charge Pump and Resistor-Capacitor (RC) Oscilla-
tor
for additional information.
4.3.4 EEPROM Programming Register (PPROG)
The PPROG register controls programming and erasure of the on-chip EEPROM. The
PPROG register may be read or written at any time, but programming and erase se-
quences are strictly controlled by logic to prevent unintentional changes to EEPROM
data. In the MC68HC11A8, the CONFIG register EEPROM location cannot be pro-
grammed or erased unless the MCU is operating in special test or special bootstrap
mode. The V
PP
power supply voltage is not enabled to the EEPROM array until all se-
quence requirements are met for a programming or erase operation. The required se-
quence consists of the following steps: 1) write to PPROG with EEPROM latch control
(EELAT) bit equals one and EEPROM programming voltage enable (EEPGM) bit
equals zero; 2) write to a valid EEPROM location or the CONFIG address; 3) write to
PPROG with EELAT and EEPGM bits equal one. Hardware logic enforces this se-
quence by imposing the following restrictions. If an attempt is made to change both
EELAT and EEPGM to ones with the same write operation, neither bit is set (enforces
step 1). Writes to EEPROM addresses are inhibited while EEPGM is one, which pre-
vents two kinds of errors. First, step 2 must be performed before step 3, or no EE-
PROM changes will occur. Second, a write to a different EEPROM location is
prevented while a programming or erase operation is in progress.
In some members of the M68HC11 Family, there is a block protection mechanism that
can inhibit programming and erasure of the CONFIG register or selected areas of EE-
PROM. After reset, these block protect control bits (in a block protect (BPROT) regis-
ter) are set to inhibit EEPROM changes. A user can write these bits to zero to enable
programming and erase operations, but this write must be performed within 64 cycles
after reset. The user may write these bits back to one at any time to inhibit further EE-
PROM changes. Once this protection is re-enabled, it remains in effect until another
reset. There is no BPROT register in the MC68HC11A8.